O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with ... more O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al(2)O(3). To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R-S switch system is provided.
The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employ... more The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employed as chiral modifiers for heterogeneous enantioselective hydrogenations on supported Pt catalysts, has been investigated by using attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory (DFT) electronic structure calculations. The ATR-IR spectroscopic investigation provided detailed insight of the adsorbed modifiers under conditions close to those employed during catalytic processes, and electronic structure calculations were used as a complement to the experiments to uncover the implications of conformational changes in generating the topology of the surface chiral site. The structural investigation of the adsorbed modifiers revealed a relationship between the spatial positions of the ether substituents and the enantiodifferentiation induced by the modified catalyst observed in the hydrogenation of alpha-activated ketones. Experiments and calculations corroborate a model, according to which the addition of a bulky ether group to CD reshapes the chiral sites, thus generating catalytic chiral surfaces with different and, in some cases (e.g. hydrogenation of ketopantolactone), even opposite enantioselective properties to those obtained with CD without altering the absolute configuration of the modifier. The study also confirms that active surface conformations of cinchona modifiers are markedly different from those existing in vacuum and in solution, thus underlying the necessity of investigating the surface-modifier interaction in order to understand enantioselectivity.
O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with ... more O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al(2)O(3). To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R-S switch system is provided.
The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employ... more The adsorption behavior of O-methyl and O-trimethylsilyl derivatives of cinchonidine (CD), employed as chiral modifiers for heterogeneous enantioselective hydrogenations on supported Pt catalysts, has been investigated by using attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory (DFT) electronic structure calculations. The ATR-IR spectroscopic investigation provided detailed insight of the adsorbed modifiers under conditions close to those employed during catalytic processes, and electronic structure calculations were used as a complement to the experiments to uncover the implications of conformational changes in generating the topology of the surface chiral site. The structural investigation of the adsorbed modifiers revealed a relationship between the spatial positions of the ether substituents and the enantiodifferentiation induced by the modified catalyst observed in the hydrogenation of alpha-activated ketones. Experiments and calculations corroborate a model, according to which the addition of a bulky ether group to CD reshapes the chiral sites, thus generating catalytic chiral surfaces with different and, in some cases (e.g. hydrogenation of ketopantolactone), even opposite enantioselective properties to those obtained with CD without altering the absolute configuration of the modifier. The study also confirms that active surface conformations of cinchona modifiers are markedly different from those existing in vacuum and in solution, thus underlying the necessity of investigating the surface-modifier interaction in order to understand enantioselectivity.
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