- University of St. Andrews, EaStCHEM School of Chemistry, St. Andrews, Fife, KY16 9ST, UK
- 01334474244
John C Walton
University of St Andrews, Chemistry, Faculty Member
Research Interests: History, Archaeology, Geology, Paleontology, Chronology, and 3 moreRadiocarbon Dating, Kerygma, and Flood Myth
The scope and limitations of a photoinitiated N- to C-sulfonyl migration process within a range of dihydropyridinones is assessed. This sulfonyl transfer proceeds without erosion of either diastereo- or enantiocontrol, and is general... more
The scope and limitations of a photoinitiated N- to C-sulfonyl migration process within a range of dihydropyridinones is assessed. This sulfonyl transfer proceeds without erosion of either diastereo- or enantiocontrol, and is general across a range of N-sulfonyl substituents (SO2R; R = Ph, 4-MeC6H4, 4-MeOC6H4, 4-NO2C6H4, Me, Et) as well as C(3)-(aryl, heteroaryl, alkyl and alkenyl) and C(4)-(aryl and ester) substitution. Crossover reactions indicate an intermolecular step is operative within the formal migration process, although no crossover from C-sulfonyl products was observed. EPR studies indicate the intermediacy of a sulfonyl radical and a mechanism is proposed based upon these observations.
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The radical reaction of alkanethiols with tert-butyl isocyanide in boiling benzene gives rise to intermediate thioimidoyls radicals which undergo easy fragmentation yielding tert-butyl isothiocyanate and corresponding alkyl radicals.... more
The radical reaction of alkanethiols with tert-butyl isocyanide in boiling benzene gives rise to intermediate thioimidoyls radicals which undergo easy fragmentation yielding tert-butyl isothiocyanate and corresponding alkyl radicals. These reactions allow a new protocol for the generation of alkyl radicals under tin-free conditions
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ABSTRACT Synthetic routes to polyradicals are briefly reviewed; in addition the synthesis and electrochemical characterization of a novel anthraquinone derivatized thiophene monomer, 1-[4-(3-thienyl)butyl]2-anthraquinone carboxylate (I)... more
ABSTRACT Synthetic routes to polyradicals are briefly reviewed; in addition the synthesis and electrochemical characterization of a novel anthraquinone derivatized thiophene monomer, 1-[4-(3-thienyl)butyl]2-anthraquinone carboxylate (I) is discussed. This monomer may be electropolymerized onto Pt in CH2Cl2 by successively more anodic potential sweeps from 0.0V, finally reaching a steady growth at a limit of +2.0V vs Ag+/0. Thhe voltammogram of the resulting conducting polymer displaysa polymer waves at Epa + 1.0 and Epc + 0.84V, and quinone waves at E0′ = −0.80 and −1.24V vs Ag.+/0 The polymer is active towards O2 reduction in pH7 buffered aqueous solution.
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An electron paramagnetic resonance study of intermediates generated from aromatic aldiminesElectronic supplementary information (ESI) available: Gaussian98 or Jaguar optimised structures for precursor imines and imidoyl radicals together with their energies at all levels of theory. See http://www...more
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An Electron Spin Resonance Study of Pentadienyl and Related Radicals : Homolytic Fission of Cyclobut-2-enylmethyl Radicals ... By Alwyn G. Davies, Chemistry Department, University College London, 20 Gordon Street, London WC1 H OAJ David... more
An Electron Spin Resonance Study of Pentadienyl and Related Radicals : Homolytic Fission of Cyclobut-2-enylmethyl Radicals ... By Alwyn G. Davies, Chemistry Department, University College London, 20 Gordon Street, London WC1 H OAJ David Griller, Keith U. Ingold, ...
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ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full... more
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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Here, we highlight the ability of peri-substitution chemistry to promote a series of unique P-P/P-As coupling reactions, which proceed with concomitant C-H bond formation. This dealkanative reactivity represents an interesting and... more
Here, we highlight the ability of peri-substitution chemistry to promote a series of unique P-P/P-As coupling reactions, which proceed with concomitant C-H bond formation. This dealkanative reactivity represents an interesting and unexpected expansion to the established family of main-group dehydrocoupling reactions. These transformations are exceptionally clean, proceeding essentially quantitatively at relatively low temperatures (70-140 °C), with 100% diastereoselectivity in the products. The reaction appears to be radical in nature, with the addition of small quantities of a radical initiator (azobisisobutyronitrile) increasing the rate dramatically, as well as altering the apparent order of reaction. DFT calculations suggest that the reaction involves dissociation of a phosphorus centred radical (stabilised by the peri-backbone) to the P-P coupled product and a free propyl radical, which carries the chain. This unusual reaction demonstrates the powerful effect that geometric con...
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Keywords: carboxylic acids; free radicals; organic synthesis; photocatalysis; titania
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... Journal of Crystal Growth 107 (1991) 325 330 North-Holland Synthesis and decomposition studies of dialkyltellurides of type RTeR A. Ewan D. McQueen ... both) a radical or fl-hydrogen elimina-tion mechanism(s). In both mechanisms,... more
... Journal of Crystal Growth 107 (1991) 325 330 North-Holland Synthesis and decomposition studies of dialkyltellurides of type RTeR A. Ewan D. McQueen ... both) a radical or fl-hydrogen elimina-tion mechanism(s). In both mechanisms, cleavage of the first Te C bond is assumed ...
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J. Chem. Soc., Perkin Trans. 2, 2001, 1758-1764 DOI:10.1039/B104859G (Paper). Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: an EPR spectroscopic study of chain propagation. Leon V. Jackson and John C.... more
J. Chem. Soc., Perkin Trans. 2, 2001, 1758-1764 DOI:10.1039/B104859G (Paper). Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: an EPR spectroscopic study of chain propagation. Leon V. Jackson and John C. Walton. University of St. ...
Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N-O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N- and O-centred... more
Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N-O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N- and O-centred radicals. This article reviews the use of these precursors for studying the structures, reactions and kinetics of the released radicals. Two classes have been exploited for radical generation; one comprises carbonyl oximes, principally oxime esters and amides, and the second comprises oxime ethers. Both classes release an iminyl radical together with an equal amount of a second oxygen-centred radical. The O-centred radicals derived from carbonyl oximes decarboxylate giving access to a variety of carbon-centred and nitrogen-centred species. Methods developed for homolytically dissociating the oxime derivatives include UV irradiation, conventional thermal and microwave heating. Photoredox catalytic methods succeed well with specially functionalised o...
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The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical,... more
The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical, complemented by DFT computations at the CAM-B3LYP level applied to the bicarbonate radical itself. Methyl carbonate radicals were generated by UV irradiation of oxime carbonate precursors. Kinetic EPR was employed to measure rate constants and Arrhenius parameters for their dissociation to CO2 and methoxyl radicals. With oleate and cholesterol lipid components, methyl carbonate radicals preferentially added to their double bonds; with linoleate and linolenate substrates, abstraction of the bis-allylic H atoms competed with addition. This contrasts with the behavior of ROS such as hydroxyl radicals that selectively abstract allylic and/or bis-allylic H atoms. The thermodynamic and activation parameters for bicarbonate radical dissociation, obtained from D...
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Sensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals, which were characterised by EPR spectroscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent... more
Sensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals, which were characterised by EPR spectroscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent cyclisations yielding phenanthridine-type products from ortho-additions. For benzofuran and benzothiophene acceptors, spiro-cyclisation predominated at low temperatures, but thermodynamic control ensured ortho-products, benzofuro- or benzothieno-isoquinolines, formed at higher temperatures. Estimates by steady-state kinetic EPR established that iminyl radical cyclisations onto aromatics took place about an order of magnitude more slowly than prototypical C-centred radicals. The cyclisation energetics were investigated by DFT computations, which gave insights into factors influencing the two cyclisation modes.
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The concept behind the research described in this article was that of marrying the 'soft' methods of radical generation with the effectiveness and flexibility of nucleophile/electrophile synthetic procedures. Classic studies with... more
The concept behind the research described in this article was that of marrying the 'soft' methods of radical generation with the effectiveness and flexibility of nucleophile/electrophile synthetic procedures. Classic studies with pulse radiolysis and laser flash photolysis had shown that free radicals could be more acidic than their closed shell counterparts. QM computations harmonised with this and helped to define which radical centres and which structural types were most effective. Radicals based on the sulfonic acid moiety and on the Meldrum's acid moiety (2,2-dimethyl-1,3-dioxane-4,6-dione) were found to be extreme examples in the superacid class. The ethyne unit could be used as a very effective spacer between the radical centre and the site of proton donation. The key factor in promoting acidity was understood to be the thermodynamic stabilisation of the conjugate anion-radicals released on deprotonation. Solvation played a key part in promoting this and theoretic...
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Radical centers close to protons are known to enhance their dissociation. Investigation of the generality of this radical enhanced deprotonation (RED-shift) phenomenon, and the kinds of structures in which it operates, are reported. The... more
Radical centers close to protons are known to enhance their dissociation. Investigation of the generality of this radical enhanced deprotonation (RED-shift) phenomenon, and the kinds of structures in which it operates, are reported. The pKs for sulfinic, sulfonic, pentan-2,4-dione, and Meldrum's acid species, with adjacent radicals centered on C-, N-, and O atoms, were computed by a DFT method from free energies of deprotonation. All series showed significant RED-shifts that increased with the electronegativity of the radical center. The hugely negative pKobtained for a Meldrum's acid with an alkoxyl radical substituent showed it to belong to the superacid class. The ethyne unit was found to be uniquely effective at enhancing acidity and conducting RED-shifts through chains up to and beyond 20 atoms. These connector units enable a radical center to alter the pKof a spatially remote acidic group. RED-shifted species were characterized by conjugate radical anions displaying si...
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Recent theoretical research employing a continuum solvent model predicted that radical centers would enhance the acidity (RED-shift) of certain proton-donor molecules. Microhydration studies employing a DFT method are reported here with... more
Recent theoretical research employing a continuum solvent model predicted that radical centers would enhance the acidity (RED-shift) of certain proton-donor molecules. Microhydration studies employing a DFT method are reported here with the aim of establishing the effect of the solvent micro-structure on the acidity of radicals with and without RED-shifts. Microhydration cluster structures were obtained for carboxyl, carboxy-ethynyl, carboxy-methyl, and hydroperoxyl radicals. The numbers of water molecules needed to induce spontaneous ionization were determined. The hydration clusters formed primarily round the CO₂ units of the carboxylate-containing radicals. Only 4 or 5 water molecules were needed to induce ionization of carboxyl and carboxy-ethynyl radicals, thus corroborating their large RED-shifts.
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Knitting: Recently Antonchick and Manna described a unique annulation that knits together three acetophenones to construct cyclopropanes. The cascade is mediated by organocopper and free radical species, and amounts to the first known... more
Knitting: Recently Antonchick and Manna described a unique annulation that knits together three acetophenones to construct cyclopropanes. The cascade is mediated by organocopper and free radical species, and amounts to the first known [1+1+1] cyclotrimerization. It works well for ketones having electron-deficient or electron-rich substituents in their aryl rings. DTBP=di-tert-butylperoxide.
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The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole,... more
The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole, indole, pyridine, quinoline, quinazoline and related 6-membered ring-containing aza-arenes figure prominently. This review focuses on the search for fast, efficient and environmentally friendly preparative methods for these rings with specific emphasis on iminyl radical-mediated procedures. Oxime derivatives, particularly oxime esters and oxime ethers, are attractive precursors for these radicals. Their use is described in conventional thermolytic, microwave-assisted and UV-vis based preparative procedures. Photoredox-catalyzed protocols involving designer oxime ethers are also covered. Choice can be made amongst these synthetic strategies for a wide variety of 5- and 6-membered ring heterocycles including phenanthridine and related aza-arenes. Appli...
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This article reviews the excited-state quenching, pro-vitamin A activity and anticarcinogenicity of carotenes and xanthophylls in relation to their chemical structures. Excited-state quenching improved with the length of the conjugated... more
This article reviews the excited-state quenching, pro-vitamin A activity and anticarcinogenicity of carotenes and xanthophylls in relation to their chemical structures. Excited-state quenching improved with the length of the conjugated chain structure. Pro-vitamin A activity was dependent on the presence of at least one beta-ionyl ring structure. The effectiveness of carotenoids as antioxidants depended on their ability to trap peroxyl radicals with production of resonance-stabilized carotenyl radicals. The products identified from oxidations of carotenes and xanthophylls with molecular oxygen and other oxidizing agents are presented. The free radical-mediated mechanisms that have been proposed to account for the different classes of products are reviewed.
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Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled... more
Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled boriranyl radicals to be detected and characterized by EPR spectroscopy for the first time. Their EPR parameters indicated they had planar pi-character. From competition experiments the rate constant for H-atom abstraction was determined and found to be about two orders of magnitude less than for NHC-boranes. The B-H BDE of an NHC-borirane was estimated to be about 95 kcal mol-1.
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5- to 6-member ring enlargements of 3-oxa-2-silacyclopentylmethyl to 4-oxa-3-silacyclohexyl radicals were investigated by EPR spectroscopy and QM computations of model indano-oxasilacyclopentane and oxasilinanyl compounds. Both... more
5- to 6-member ring enlargements of 3-oxa-2-silacyclopentylmethyl to 4-oxa-3-silacyclohexyl radicals were investigated by EPR spectroscopy and QM computations of model indano-oxasilacyclopentane and oxasilinanyl compounds. Both experimental and computational evidence favored a mechanism via a concerted 1,2-migration of the "tethered" Si-group. Thus, the "forbidden" 1,2-Si-group migration from carbon to carbon becomes allowed when the Si-group is "tethered". The EPR data from 3-oxa-2-silacyclopentylmethyl radicals disclosed ground state conformations having semioccupied p-orbitals close to antiperiplanar with respect to their β-Si-C bonds, but indicated Si-hyperconjugation (β-silicon effect) was insignificant in radicals. Kinetic data was obtained by the steady state EPR method for ring enlargement of indano-3-oxa-2-silacyclopentylmethyl radicals. The scope of the novel rearrangement in terms of other ring types and sizes, as well as the analogous 1,2-migration of "tethered" C-centered groups, was explored computationally.
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Tricyclo [3.1.1.03,6]hept-6-yl radicals were generated by bromine abstraction from 6-bromo-tricyclo[3.1.1.03,6] heptane , and observed by e.p.r. spectroscopy. In spite of their high ring strain the radicals were found not to rearrange... more
Tricyclo [3.1.1.03,6]hept-6-yl radicals were generated by bromine abstraction from 6-bromo-tricyclo[3.1.1.03,6] heptane , and observed by e.p.r. spectroscopy. In spite of their high ring strain the radicals were found not to rearrange readily but to take part in bimolecular combination reactions and to abstract hydrogen from the methylene groups of triethylsilane. Similar treatment of 6-bromomethyltricyclo[3.1.1.03,6] heptane gave tricyclo [3.1.1.03,6]hept-6- ylmethyl radicals, which rearranged so rapidly that only the product of β-scission, the 6-methylenebicyclo[3.1.1]hept-3-yl radical, was observed spectroscopically . The rate constant for β-scission of the tricyclo [3.1.1.03,6]hept-6-ylmethyl radical, estimated from e.p.r. experiments and also from reduction of 6-bromomethyltricyclo[3.1.1.03,6] heptane with tributyltin hydride, was found to be >5×109 s-1 at 298 K.
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Part 1 $.-Addition of Trifluoromethyl Radicals to Fluoroethylenes ... BY JOHN N. CAPE (in part), ALAN C. GREIG, JOHN M. TEDDER AND JOHN C. WALTON* Department of Chemistry, The University, St. Andrews, Fife ICY16 9ST ... A competitive... more
Part 1 $.-Addition of Trifluoromethyl Radicals to Fluoroethylenes ... BY JOHN N. CAPE (in part), ALAN C. GREIG, JOHN M. TEDDER AND JOHN C. WALTON* Department of Chemistry, The University, St. Andrews, Fife ICY16 9ST ... A competitive method has ...
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Page 1. J. CHEM. SOC. PERKIN TRANS. II 1989 2169 Structural Fluxionality in the Tricyclo[3.3.1 .02v8]nona-3,6-dienyl and Bicyclo[3.2.2]nona-2,6,8-trienyl Radicals ... 9-Deuterio-and 2-deuterio-9-bromotricyclo[3.3.1 .021*] nona-3,6-diene... more
Page 1. J. CHEM. SOC. PERKIN TRANS. II 1989 2169 Structural Fluxionality in the Tricyclo[3.3.1 .02v8]nona-3,6-dienyl and Bicyclo[3.2.2]nona-2,6,8-trienyl Radicals ... 9-Deuterio-and 2-deuterio-9-bromotricyclo[3.3.1 .021*] nona-3,6-diene were reduced with tributyltin hydride. ...
Under dry, anaerobic conditions, TiO(2) photocatalysis of carboxylic acid precursors resulted in carbon-carbon bond-forming processes. High yields of dimers were obtained from TiO(2) treatment of carboxylic acids alone. On inclusion of... more
Under dry, anaerobic conditions, TiO(2) photocatalysis of carboxylic acid precursors resulted in carbon-carbon bond-forming processes. High yields of dimers were obtained from TiO(2) treatment of carboxylic acids alone. On inclusion of electron-deficient alkenes, efficient alkylations were achieved with methoxymethyl and phenoxymethyl radicals. In reactions with maleic anhydride or maleimides, phenoxyacetic acid produced chromenedione derivatives in addition to adducts. These photocatalytic reactions are simple and cheap to perform, and the TiO(2) is easily removed by filtration. The anaerobic photocatalysis strategy offers a range of synthetic possibilities.
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N-Heterocyclic carbene boranes (NHC-boranes) are a new... more
N-Heterocyclic carbene boranes (NHC-boranes) are a new "clean" class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH(3) hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH(2)* radicals are planar pi-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC-boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC-BH(3) centers. Combinations of two NHC-boryl radicals afforded 1,2-bis-NHC-diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC-boryls. Bromine atoms were rapidly transferred to imidazole-based NHC-boryl radicals from alkyl, allyl, and benzyl bromides. Chlorine-atom abstraction was, however, much less efficient and only observed for sterically unhindered NHC-boryls reacting with allylic and benzylic chlorides. For an NHC-borane containing a bulky thexyl substituent at boron, the tertiary H atom of the thexyl group was selectively removed. The resulting beta-boron-containing alkyl radical rapidly underwent beta scission of the B-C bond with production of an NHC-boryl radical and an alkene.
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JA Martinho Sim&s,l and John C. Walton*J Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1 A ORs, Department of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST UK, Faculty... more
JA Martinho Sim&s,l and John C. Walton*J Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1 A ORs, Department of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST UK, Faculty of Science, University of Ottawa, 32 ...
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Microwave irradiations of 2-(aminoaryl)alkanone O-phenyl oximes and carbonyl compounds generate iminyl radicals in company with imines; iminyl on imine ring closure yields dihydroquinazolines or quinazolines when ZnCl2 is included in the... more
Microwave irradiations of 2-(aminoaryl)alkanone O-phenyl oximes and carbonyl compounds generate iminyl radicals in company with imines; iminyl on imine ring closure yields dihydroquinazolines or quinazolines when ZnCl2 is included in the mixture.
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Nitric oxide did not react directly with retinyl acetate but, in the presence of an initiator, a series of nitroxide radicals was produced and observed by EPR spectroscopy. Product analyses showed that rapid oxidative degradation of... more
Nitric oxide did not react directly with retinyl acetate but, in the presence of an initiator, a series of nitroxide radicals was produced and observed by EPR spectroscopy. Product analyses showed that rapid oxidative degradation of retinyl acetate gave a series of carbonyl compounds. ...
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ABSTRACT Dringend gesucht werden neue Reagentien, die Reaktionen freier Radikale vermitteln, um den Problemen ein Ende zu setzen, die mit Organozinnhydriden einhergehen (vor allem deren hohe Toxizitt). Zwar wurden schon Fortschritte bei... more
ABSTRACT Dringend gesucht werden neue Reagentien, die Reaktionen freier Radikale vermitteln, um den Problemen ein Ende zu setzen, die mit Organozinnhydriden einhergehen (vor allem deren hohe Toxizitt). Zwar wurden schon Fortschritte bei zinnorganischen Reagentien erzielt, z. B. hinsichtlich der Abtrennbarkeit von den Produkten, doch genießen Organosiliciumverbindungen und etliche Thiocarbonylverbindungen ein weitaus besseres Ansehen. Radikalische Reaktionssequenzen ausgehend von Cyclohexadienderivaten als „proaromatischen” Verbindungen wurden entwickelt (siehe Schema), und mit Tetrathiafulvalenen lassen sich Umwandlungen im Grenzgebiet zwischen radikalischen und ionischen Reaktionen durchführen. Z=Alken
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An urgent search is being made for generic reagents that promote free radical synthetic transformations as means of banishing the toxic threat of organotin hydrides. Although second-generation tin reagents are beguiling, organosilanes and... more
An urgent search is being made for generic reagents that promote free radical synthetic transformations as means of banishing the toxic threat of organotin hydrides. Although second-generation tin reagents are beguiling, organosilanes and a range of thiocarbonyl compounds are more intrinsically benign. Metal-free radical chain sequences based around cyclohexadiene derivatives are being developed (see reaction scheme), and tetrathiafulvalenes mimic metals and allow a crossover from homolytic to ionic chemistry. Z=alkene.
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Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled... more
Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled boriranyl radicals to be detected and characterized by EPR spectroscopy for the first time. Their EPR parameters indicated they had planar pi-character. From competition experiments the rate constant for H-atom abstraction was determined and found to be about two orders of magnitude less than for NHC-boranes. The B-H BDE of an NHC-borirane was estimated to be about 95 kcal mol-1.
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Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled... more
Three-member ring boracyclopropanes (boriranes) with N-heterocyclic carbene substituents were prepared by a recently discovered route. H-atoms were selectively abstracted from the boron atoms by t-butoxyl radicals and this enabled boriranyl radicals to be detected and characterized by EPR spectroscopy for the first time. Their EPR parameters indicated they had planar pi-character. From competition experiments the rate constant for H-atom abstraction was determined and found to be about two orders of magnitude less than for NHC-boranes. The B-H BDE of an NHC-borirane was estimated to be about 95 kcal mol-1.
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ABSTRACT A range of 1-alkylcyclohexa-2,5-diene-1-carboxylic acids were prepared by Birch reduction–alkylation of benzoic acid and their efficiency as mediators of alkyl radical chain addition and cyclisation processes was investigated.... more
ABSTRACT A range of 1-alkylcyclohexa-2,5-diene-1-carboxylic acids were prepared by Birch reduction–alkylation of benzoic acid and their efficiency as mediators of alkyl radical chain addition and cyclisation processes was investigated. Reductive alkylations were respectably successful, even with only one or two equivalents of alkene, for secondary, tertiary and benzylic radicals. Reaction of 1-[2-(cyclohex-2-enyloxy)ethyl]cyclohexa-2,5-diene-1-carboxylic acid yielded the product of exo-trig-cyclisation, i.e. 7-oxabicyclo[4.3.0]nonane, in a yield comparable to that obtained from the tributyltin hydride induced cyclisation of 3-(2′-iodoethoxy)cyclohexene. This, together with the isolation of both exo- and endo-cyclisation products from 1-[2-(6,6-dimethylbicyclo[3.1.1]hept-2-en-2-ylmethoxy)ethyl]cyclohexa-2,5-diene-1-carboxylic acid established that ring closures could also be satisfactorily mediated with these reagents. Preparations were completely free of metal contaminants and direct reduction of the alkyl radicals, prior to addition or cyclisation, was completely absent. However, the desired products were accompanied by alkylbenzenes, together with by-products from the initiator decompositions, and this complicated work-up. Failure to obtain 1-[2-(prop-2-yn-1-yloxy)cyclohexyl]cyclohexa-2,5-diene-1-carboxylic acid in Birch reductive alkylations with trans-1-iodo-2-(prop-2-yn-1-yloxy)cyclohexane (and the corresponding bromide) indicated a limitation on precursor synthesis. The Birch reduction–alkylation was not of universal applicability and was suppressed for alkyl halides having β-substituents.
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ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select... more
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Complex perovskites from the system Sr3Ca1(Zr(1−x)Ta(1+y))O8.5−x/2 offer a high concentration of oxygen vacancies and show promise as good proton conductors for SOFC and related applications. The oxygen-ion vacancies can be filled by O–H... more
Complex perovskites from the system Sr3Ca1(Zr(1−x)Ta(1+y))O8.5−x/2 offer a high concentration of oxygen vacancies and show promise as good proton conductors for SOFC and related applications. The oxygen-ion vacancies can be filled by O–H groups, by exposing the sample to a wet 5% H2/Ar atmosphere at intermediate temperatures (350–400 °C). However, by using high temperatures (>1000 °C) and/or pressures, we present
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The conformations of some six-membered oxygen-and sulphur-containing heterocyclic rings have been investigated by EPR spectroscopy using the methylenyl group, CH 2, directly attached to a ring carbon atom as a,'spin probe'. For... more
The conformations of some six-membered oxygen-and sulphur-containing heterocyclic rings have been investigated by EPR spectroscopy using the methylenyl group, CH 2, directly attached to a ring carbon atom as a,'spin probe'. For the 2-oxacyclohexylmethyl radical the ...
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2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0-4) were prepared and their SRN1-type reactions with several base systems examined. The best conditions to promote cyclo-coupling to the corresponding... more
2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0-4) were prepared and their SRN1-type reactions with several base systems examined. The best conditions to promote cyclo-coupling to the corresponding benzocycloalkane derivatives involved use of LDA in THF. The precursors with 3-C-atom and 4-C-atom spacers gave good yields of 2-(1'-phenylindan-1'-yl)-2-oxazolines and 2-(1-phenyl-1,2,3,4-tetrahydronaphthalen-1-yl)-2-oxazoline, respectively. The major products from the precursor with a 5-C-atom spacer were derivatives of benzocycloheptane in which the oxazoline group had undergone a novel areneotropic migration from the end of the spacer to the benzo ring. The product from reaction of the corresponding 2-C-atom precursor was a 9-oxazolinophenanthrene derivative. EPR spectroscopy showed the intermediates of the LDA-promoted reactions to be radical anions of the product benzocycloalkanes. This supported an SRN1-type chain mechanism involving initial production of aryl radicals connected to azaenolate ions via the spacer groups. Intramolecular radical to carbanion coupling then generated ring-closed benzocycloalkane radical anions that transferred an electron to more precursor. Diastereoselective radical to carbanion cyclo-coupling reactions were carried out with 2-bromophenylpropyl precursors containing chiral 2-oxazolines. The diastereoselectivity achievable was modest, but the product diastereoisomeric Indane derivatives were easily separable by chromatography.
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Page 1. J. Am. Chem. Soc. 1994,116, 2159-2166 2759 Homolytic Reactions of Homocubane and Basketane: Rearrangement of the 9-Basketyl Radical by Multiple @-Scissions Gavin T. Binmore,? Ernest W. DeUa,*a Gordon M. Elseyb Nicbolas J. Head,#... more
Page 1. J. Am. Chem. Soc. 1994,116, 2159-2166 2759 Homolytic Reactions of Homocubane and Basketane: Rearrangement of the 9-Basketyl Radical by Multiple @-Scissions Gavin T. Binmore,? Ernest W. DeUa,*a Gordon M. Elseyb Nicbolas J. Head,# and John C. Walton*,f ...
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[reaction: see text] But-3-enylthioimidoyl radicals were shown by EPR spectroscopy and end product analysis to ring-close predominantly in the 5-exo mode with a rate constant of 2.4 x 10(4) s(-)(1) at 300 K to afford substituted... more
[reaction: see text] But-3-enylthioimidoyl radicals were shown by EPR spectroscopy and end product analysis to ring-close predominantly in the 5-exo mode with a rate constant of 2.4 x 10(4) s(-)(1) at 300 K to afford substituted dihydrothiophenylmethyl radicals. This ring closure was in competition with dissociation to but-3-enyl radicals and an isothiocyanate. The dissociation predominated at temperatures above ca. 300 K.
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Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(3)O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in... more
Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(3)O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in the presence of various concentrations of the amines. The reaction is a non-chain process in which the 2-methyl-2-phenylpropyl radical and its rearrangement product, the 2-benzylpropan-2-yl radical, abstract hydrogen from both the solvent and the amine. Cross-disproportionation reactions of the rearranged radical led to the formation of significant amounts of beta,beta-dimethylstyrene. Rate constants for hydrogen abstraction by the unrearranged, primary alkyl radical from n-dodecane (k(373K) = 3.5 x 10(3) M(-)(1) s(-)(1)), diphenylamine (k(373K) = 1.3 x 10(6) M(-)(1) s(-)(1)), and the substituted diarylamines were determined from the product yields and the known rate constant for the radical rearrangement. From kinetic experiments with N-deuteriodiphenylamine the deuterium kinetic isotope effect,k(NH)/k(ND), was found to be 2.3 at 373 K.
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Page 1. 10730 J. Am. Chem. SOC. 1992, 114, 10730-10738 Homolytic Reactions of Cubanes. Generation and Characterization of Cubyl and Cubylcarbinyl Radicals Ernest W. Della,*$+ Nicholas J. Head,+ Philip Mallon,' and ...
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ABSTRACT A series of 3-substituted bicyclo[1.1.1]pent-1-yl radicals, including the 3-fluoro derivative, was generated by bromine atom abstraction from 1-bromo-3-substituted-bicyclo[1.1.1]pent and examined by EPR spectroscopy. The... more
ABSTRACT A series of 3-substituted bicyclo[1.1.1]pent-1-yl radicals, including the 3-fluoro derivative, was generated by bromine atom abstraction from 1-bromo-3-substituted-bicyclo[1.1.1]pent and examined by EPR spectroscopy. The exceptionally large hyperfine splittings obtained from magnetic nuclei of the 3-substituents indicated that cross cage electronic interactions were substantial in these species. Bromine atom abstraction by triethylsilyl radicals from 1-bromo-3-fluorobicyclo[1.1.1]pentane was found to take place more rapidly than bromine abstraction from the unsubstituted parent, i.e., the fluorine substituent mediated a significant polar effect. Evidence was found of a novel disproportionation process in which the gamma-fluorine atom was transferred from the 3-fluoro radical to a triethylsilyl or to a second bicyclo[1.1.1]pent-1-yl radical; an analogous chlorine atom transfer process was found for the 3-chloro radical. Ab initio MO calculations (6-31G* basis with electron correlation up to MP4) on the 3-fluoro- and 3-methyl-substituted radicals indicated that loss of the substituent to give [1.1.1]propellane would be comparatively easy for both species.
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ABSTRACT Alk-4-enesulphonyl chlorides and alk-5-enesulphonyl chlorides cyclise under free radical conditions mainly in the mode to give tetrahydrothiopyran-1,1-dioxide and thiepane- 1,1-dioxide respectively.
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ABSTRACT Thioimidoyl radicals were generated by addition of alkylsulfanyl radicals to alkyl isonitriles and were characterized by electron paramagnetic resonance (EPR) spectroscopy. The beta-scissions of their C.S-C bonds were studied by... more
ABSTRACT Thioimidoyl radicals were generated by addition of alkylsulfanyl radicals to alkyl isonitriles and were characterized by electron paramagnetic resonance (EPR) spectroscopy. The beta-scissions of their C.S-C bonds were studied by variable-temperature EPR spectroscopy and the fragmentation rate constants and activation energies were calculated. The scission rates depend on the stability of the released alkyl radicals but in any case, at room temperature, the processes were fast. Data collected on similar oxyimidoyls showed that their fragmentations are slightly slower compared to those of analogous thioimidoyls. The scission rates of selenoimidoyls could not be studied by EPR and were evaluated by theoretical calculations. EPR experiments also enabled both beta-scission and 5-exo ring closure rate constants of two S-but-3-enyl-substituted imidoyl radicals to be determined, showing that cyclization prevails only at low temperatures. Density functional theory (DFT) theoretical calculations predicted that the fragmentation process preferentially occurs from the s-cis rotamers (X-C bond) of the imidoyl radicals. Thio- and seleno-imidoyls (but not oxyimidoyls) prefer s-trans conformations so that their fragmentations involve prior rotation about the X-C bond.
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In the self-initiated oxidation of β-carotene with molecular oxygen the rate of oxygen uptake was shown to depend on the oxygen partial pressure. Epoxides, dihydrofurans, carbonyl compounds, carbon dioxide, oligomeric material, traces of... more
In the self-initiated oxidation of β-carotene with molecular oxygen the rate of oxygen uptake was shown to depend on the oxygen partial pressure. Epoxides, dihydrofurans, carbonyl compounds, carbon dioxide, oligomeric material, traces of alcohols, and probably ...
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... Soc., Chem. Commun., 1995, 27 RSC . 2, G. Binmore, L. Cardellini and JC Walton, J. Chem. Soc., Perkin Trans. ... Chem. Soc. Jpn., 1985, 58, 3335 Search PubMed . 11, B. Schroeder, WP Neumann, J. Hollaender and H.-P. Becker, Angew.... more
... Soc., Chem. Commun., 1995, 27 RSC . 2, G. Binmore, L. Cardellini and JC Walton, J. Chem. Soc., Perkin Trans. ... Chem. Soc. Jpn., 1985, 58, 3335 Search PubMed . 11, B. Schroeder, WP Neumann, J. Hollaender and H.-P. Becker, Angew. Chem., 1972, 84, 894 Search PubMed . ...
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Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor... more
Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or (ii) interact with an acceptor precursor that picks up an electron with production of a radical anion. The radical cations of appropriate donors convert to neutral radicals usually by loss of a proton. The most efficient donors for synthetic purposes contain adjacent functional groups such that the neutral radicals are resonance stabilized. Thus, ET from allylic alkenes and enol ethers generated allyl type radicals that reacted with 1,2-diazine or imine co-reactants to yield functionalized hydrazones or benzylanilines. SCPC with tertiary amines enabled electron-deficient alkenes to be alkylated and furoquinolinones to be accessed. Primary amines on their own led to self-...
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This paper describes how the rates of 5-exo-ring closures of unsaturated iminyl radicals to pyrrolomethyl radicals respond to substituents in the pentenyl chain and at the C=N bond. Benzyl- and acyl oxime esters, as well as dioxime... more
This paper describes how the rates of 5-exo-ring closures of unsaturated iminyl radicals to pyrrolomethyl radicals respond to substituents in the pentenyl chain and at the C=N bond. Benzyl- and acyl oxime esters, as well as dioxime oxalates, were identified as suitable iminyl radical sources for electron paramagnetic resonance (EPR) spectroscopy. Pentenyliminyl radicals with aryl substituents at their C=N bonds, and one with an alkyl substituent at its C=N bond, were studied in solution by steady-state continuous wave EPR spectroscopy. All the pentenyliminyls selectively ring closed in the 5-exo-mode rather than the 6-endo-mode. EPR monitoring of the decay of the 2,2-dimethyl-1-phenylpent-4-enyliminyl radical showed that it underwent bimolecular combination at about the diffusion controlled limit (2kt approximately 3 x 10(8) M(-1) s(-1) at 245 K). The rate constant for 5-exo-ring closure of phenylpentenyliminyl (8.8 x 10(3) s(-1) at 300 K) was a factor of 25 smaller than the rate constant for hex-5-enyl radical cyclization. The rate of cyclization was slower for an iminyl having a Me group at the site of 5-exo-cyclization but faster for an iminyl with an Et substituent at the terminus of the C=C double bond. Surprisingly, the 2,2-dimethyl-1-phenylpent-4-enyliminyl radical, with a bismethyl group in its pentenyl chain, ring closed more slowly than the unsubstituted analogue. DFT computations were in accord with this inverse gem-dimethyl effect and suggested it resulted from steric interaction of the Ph and bis-Me groups which forced the aromatic ring out of the plane of the imine moiety. To check on the role of the Ph substituent, pentenyliminyls lacking this group were sought. A pentenyliminyl radical with an alkyl group in place of the Ph group, and a single Me group in its pentenyl chain, was generated by means of an unsymmetrical dioxime oxalate precursor. The k(c) for this species was a factor of 2.5 larger than k(c) for the original pentenyliminyl, suggesting that the normal positive gem-dimethyl effect does operate for pentenyliminyls lacking the aromatic substituent at the C=N bond. DFT computations also successfully reproduced this trend for model iminyls. It appears that for alkenyliminyl radicals positive or negative gem-dimethyl effects on the cyclization can be induced by appropriate choice of the second substituent on the C=N bond.
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The O-H bond dissociation enthalpies (BDEs) of 13 oximes,... more
The O-H bond dissociation enthalpies (BDEs) of 13 oximes, RR'C=NOH, having R and/or R' = H, alkyl, and aryl are reported. Experimental anchor points used to validate the results of theoretical calculations include (1) the O-H BDEs of (t-Bu)2C=NOH, t-Bu(i-Pr)C=NOH, and t-Bu(1-Ad)C=NOH determined earlier from the heat released in the reaction of (t-Bu)2C=NO* with (PhNH)2 in benzene and EPR spectroscopy (Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J. Am. Chem. Soc. 1973, 95, 8610), all of which were decreased by 1.7 kcal/mol to reflect a revision to the heat of formation of (E)-azobenzene (which has significant ramifications for other BDEs) and to correct for the heat of hydrogen bonding of (t-Bu)2C=NOH (alphaH2 = 0.43 measured in this work) to benzene, and (2) the measured rates of thermal decomposition of six RR'C=NOCH2Ph at 423 or 443 K, which were used to derive O-H BDEs for the corresponding RR'C=NOH. Claims (Bordwell, F. G.; Ji, G. Z. J. Org. Chem. 1992, 57, 3019; Bordwell, F. G.; Zhang, S. J. Am. Chem. Soc. 1995, 117, 4858; and Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819) that the O-H BDEs in mono- and diaryloximes are significantly lower than those for alkyloximes due to delocalization of the unpaired electron into the aromatic ring have always been inconsistent with the known structures of iminoxyl radicals as are the purported perpendicular structures, i.e., phi(Calpha-C=N-O*) = 90 degrees, for sterically hindered dialkyl iminoxyl radicals. The present results confirm the 1973 conclusion that simple steric effects, not electron delocalization or dramatic geometric changes, are responsible for the rather small differences in oxime O-H BDEs.
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Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals gave distinct EPR spectra thanks to surprisingly large beta-hydrogen atom hyperfine splittings that enabled them to be characterized and monitored. DFT computations... more
Axial and equatorial cyclohexylacyl and tetrahydropyranyl-2-acyl radicals gave distinct EPR spectra thanks to surprisingly large beta-hydrogen atom hyperfine splittings that enabled them to be characterized and monitored. DFT computations indicated that the axial species (X = CH(2)) had a higher barrier to rotation about the (O)C(alpha)-C(beta) bond. The computed difference Delta H degrees for the axial and equatorial radicals (R = H, X = CH(2)) was 0.8 kcal mol(-)(1).
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ABSTRACT The thermal dissociation of bis(pentamethylcyclopentadienyl) to pentamethylcyclopentadienyl radicals has been studied by EPR spectroscopy in the temperature range 365–393 K. The enthalpy of dissociation of the dimer is found to... more
ABSTRACT The thermal dissociation of bis(pentamethylcyclopentadienyl) to pentamethylcyclopentadienyl radicals has been studied by EPR spectroscopy in the temperature range 365–393 K. The enthalpy of dissociation of the dimer is found to be 18.8 kcal mol–1 and combination of this result with empirical and semi-empirical estimates of the enthalpies of formation of the dimer and of pentamethylcyclopentadiene, leads to ΔH°f(Me5C5˙)= 10.7 kcal mol –1, DH°(Me5C5–H)= 70.6 kcal mol–1 and EgMeH(Me5C5˙)= 34.2 kcal mol–1(where EgMeH is the methane-based stabilisation energy). Steric strain is shown to be a major factor leading to weakening of the inter-ring bond of the dimer. The pentaethyl and pentamethoxycarbonyl analogues have been investigated, but neither is suitable for quantitative measurements.
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An electron paramagnetic resonance study of intermediates generated from aromatic aldiminesElectronic supplementary information (ESI) available: Gaussian98 or Jaguar optimised structures for precursor imines and imidoyl radicals together with their energies at all levels of theory. See http://www...more
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ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select... more
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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ABSTRACT A graphical abstract is available for this content
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ABSTRACT Substituted benzyloxycarbonyloxyl radicals were generated by sensitised photolyses of benzyl oxime carbonates. EPR spectroscopy showed they ring closed exclusively by spiro-cyclisation onto the ipso-C-atoms of the aromatic rings.... more
ABSTRACT Substituted benzyloxycarbonyloxyl radicals were generated by sensitised photolyses of benzyl oxime carbonates. EPR spectroscopy showed they ring closed exclusively by spiro-cyclisation onto the ipso-C-atoms of the aromatic rings. β-Scission of the alkoxycarbonyloxyls to CO2 and benzyloxyl radicals increasingly competed and became dominant above 270 K. The first rate parameters for spiro-cyclisations of O-centred radicals onto aromatics were obtained by the steady-state kinetic EPR method. Pentafluoro-substitution of the ring substantially reduced the spiro-cyclisation rate. Activation barriers of the spiro-cyclisations were computed by DFT to be about half those of the alternative ortho-cyclisations. Consideration of the TS structures suggested the preference for spiro- over ortho-cyclisation resulted from better overlap of the oxyl SOMO with the aromatic π-system during spiro closure.
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Page 1. J. CHEM. SOC., CHEM. COMMUN., 1990 1589 Cubyl and 4-Fluorocubyl Radicals Ernest W. Della," a Gordon M. Elsey,a Nicholas J. Head a and John C. Walton" b a Flinders University of South Australia, School of Physical ...
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ABSTRACT Direct photolytic and radical induced homolyses of O-alkyl arylaldoxime ethers (ArCHNOR) were studied by EPR spectroscopy and by end product analyses. Initiating radicals (X˙), including t-BuO˙, t-BuS˙, alkyl and Me3Sn˙, added... more
ABSTRACT Direct photolytic and radical induced homolyses of O-alkyl arylaldoxime ethers (ArCHNOR) were studied by EPR spectroscopy and by end product analyses. Initiating radicals (X˙), including t-BuO˙, t-BuS˙, alkyl and Me3Sn˙, added rapidly to the CN double bond to give adduct oxyaminyl radicals (ArCHXN˙OR) that could be observed and characterised by EPR spectroscopy. For O-alkyl arylaldoxime ethers containing H-atoms attached to the carbon adjacent to the ether oxygen (OR = OCHR12), t-BuO˙ radicals also abstracted this hydrogen to yield oxyalkyl radicals that underwent rapid β-scission to afford iminyl radicals (ArCHN˙) and an aldehyde or ketone (R12CO). As judged by the relative importance of ROH and ArCN amongst the products, abstraction of the iminyl hydrogen atom also took place to yield oximidoyl radicals (ArC˙NOR), although this could not be confirmed by EPR spectroscopic observation of these radicals. Thus, homolysis induced by t-BuO˙ radicals took place comparatively unselectively. Addition of the t-BuO˙ radical to the CN double bond of oxime ethers was very fast, the rate constant being comparable to that for addition of the same radical to nitrones. Direct and photosensitised UV photolysis of O-alkyl arylaldoxime ethers gave alkoxyl and aryliminyl radicals in very low yields. Although traces of 2-methyltetrahydrofuran were detected from cyclisation of the pent-4-enyloxyl radical generated by direct photolysis of O-pent-4-enyl benzaldoxime, yields were too low for preparative purposes.
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ABSTRACT Oxime carbonates have been synthesised and their potential studied as a new class of radical precursor. A general synthetic route, avoiding laboratory use of phosgene, was developed. Alkyl or aryl 1H-imidazole-1-carboxylates were... more
ABSTRACT Oxime carbonates have been synthesised and their potential studied as a new class of radical precursor. A general synthetic route, avoiding laboratory use of phosgene, was developed. Alkyl or aryl 1H-imidazole-1-carboxylates were prepared from the corresponding alcohols and 1,1-carbonyldiimidazole, then reacted with oximes in the presence of base. UV photolyses of oxime carbonates released particular N- and O-centred radicals suitable for spectroscopic examination. Spiro-cyclisations of alkoxycarbonyloxyl radicals onto aromatic acceptors and 5-exo cyclisations onto alkene acceptors were studied by EPR spectroscopy. EPR data also provided insight to the loss of CO2 from alkoxycarbonyloxyl radicals. The conformational preferences of 2-oxo-1,3-dioxolan-4-yl-methyl and related radicals were established. Modest yields of cyclic carbonates were obtained from preparative scale photolyses.
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... Ernest W. Della,"S8 Carl H. Schiesser,'nb Dennis K. Taylora and John C. Walton *ac a School of Physical Sciences, Flinders University, Bedford Park, South Australia, 5042 Department of Chemistry, Deakin University ... 16 PF... more
... Ernest W. Della,"S8 Carl H. Schiesser,'nb Dennis K. Taylora and John C. Walton *ac a School of Physical Sciences, Flinders University, Bedford Park, South Australia, 5042 Department of Chemistry, Deakin University ... 16 PF McGarry, L. J. Johnston and JC Scaiano, J. Am. Chem. ...
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A titania photoredox catalysis protocol was developed for the homocoupling of C-centered radicals derived from carboxylic acids. Intermolecular reactions were generally efficient and selective, furnishing the desired dimers in good yields... more
A titania photoredox catalysis protocol was developed for the homocoupling of C-centered radicals derived from carboxylic acids. Intermolecular reactions were generally efficient and selective, furnishing the desired dimers in good yields under mild neutral conditions. Selective cross-coupling with two acids proved unsuccessful. An intramolecular adaptation enabled macrocycles to be prepared, albeit in modest yields.
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... 0.0234] nona-6-enyl radicals, and from the ... Experimental ESR spectra were recorded with a Bruker ER 200D spectrometer on degassed solutions in Spectrosil tubes irradiated with light from a 500W super-pressure Hg arc. Unless ...
Hydrogen abstraction by t-butoxyl radicals from primary amines RCH 2 NH 2 (R = H, Me, Et, Pr i , or Bu t ) gave mainly -aminoalkyl radicals which were observed by esr spectroscopy. The barriers to rotation about the CN bonds were... more
Hydrogen abstraction by t-butoxyl radicals from primary amines RCH 2 NH 2 (R = H, Me, Et, Pr i , or Bu t ) gave mainly -aminoalkyl radicals which were observed by esr spectroscopy. The barriers to rotation about the CN bonds were determined from exchange broadening. For ...
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The rotational barriers about the CN bonds in aminoalkyl radicals, H2NCHR, were found to be significantly greater when R is an acceptor group than when R is an alkyl group; this is good evidence for capto-dative stabilisation of the... more
The rotational barriers about the CN bonds in aminoalkyl radicals, H2NCHR, were found to be significantly greater when R is an acceptor group than when R is an alkyl group; this is good evidence for capto-dative stabilisation of the ground state radicals.
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... BI incomuHiDG cooizncs AM) see DEHTDROOXASES by David Laurie, Edward Lucas, Derek C. Honhebel, and Colin J. Suckling,* Department of Pure and Applied Chemistry, University of Strathclyde, 295, Cathedral Street, Glasgow Gl 1XL,... more
... BI incomuHiDG cooizncs AM) see DEHTDROOXASES by David Laurie, Edward Lucas, Derek C. Honhebel, and Colin J. Suckling,* Department of Pure and Applied Chemistry, University of Strathclyde, 295, Cathedral Street, Glasgow Gl 1XL, Scotland, and John C. Walton ... Cheg. ...
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An Electron Spin Resonance Study of Pentadienyl and Related Radicals : Homolytic Fission of Cyclobut-2-enylmethyl Radicals ... By Alwyn G. Davies, Chemistry Department, University College London, 20 Gordon Street, London WC1 H OAJ David... more
An Electron Spin Resonance Study of Pentadienyl and Related Radicals : Homolytic Fission of Cyclobut-2-enylmethyl Radicals ... By Alwyn G. Davies, Chemistry Department, University College London, 20 Gordon Street, London WC1 H OAJ David Griller, Keith U. Ingold, ...
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ABSTRACT The EPR spectra of 1,2-dihydro-6-ethoxy-2,2,4-trimethyl- and 1,2-dihydro-6-ethoxy-2,2,4,8-tetramethyl-quinolin-1-yl radicals were observed in heptane solution. The hyperfine splittings showed that this class of radical is... more
ABSTRACT The EPR spectra of 1,2-dihydro-6-ethoxy-2,2,4-trimethyl- and 1,2-dihydro-6-ethoxy-2,2,4,8-tetramethyl-quinolin-1-yl radicals were observed in heptane solution. The hyperfine splittings showed that this class of radical is extensively delocalised with significant spin density at C(8). Both radicals decayed by second-order processes, the rate constants being 5 × 106 and 4 × 102 dm3 mol–1 s–1, respectively at 273 K. The latter reaction is much slower because the 8-methyl substituent blocks the formation of the 1,8′-dimer. Both radicals reacted with oxygen to give the corresponding nitroxides, although reaction was very slow for the 8-methyl derivative. A mechanism is proposed to rationalise product formation from 1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline when used as an antioxidant.
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A kinetic EPR study of the dissociation of 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals: release of aminoacyl radicals and their cyclisationElectronic supplementary information (ESI) available: Selected EPR spectra, measured radical concentrations and the corresponding rate constants at each...more
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Three naphtho[2,3-c]thiophene-4,9-diones (thienonaphthoquinones) have been prepared and their redox properties studied by chemical, electrochemical and EPR spectroscopic methods. All three quinones were readily reduced electrochemically... more
Three naphtho[2,3-c]thiophene-4,9-diones (thienonaphthoquinones) have been prepared and their redox properties studied by chemical, electrochemical and EPR spectroscopic methods. All three quinones were readily reduced electrochemically to the corresponding ...
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ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select... more
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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... Roy T. McBurney. Roy T. McBurney was born and raised in Carnoustie, Scotland. ... Fernando Portela Cubillo. Fernando Portela-Cubillo was born in Aranda de Duero (Burgos, Spain) in 1975. He obtained his first degree from the University... more
... Roy T. McBurney. Roy T. McBurney was born and raised in Carnoustie, Scotland. ... Fernando Portela Cubillo. Fernando Portela-Cubillo was born in Aranda de Duero (Burgos, Spain) in 1975. He obtained his first degree from the University of Valladolid in 2002. ...
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The reaction coordinates of an archetypical set of 5-exo cyclizations of C-, N- and O-centred radicals were investigated by computational methods. G4 theory, and DFT with the um062x functional, were able to rationalise counterintuitive... more
The reaction coordinates of an archetypical set of 5-exo cyclizations of C-, N- and O-centred radicals were investigated by computational methods. G4 theory, and DFT with the um062x functional, were able to rationalise counterintuitive factors such as the 'normal' order of rate constants being: N-centred < C-centred < O-centred radicals. The access angle between the radical centre and the double bond was identified as a key factor. Examination of its evolution during ring closure implied that rigidity at the N-ends of the chains, and the consequent extra energy needed to attain chair-like transition states, might be the reason for slow aminyl cyclizations. A novel linear correlation between cyclization activation energies and the access angles was discovered. The preference for cis-1,2-disubstituted product formation was also accounted for in terms of interaction between the hyperconjugatively delocalized SOMO and the alkene π* orbital.
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Page 1. J. CHEM. soc. PERKIN TRANS. 2 1993 21 19 Hydrogen Abstraction from Silylamines; an Investigation of the 1.2-Migration of the Trimethylsilyl Group in Aminyl Radicals Joanna M. Harris,8 John C. Walfon,*s8 Bernard Maillard! ...
Free Radical Addition to Olefins Part 20.-A Reinvestigation of the Addition of Methyl Radicals to Fluoroethylenes. BY HAMISH c. LOW, JOHN M. TEDDER" AND JOHN c. WALTON Department of Chemistry, University of St. Andrews, St. Andrews,... more
Free Radical Addition to Olefins Part 20.-A Reinvestigation of the Addition of Methyl Radicals to Fluoroethylenes. BY HAMISH c. LOW, JOHN M. TEDDER" AND JOHN c. WALTON Department of Chemistry, University of St. Andrews, St. Andrews, KY 16 9ST, Fife, Scotland ...
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Preparation of 1-phenylcyclohexa-2,5-diene-1-carboxylates and their use in free-radical mediated synthesesElectronic supplementary information (ESI) available: experimental procedures for the preparation of 1,4-dihydrobiphenyl,9 1-deuterio-1,4-dihydrobiphenyl, 1-phenylcyclohexa-2,5-diene-1-carbox...more
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ABSTRACT The main products in the early stages of β-carotene autoxidation were epoxides, β-ionone, β-apo-13-carotenone, retinal, and related carbonyl compounds; in the final mixture short chain carbonyl compounds predominated.
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Page 1. Tetrahedron Report Number 541 Conceptual and Synthetic Strategies for the Preparation of Organic Magnets Joe A. Crayston, John N. Devine and John C. Walton* School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16... more
Page 1. Tetrahedron Report Number 541 Conceptual and Synthetic Strategies for the Preparation of Organic Magnets Joe A. Crayston, John N. Devine and John C. Walton* School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK Received 30 June 2000 ...
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Carbamoyl radicals were generated from oxime oxalate amides, and the kinetics of their 4-exo cyclizations onto C=C and C=NO bonds, leading to beta-lactam-containing species, were studied by EPR spectroscopy. DFT computations with model... more
Carbamoyl radicals were generated from oxime oxalate amides, and the kinetics of their 4-exo cyclizations onto C=C and C=NO bonds, leading to beta-lactam-containing species, were studied by EPR spectroscopy. DFT computations with model carbamoyl radicals predicted 4-exo ring closures onto C=NO bonds to be facile, especially when tert-butyl substituents were present. The reverse ring-opening reactions were predicted to have much higher activation energies. Experimental evidence also favored slow reverse ring opening.
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A general synthetic route to oxime oxalate amides was developed and applied to the preparation of molecules incorporating N-benzyl-N-alkenyl amides linked with acetone oxime or benzaldoxime units. In addition,... more
A general synthetic route to oxime oxalate amides was developed and applied to the preparation of molecules incorporating N-benzyl-N-alkenyl amides linked with acetone oxime or benzaldoxime units. In addition, 2-substituted-thiazolidine-4-carboxylic acid methyl ester amides of oxalyl benzaldoxime were also prepared. It was shown by EPR spectroscopy that the oxalyl benzaldoxime amides dissociated to produce benziminyl and carbamoyl (aminoacyl) radicals when photolysed with 4-methoxyacetophenone as a photosensitizer. Carbamoyl radicals derived from N-alk-3-enyl oxime oxalate amides underwent ring closure to afford pyrrolidin-2-ones. The analogous N-alk-2-enyl precursors afforded azetidin-2-ones. Reactions of the cyclohexenyl and cinnamyl oxime oxalate amides afforded a bicyclic beta-lactam and a 3-benzyl-substituted beta-lactam respectively. Interestingly, both products were isolated as hydroxylated compounds. A thiazolidine-derived oxime oxalate amide containing an isobutenyl side chain also dissociated with production of the corresponding thiazolidinyl-carbamoyl radical, as shown by EPR spectroscopy. GC-MS evidence indicated that this radical cyclised to afford some of the corresponding penicillin derivative
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1-Carbamoyl-1-methylcyclohexa-2,5-dienes produced the corresponding delocalised 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals on treatment with radical initiators. At temperatures above ca. 300 K dissociation to produce toluene and... more
1-Carbamoyl-1-methylcyclohexa-2,5-dienes produced the corresponding delocalised 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals on treatment with radical initiators. At temperatures above ca. 300 K dissociation to produce toluene and aminoacyl (carbamoyl) radicals took place. The alternative dissociation of the 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals to release methyl radicals and an aromatic amide did not compete. Aminoacyl radicals with allyl, butenyl or similar side chains underwent cyclisations. Moderate yields of N-benzyl-azetidin-2-ones and N-benzyl-pyrrolidin-2-ones were isolated for a range of substituents. The main by-products were N-benzyl-N-alkenylformamides. Ring closure did not take place to a significant extent for precursors with alk-2-ynyl or 2-cyanoalkyl side chains. An improved yield of 1,3-dibenzylazetidin-2-one was obtained by use of lauroyl peroxide as initiator and by inclusion of methyl thioglycolate as polarity reversal catalyst.
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Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and... more
Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and 9,10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., R(1)R(2)C[double bond, length as m-dash]NOH and PhCH(3)) and N-O bond (viz., R(1)R(2)C[double bond, length as m-dash]NH and PhCH(2)OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9,10-dihydrophenanthrene and 9,10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by [small beta]-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by [small beta]-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.
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The reactions of gallium trichloride with phenyl and deuterio-phenyl azides, as well as with 4-methoxyphenyl azide and deuterium isotopomers, were examined by product analysis, CW EPR spectroscopy and pulsed ENDOR spectroscopy. The... more
The reactions of gallium trichloride with phenyl and deuterio-phenyl azides, as well as with 4-methoxyphenyl azide and deuterium isotopomers, were examined by product analysis, CW EPR spectroscopy and pulsed ENDOR spectroscopy. The products included the corresponding anilines together with 4-aminodiphenylamine type dimers, and polyanilines. Complex CW EPR spectra of the radical cations of the dimers [ArNHC(6)H(4)NH(2)](+)˙ and trimers [ArNHC(6)H(4)NHC(6)H(4)NH(2)](+)˙ were obtained. These EPR spectra were analysed with the help of data from the deuterium-substituted analogues as well as the pulse Davies ENDOR spectra. DFT computations of the radical cations provided corroborating evidence and suggested the unpaired electrons were accommodated in extensive π-delocalised orbitals. A mechanism to account for the reductive conversion of aromatic azides to the corresponding anilines and thence to the dimers and trimers is proposed.
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The idea that ring closures of C-centred radicals onto isocyanates could be made permanent by designing the cyclised radical to undergo a rapid onward beta-scission, was investigated for the 2-(2-isocyanato)cyclopropylphenyl and... more
The idea that ring closures of C-centred radicals onto isocyanates could be made permanent by designing the cyclised radical to undergo a rapid onward beta-scission, was investigated for the 2-(2-isocyanato)cyclopropylphenyl and 2-(2-isocyanato)oxiranylphenyl radicals. The radical precursors, trans- and cis-1-bromo-(2-isocyanatocyclopropyl)benzene and (2-bromophenyl)-3-isocyanatooxirane, were prepared from the corresponding bromophenylcyclopropane and bromophenyloxirane carboxylic acids via Curtius rearrangements of the derived azides. The structure of the trans-2-(2-isocyanato)cyclopropylphenyl radical prevents cyclization, however, it was shown that isomerisation to the analogous cis-radical occurred, probably by scission of the disubstituted cyclopropane bond followed by internal rotation of the resulting resonance stabilised diradical. It was found, however, that the main product from homolytic reactions of both trans- and cis-isocyanatocyclopropyl compounds, with tributyltin hydride and tris(trimethylsilyl)silane, was the direct reduction product, trans-(2-isocyanatocyclopropyl)benzene. Only traces of cyclised products, that were probably 4,5-dihydrobenzo[c]azepin-1-one from the cyclopropane precursor and 5H-6-oxa-8-azabenzocyclohepten-9-one from the oxirane precursor, were detected. We conclude, therefore, that the rate of cyclization onto isocyanate acceptor groups must be slower in these systems than hex-5-enyl cyclization or that the reverse ring-opening process must be faster than for analogous radicals.
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... Raphael C. Mordi and John C. Walton* Department of Chemictry, University of SI. ... However, it has been suggested on tlie basis of senii-empirical SCF MO calculations that radicals containing both electron-donating and... more
... Raphael C. Mordi and John C. Walton* Department of Chemictry, University of SI. ... However, it has been suggested on tlie basis of senii-empirical SCF MO calculations that radicals containing both electron-donating and eiectron-accepting substituents at the radical centre, ie ...
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ABSTRACT Pyrrolyl-1-oxyl, 3-phenylpyrrolyl-1-oxyl and a number of deuterium-substituted derivatives were generated by hydrogen abstraction from the corresponding N-hydroxypyrroles, and observed by EPR spectroscopy. They were all transient... more
ABSTRACT Pyrrolyl-1-oxyl, 3-phenylpyrrolyl-1-oxyl and a number of deuterium-substituted derivatives were generated by hydrogen abstraction from the corresponding N-hydroxypyrroles, and observed by EPR spectroscopy. They were all transient with lifetimes ≤ 10−2 s. The hyperfine splittings showed that unpaired spin was delocalized around the five-membered ring with significant density at N-1, C-3 and C-4, but very low density at C-2 and C-5. N-Hydroxpyrrole and pyrrolyl-1-oxyl were studied by the semi-empirical AM1 method, and the radical by the INDO method. The transient pyrrolinyl-1-oxyl was also generated from the corresponding N-hydroxypyrroline and observed by EPR spectroscopy.
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ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select... more
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom... more
Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC-BH(2)Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC-B·HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC-boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The β-borylalkyl radicals obtained from N,N&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;-dipp-substituted boranes underwent exothermic β-scissions with production of dipp-Imd-BH(2)· radicals and alkenes. The reverse additions of NHC-boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC-boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.
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... Radicals. Dissociation to [ 1. 1. 11Propellane William Adcock,**+ Gavin T, Binmore: Alexander R. KrsticJ John C. Walton,*j* and John Wilkies ... 1993, (33) Adcock, W.; Clark, C. I.; Houman, A,; Krstic, AR; Pinson, J.; SavCant, J.-M.;... more
... Radicals. Dissociation to [ 1. 1. 11Propellane William Adcock,**+ Gavin T, Binmore: Alexander R. KrsticJ John C. Walton,*j* and John Wilkies ... 1993, (33) Adcock, W.; Clark, C. I.; Houman, A,; Krstic, AR; Pinson, J.; SavCant, J.-M.; Taylor, DK; Taylor, J. F. J. Am. Chem. Soc. ...
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The O-H bond dissociation enthalpies (BDEs) of 13 oximes,... more
The O-H bond dissociation enthalpies (BDEs) of 13 oximes, RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=NOH, having R and/or R&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; = H, alkyl, and aryl are reported. Experimental anchor points used to validate the results of theoretical calculations include (1) the O-H BDEs of (t-Bu)2C=NOH, t-Bu(i-Pr)C=NOH, and t-Bu(1-Ad)C=NOH determined earlier from the heat released in the reaction of (t-Bu)2C=NO* with (PhNH)2 in benzene and EPR spectroscopy (Mahoney, L. R.; Mendenhall, G. D.; Ingold, K. U. J. Am. Chem. Soc. 1973, 95, 8610), all of which were decreased by 1.7 kcal/mol to reflect a revision to the heat of formation of (E)-azobenzene (which has significant ramifications for other BDEs) and to correct for the heat of hydrogen bonding of (t-Bu)2C=NOH (alphaH2 = 0.43 measured in this work) to benzene, and (2) the measured rates of thermal decomposition of six RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=NOCH2Ph at 423 or 443 K, which were used to derive O-H BDEs for the corresponding RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=NOH. Claims (Bordwell, F. G.; Ji, G. Z. J. Org. Chem. 1992, 57, 3019; Bordwell, F. G.; Zhang, S. J. Am. Chem. Soc. 1995, 117, 4858; and Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819) that the O-H BDEs in mono- and diaryloximes are significantly lower than those for alkyloximes due to delocalization of the unpaired electron into the aromatic ring have always been inconsistent with the known structures of iminoxyl radicals as are the purported perpendicular structures, i.e., phi(Calpha-C=N-O*) = 90 degrees, for sterically hindered dialkyl iminoxyl radicals. The present results confirm the 1973 conclusion that simple steric effects, not electron delocalization or dramatic geometric changes, are responsible for the rather small differences in oxime O-H BDEs.
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C25 Journal of Organometallic Chemistry, 403 (1991) C25C28 Elsevier Sequoia SA, Lausanne JOM 21591PC Preliminary communication 1,2Migration of the trimethylsilyl group in free radicals Joanna M. Harris a, lain MacInnes a, John C. Walton a... more
C25 Journal of Organometallic Chemistry, 403 (1991) C25C28 Elsevier Sequoia SA, Lausanne JOM 21591PC Preliminary communication 1,2Migration of the trimethylsilyl group in free radicals Joanna M. Harris a, lain MacInnes a, John C. Walton a and Bernard Maillard b ...
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A wide range of biologically active compounds contain the quinazoline ring system. A new free-radical-based method of making functionalized quinazolines is described, which relies on microwave-promoted reactions of O-phenyl oximes with... more
A wide range of biologically active compounds contain the quinazoline ring system. A new free-radical-based method of making functionalized quinazolines is described, which relies on microwave-promoted reactions of O-phenyl oximes with aldehydes. A small set of 2-aminoaryl alkanone O-phenyl oximes was prepared and shown to produce dihydroquinazolines when mixed with an aldehyde in toluene and subjected to microwave heating. When ZnCl(2) was included in the reaction mixture, fully aromatic quinazolines were produced in high yields by a rapid and convenient process. The method worked well with alkyl, aryl, and heterocyclic aldehydes and for a variety of substituents in the benzenic part of the molecule. Similar reactions employing ketones instead of aldehydes were less efficient. Although some dihydroquinazolines did form, they were accompanied by several byproducts. Surprisingly, in each case, one of the byproducts was a quinoline derivative, and a plausible mechanism to account for this rearrangement is proposed.
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2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0-4) were prepared and their SRN1-type reactions with several base systems examined. The best conditions to promote cyclo-coupling to the corresponding... more
2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0-4) were prepared and their SRN1-type reactions with several base systems examined. The best conditions to promote cyclo-coupling to the corresponding benzocycloalkane derivatives involved use of LDA in THF. The precursors with 3-C-atom and 4-C-atom spacers gave good yields of 2-(1&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;-phenylindan-1&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;-yl)-2-oxazolines and 2-(1-phenyl-1,2,3,4-tetrahydronaphthalen-1-yl)-2-oxazoline, respectively. The major products from the precursor with a 5-C-atom spacer were derivatives of benzocycloheptane in which the oxazoline group had undergone a novel areneotropic migration from the end of the spacer to the benzo ring. The product from reaction of the corresponding 2-C-atom precursor was a 9-oxazolinophenanthrene derivative. EPR spectroscopy showed the intermediates of the LDA-promoted reactions to be radical anions of the product benzocycloalkanes. This supported an SRN1-type chain mechanism involving initial production of aryl radicals connected to azaenolate ions via the spacer groups. Intramolecular radical to carbanion coupling then generated ring-closed benzocycloalkane radical anions that transferred an electron to more precursor. Diastereoselective radical to carbanion cyclo-coupling reactions were carried out with 2-bromophenylpropyl precursors containing chiral 2-oxazolines. The diastereoselectivity achievable was modest, but the product diastereoisomeric Indane derivatives were easily separable by chromatography.
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Six O-phenyl ketoxime ethers,... more
Six O-phenyl ketoxime ethers, RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=NOPh 1-6, with RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; = diaryl, dialkyl, and arylalkyl, together with N-phenoxybenzimidic acid phenyl ether, PhO(Ph)C=NOPh, 7, have been shown to thermolyze at moderate temperatures with &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;clean&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; N-O bond homolyses to yield iminyl and phenoxyl radicals, RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=N(*) and PhO(*). Since 1-6 can be synthesized at room temperature, these compounds provide a new and potentially useful source of iminyls for syntheses. The iminyl from 7 undergoes a competition between beta-scission, to PhCN and PhO(*), and cyclization to an oxazole. Rate constants, 10(6) k/s(-1), at 90 degrees C for 1-6 range from 4.2 (RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; = 9-fluorenyl) to 180 (RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; = 9-bicyclononanyl), and that for 7 is 0.61. The estimated activation enthalpies for N-O bond scission are in satisfactory agreement with the results of DFT calculations of N-O bond dissociation enthalpies, BDEs, and represent the first thermochemical data for any reaction yielding iminyl radicals. The small range in k (N-O homolyses) is consistent with the known sigma structure of these radicals, and the variations in k and N-O BDEs with changes in RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; are rationalized in terms of iminyl radical stabilization by hyperconjugation: RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=N(*) &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;--&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; R(*)R&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C[triple bond]N. Calculated N-H BDEs in the corresponding RR&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;C=NH are also presented.
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The 2-(2-isocyanatophenyl)ethyl radical was generated from the corresponding bromide with tributyltin and tris(trimethylsilyl)silyl radicals and shown to ring close in the 6-endo-mode to afford 3,4-dihydro-1H-quinolin-2-one as the major... more
The 2-(2-isocyanatophenyl)ethyl radical was generated from the corresponding bromide with tributyltin and tris(trimethylsilyl)silyl radicals and shown to ring close in the 6-endo-mode to afford 3,4-dihydro-1H-quinolin-2-one as the major product. Cyclization in the 5-exo-mode to produce 2,3-dihydroindole-1-carbaldehyde, after hydrogen abstraction, was a minor reaction. Rate constants for the two processes were estimated and compared with reaction enthalpies computed by the DFT method.
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Thioimidoyl radicals were generated by addition of alkylsulfanyl radicals to alkyl isonitriles and were characterized by electron paramagnetic resonance (EPR) spectroscopy. The beta-scissions of their C.S-C bonds were studied by... more
Thioimidoyl radicals were generated by addition of alkylsulfanyl radicals to alkyl isonitriles and were characterized by electron paramagnetic resonance (EPR) spectroscopy. The beta-scissions of their C.S-C bonds were studied by variable-temperature EPR spectroscopy and the fragmentation rate constants and activation energies were calculated. The scission rates depend on the stability of the released alkyl radicals but in any case, at room temperature, the processes were fast. Data collected on similar oxyimidoyls showed that their fragmentations are slightly slower compared to those of analogous thioimidoyls. The scission rates of selenoimidoyls could not be studied by EPR and were evaluated by theoretical calculations. EPR experiments also enabled both beta-scission and 5-exo ring closure rate constants of two S-but-3-enyl-substituted imidoyl radicals to be determined, showing that cyclization prevails only at low temperatures. Density functional theory (DFT) theoretical calculations predicted that the fragmentation process preferentially occurs from the s-cis rotamers (X-C bond) of the imidoyl radicals. Thio- and seleno-imidoyls (but not oxyimidoyls) prefer s-trans conformations so that their fragmentations involve prior rotation about the X-C bond.
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Acyl oximes derived from a variety of indolylalkanones underwent a ring closure sequence during FVP to afford 9H-pyrido[2,3-b]indoles. Unlike UV light promoted reactions of oxime esters, the mechanism is almost certainly not mediated by... more
Acyl oximes derived from a variety of indolylalkanones underwent a ring closure sequence during FVP to afford 9H-pyrido[2,3-b]indoles. Unlike UV light promoted reactions of oxime esters, the mechanism is almost certainly not mediated by iminyl radicals but probably involves tautomerism, elimination of acetic acid, and a final electrocyclic ring closure.
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Spectroscopic data are consistent with computations that show that, in their most stable conformations, the peroxyl moiety is equatorial in cyclohexylperoxyl radicals and axial in oxa- and most polyoxacyclohexyl-2-peroxyl radicals.
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ABSTRACT
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ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select... more
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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The relative selectivity of atomic halogenation of 2-substituted butanes is influenced by the phase and by solvents. There are solvents which increase the selectivity compared with the gas phase and solvents which decrease the relative... more
The relative selectivity of atomic halogenation of 2-substituted butanes is influenced by the phase and by solvents. There are solvents which increase the selectivity compared with the gas phase and solvents which decrease the relative selectivity. However the most striking ...
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Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a... more
Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.
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ABSTRACT 3-Substituted cyclohexadienyl radical generated by hydrogen abstraction from 3-alkylcyclohexa-1,4-diene-3-carboxylic acids readily fragment to produce alkyl radicals and benzoic acid; suitably functionalised alkyl groups cyclize... more
ABSTRACT 3-Substituted cyclohexadienyl radical generated by hydrogen abstraction from 3-alkylcyclohexa-1,4-diene-3-carboxylic acids readily fragment to produce alkyl radicals and benzoic acid; suitably functionalised alkyl groups cyclize in moderate yields.
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Oxime carbonates were found to be excellent precursors for the clean and direct generation of iminyl radicals under UV irradiation. Suitably functionalised iminyls underwent cyclisations yielding various phenanthridines and also... more
Oxime carbonates were found to be excellent precursors for the clean and direct generation of iminyl radicals under UV irradiation. Suitably functionalised iminyls underwent cyclisations yielding various phenanthridines and also substituted quinolines and isoquinolines. EPR and X-ray analyses of oxime carbonates provided insight into the mechanism.
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Microwave irradiation of alkenone O-phenyl oximes produces iminyl radicals that ring close to yield dihydropyrrole derivatives; pyrroles and pyridines can be obtained from related precursors.
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Dioxime oxalates are useful precursors for the clean generation of iminyl radicals by sensitised UV photolysis and can be adapted for serviceable preparations of 3,4-dihydro-2H-pyrroles and phenanthridines.
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HASS el al. were the first to report that atomic chlorination of alkanes was less selective in the liquid phase than in the gas phase.2 Twenty years later Russell described a more pronounced solvent effect; hc observed that when aromatic... more
HASS el al. were the first to report that atomic chlorination of alkanes was less selective in the liquid phase than in the gas phase.2 Twenty years later Russell described a more pronounced solvent effect; hc observed that when aromatic compounds are used as solvents the ...
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Cascade, domino, or tandem processes, that link together two or more transformations in one pot, are increasing in popularity because they lead to improvements in synthetic efficiency and decreases in environmental impact. Not only do... more
Cascade, domino, or tandem processes, that link together two or more transformations in one pot, are increasing in popularity because they lead to improvements in synthetic efficiency and decreases in environmental impact. Not only do these cascades contain choice mechanistic gems but they also deliver compact and elegant syntheses of complex natural products. Longer cascades require more functional groups precisely configured within carefully designed initial molecular architectures. Such &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;purposeful&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; molecules can be thought of as chemical algorithms.This article surveys the phenomenal range of unimolecular free-radical cascades. A convenient system for classifying free-radical cascades is described that is useful for evaluating and comparing cascades and aids the design of synthetic routes to polycyclic structures.Double cyclization cascades lead to cyclopentylcyclopentane or bicyclo[3.3.0]octane derivatives. Precursors that contain a ring as a template have been used to control stereochemistry in syntheses of triquinanes and many related compounds. Of the cascades containing ring-cleavage steps, the most useful are the ring expansions which have opened up new synthetic routes to medium ring polycycles.The key design features of three-stage unimolecular free-radical cascades that yielded steroid structures, are linear arrays of radical acceptor units associated with methyl groups distributed every fifth C-atom in the precursor polyenes. Ring cleavage is the reverse of cyclization. In special, symmetrical structures, therefore, this led to sequences that were reversible, thus launching endlessly repeating cascades supported by delightfully fluxional structures. The science of &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;programming&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; organic molecules to achieve particular target structures is maturing rapidly. Coordination and classification of the welter of information in this area is intended to facilitate design and hence to extend the range and complexity of attainable structures.
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Chemistry and Physics have aptly been described as " most excellent children of Intellect and Art " [1]. Both these " children " engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which... more
Chemistry and Physics have aptly been described as " most excellent children of Intellect and Art " [1]. Both these " children " engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which are amongst the most lively and exciting. Checking out radicals dancing to the music of entropy round their potential energy ballrooms is surely both entertaining and enlightening. Radicals' old favorite convolutions are noteworthy, but the new styles, modes and arrangements appearing on the scene are even more interesting. Some of these are ephemeral and enjoy only a brief appearance, others are retro-types reappearing in new guises, still others are genuinely new and " go viral " in the scientific world. This Special Issue of Molecules contains the observations and reflections of a select group of chemists and physicists fascinated by this spectacle. It contains an eclectic mix reflecting on new modes and advances as well as on permutations and combinations that revive mature themes. Many types of radicals rapidly couple with suitably accoutered acceptor partners. On encountering an alkene the outcome may well be a recurrent cycle in which more and more alkene is assimilated into a growing polymer chain. Resolving what was going on in these processes played a major part in the acceptance of free radicals as key players in organic chemistry. Furthermore the polymer products have found many valuable uses in textiles, as structural materials and in electronic devices. It is no surprise, therefore, to find the polymerization theme well represented in this Special Issue. Instead of polymer, the outcome of a radical and acceptor encounter may be a conveniently functionalized product. When the radical is so constructed that the acceptor group forms part of its own architecture; their encounter
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Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane... more
Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 × 10 4 M-1 s-1 , and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a π-radical, analogous to the benzyl radical.
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A set of oxime carbamates having N-alkyl and N,N-dialkyl substituents were prepared via carbonyldiimidazole intermedi-ates. It was shown by EPR spectroscopy that they underwent clean homolysis of their N−O bonds upon UV photolysis. During... more
A set of oxime carbamates having N-alkyl and N,N-dialkyl substituents were prepared via carbonyldiimidazole intermedi-ates. It was shown by EPR spectroscopy that they underwent clean homolysis of their N−O bonds upon UV photolysis. During photolysis of acetophenone O-allylcarbamoyl oxime, the corresponding oxazolidin-2-onylmethyl radical was detected by EPR spectrosco-py, providing the first evidence that N-monosubstituted carbamoyloxyl radicals can hold their structural integrity. N,N-Disubstituted carbamoyloxyl radicals dissociated rapidly at the lowest accessible temperatures. Above room temperature, both types of oxime carbamate acted as selective new precursors for aminyl and iminyl radicals. Rate parameters were measured for 5-exo cyclization of N-benzyl-N-pent-4-enylaminyl radicals; the rate constant was smaller than for C-centered and O-centered analogues. Oxime carbamates derived from the volatile diethylamine afforded aryliminyl radicals that proved convenient for phenanthridine preparations. ■ INTRODUCTION Oxime carbamates (O-carbamoyl oximes) have been known for a considerable time for their antimicrobial activity 1 and as inhibitors of various enzymes. 2 End-product analyses of complex mixtures obtained from photolyses of a few carbamate pesticides were reported, 3 but the notion of oxime carbamates as selective radical precursors had not been tested. We recently discovered that oxime carbonates efficiently dissociate upon photolysis to generate iminyl and alkoxycarbonyloxyl radicals. These oxime derivatives proved to be valuable precursors for clean syntheses of several types of heterocycles. 4 In view of the structural similarity of oxime carbamates [ArC(R 1)N− OC(O)NR 2 R 3 ], we conjectured that their N−O bonds would also break upon photolysis. In this way, access to iminyl radicals and the much more exotic carbamoyloxyl radicals might be gained. The latter radicals were essentially unknown. Ingold and co-workers' attempts to observe them by laser flash photolysis of tert-butyl percarbamates [RR′NC(O)OOBu-t] disclosed only aminyls (RR′N •), 5 as did Newcomb's study of N-hydroxypyridine-2-thione carbamates. 6 No other reports of carbamoyloxyl radicals have appeared. They were expected to dissociate rapidly to CO 2 and aminyl radicals, so there were question marks concerning their structural integrity. Suitable tuning of the oxime carbamate structure might enable individual members of this triad of radicals (iminyl, carbamoyloxyl, aminyl) to be highlighted and examined. A distinct advantage of oxime carbamate precursors would be the opportunity to search for evidence of carbamoyloxyl radicals at much lower temperatures using electron paramagnetic resonance (EPR) spectroscopy. We carried out an exploratory density functional theory (DFT) study of the reactivity of prototypical carbamoyloxyls. We also prepared a set of model oxime carbamates (see Scheme 1) and investigated the radicals released by each upon photolysis. In this article, we show that by an appropriate choice of substituents the rare carbamoyloxyl radicals could indeed be generated. They were cleanly transformed to aminyl radicals at higher temperatures, thus providing a new and promising source of these intermediates. Information about the cyclization behavior of both carbamoy-loxyl and aminyl radicals was obtained. Furthermore, with a different substitution pattern, oxime carbamates could be adapted for release of iminyl radicals and hence for subsequent preparations of N-heterocycles.
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CONSPECTUS: Selective syntheses are now available for compounds of many classes, based on C-centered radicals, exploiting a diverse range of mechanisms. The prospect for chemistry based around N-and O-centered radicals is probably more... more
CONSPECTUS: Selective syntheses are now available for compounds of many classes, based on C-centered radicals, exploiting a diverse range of mechanisms. The prospect for chemistry based around N-and O-centered radicals is probably more favorable because of the importance of heterocycles as biologically active materials. Heteroradical chemistry is still comparatively underdeveloped due to the need for safe and easy ways of generating them. Oxime esters appeared promising candidates to meet this need because literature reports and our EPR spectroscopic examinations showed they readily dissociated on photolysis with production of a pair of N-and O-centered radicals. It soon became apparent that a whole suite of benign oxime-containing molecules could be pressed into service. The bimodality of the oxime motif meant that by suitable choice of functionality the reactions could be directed to yield selectively products from either the N-centered radicals or from the O-centered radicals. We found that on one hand photolyses of acetophenone oxime esters of carboxylic acids yielded alicyclics. On the other hand, aromatic and heteroaromatic acyl oximes (as well as dioxime oxalates) afforded good yields of phenanthridines and related heterocycles. Easily prepared oxime oxalate amides released carbamoyl radicals, and pleasingly, β-lactams were thereby obtained. Oxime carbonates and oxime carbamates, available via our novel 1,1'-carbonyldiimidazole (CDI)-based preparations, were accessible alternatives for iminyl radicals and hence for phenanthridine preparations. In their second modes, these compounds proved their value as precursors for exotic alkoxycarbonyloxyl and carbamoyloxyl radicals. Microwave-assistance was shown to be a particularly convenient procedure with O-phenyl oxime ethers. The iminyl radicals generated from such precursors with alkene, alkyne, and aromatic acceptor substituents furnished pyrrole, quinoline, phenanthridine, benzonaphthiridine, indolopyridine, and other systems. Microwave irradiations with 2-(aminoaryl)alkanone O-phenyl oximes enabled either dihydroquinazolines or quinazolines to be obtained in very good yields. The fine quality of the EPR spectra, acquired during photolyses of all the O-carbonyl oxime types, marked this as an important complement to existing ways of obtaining such spectra in solution. Quantifications enabled SARs to be obtained for key reaction types of N-and O-centered radicals, thus putting mechanistic chemistry in this area on a much firmer footing. Surprises included the inverse gem-dimethyl effect in 5-exo-cyclizations of iminyls and the interplay of spiro-with ortho-cyclization onto aromatics. Insights into unusual 4-exo-cyclizations of carbamoyl radicals showed the process to be more viable than pent-4-enyl 4-exo-ring closure. Another surprise was the magnitude of the difference in CO 2 loss rate from alkoxycarbonyloxyl radicals as compared with acyloxyl radicals. Their rapid 5-exo-cyclization was charted, as was their preferred spiro-cyclization onto aromatics. The first evidence that N-monosubstituted carbamoyloxyls had finite lifetimes was also forthcoming. It is evident that oxime derivatives have excellent credentials as reagents for radical generation and that there is ample room to extend their applications to additional radical types and for further heterocycle syntheses. There is also clear scope for the development of preparative procedures based around the alkoxyl and aminyl radicals that emerge downstream from oxime carbonate and oxime carbamate dissociations.
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Sensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals, which were char-acterised by EPR spectroscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent... more
Sensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals, which were char-acterised by EPR spectroscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent cyclisations yielding phenanthridine-type products from ortho-additions. For benzofuran and benzothiophene acceptors, spiro-cyclisation predominated at low temperatures, but thermodynamic control ensured ortho-products, benzofuro-or benzothieno-isoquinolines, formed at higher temperatures. Estimates by steady-state kinetic EPR established that iminyl radical cyclisations onto aromatics took place about an order of magnitude more slowly than prototypical C-centred radicals. The cyclisation energetics were investigated by DFT computations , which gave insights into factors influencing the two cyclisation modes.
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Photochemical reactions employing TiO 2 and carboxylic acids under dry anaerobic conditions led to several types of C−C bond-forming processes with electron-deficient alkenes. The efficiency of alkylation varied appreciably with... more
Photochemical reactions employing TiO 2 and carboxylic acids under dry anaerobic conditions led to several types of C−C bond-forming processes with electron-deficient alkenes. The efficiency of alkylation varied appreciably with substituents in the carboxylic acids. The reactions of aryloxyacetic acids with maleimides resulted in a cascade process in which a pyrrolochromene derivative accompanied the alkylated succinimide. The selectivity for one or other of these products could be tuned to some extent by employing the photoredox catalyst under different conditions. Aryloxy-acetic acids adapted for intramolecular ring closures by inclusion of 2-alkenyl, 2-aryl, or 2-oximinyl functionality reacted rather poorly. Profiles of reactant consumption and product formation for these systems were obtained by an in situ NMR monitoring technique. An array of different catalyst forms were tested for efficiency and ease of use. The proposed mechanism, involving hole capture at the TiO 2 surface by the carboxylates followed by CO 2 loss, was supported by EPR spectroscopic evidence of the intermediates. Deuterium labeling indicated that the titania likely donates protons from surface hydroxyl groups as well as supplying electrons and holes, thus acting as both a catalyst and a reaction partner.
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N-Heterocyclic carbene boranes (NHC-boranes) are a new " clean " class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring... more
N-Heterocyclic carbene boranes (NHC-boranes) are a new " clean " class of reagents suitable for reductive radical chain transformations. Their structures are well suited for their reactivity to be tuned by inclusion of different NHC ring units and by appropriate placement of diverse substituents. EPR spectra were obtained for the boron-centered radicals generated on removal of one of the BH 3 hydrogen atoms. This spectroscopic data, coupled with DFT computations, demonstrated that the NHC-BH 2 • radicals are planar π-delocalized species. tert-Butoxyl radicals abstracted hydrogen atoms from NHC-boranes more than 3 orders of magnitude faster than did C-centered radicals, although the rate decreased markedly for sterically shielded NHC-BH 3 centers. Combinations of two NHC-boryl radicals afforded 1,2-bis-NHC-diboranes at rates which also depended strongly on steric shielding. The termination rate increased to the diffusion-controlled limit for sterically unhindered NHC-boryls. Bromine atoms were rapidly transferred to imidazole-based NHC-boryl radicals from alkyl, allyl, and benzyl bromides. Chlorine-atom abstraction was, however, much less efficient and only observed for sterically unhindered NHC-boryls reacting with allylic and benzylic chlorides. For an NHC-borane containing a bulky thexyl substituent at boron, the tertiary H atom of the thexyl group was selectively removed. The resulting-boron-containing alkyl radical rapidly underwent scission of the B-C bond with production of an NHC-boryl radical and an alkene.
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Under dry, anaerobic conditions, TiO 2 photocatalysis of carboxylic acid precursors resulted in carbon−carbon bond-forming processes. High yields of dimers were obtained from TiO 2 treatment of carboxylic acids alone. On inclusion of... more
Under dry, anaerobic conditions, TiO 2 photocatalysis of carboxylic acid precursors resulted in carbon−carbon bond-forming processes. High yields of dimers were obtained from TiO 2 treatment of carboxylic acids alone. On inclusion of electron-deficient alkenes, efficient alkylations were achieved with methoxymethyl and phenoxymethyl radicals. In reactions with maleic anhydride or maleimides, phenoxyacetic acid produced chromenedione derivatives in addition to adducts. These photocatalytic reactions are simple and cheap to perform, and the TiO 2 is easily removed by filtration. The anaerobic photocatalysis strategy offers a range of synthetic possibilities.
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Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N–O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N-and O-centred... more
Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N–O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N-and O-centred radicals. This article reviews the use of these precursors for studying the structures, reactions and kinetics of the released radicals. Two classes have been exploited for radical generation; one comprises carbonyl oximes, principally oxime esters and amides, and the second comprises oxime ethers. Both classes release an iminyl radical together with an equal amount of a second oxygen-centred radical. The O-centred radicals derived from carbonyl oximes decarboxylate giving access to a variety of carbon-centred and nitrogen-centred species. Methods developed for homolytically dissociating the oxime derivatives include UV irradiation, conventional thermal and microwave heating. Photoredox catalytic methods succeed well with specially functionalised oximes and this aspect is also reviewed. Attention is also drawn to the key contributions made by EPR spectroscopy, aided by DFT computations, in elucidating the structures and dynamics of the transient intermediates.
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The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical,... more
The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical, complemented by DFT computations at the CAM-B3LYP level applied to the bicarbonate radical itself. Methyl carbonate radicals were generated by UV irradiation of oxime carbonate precursors. Kinetic EPR was employed to measure rate constants and Arrhenius parameters for their dissociation to CO 2 and methoxyl radicals. With oleate and cholesterol lipid components, methyl carbonate radicals preferentially added to their double bonds; with linoleate and linolenate substrates, abstraction of the bis-allylic H atoms competed with addition. This contrasts with the behavior of ROS such as hydroxyl radicals that selectively abstract allylic and/or bis-allylic H atoms. The thermodynamic and activation parameters for bicarbonate radical dissociation, obtained from DFT computations, predicted it would indeed have substantial lifetime in gas and nonpolar solvents. The acidity of bicarbonate radicals was also examined by DFT methods. A noteworthy linear relationship was discovered between the known pK a 's of strong acids and the computed numbers of microsolvating water molecules needed to bring about their ionization. DFT computations with bicarbonate radicals, solvated with up to eight water molecules, predicted that only five water molecules were needed to bring about its complete ionization. On comparing with the correlation, this indicated a pK a of about −2 units. This marks the bicarbonate radical as the strongest known carboxylic acid.