- 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.