Asymmetric induction during electron transfer mediated photoreduction of carbonyl compounds: role of zeolites

Photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined. In solution five of the 17 ketones undergo intramolecular hydrogen abstraction reaction even in the presence of a chiral amine and the rest are photoreduced to the corresponding alcohol. Within zeolites all 17 ketones yielded in presence of a chiral amine, the corresponding alcohol as the major product. When a chiral amine was used as the coadsorbent within alkali ion exchanged zeolites, enantiomerically enriched alcohol was formed in all cases. The best chiral induction was obtained with phenyl cyclohexyl ketone (enantiomeric excess: 68%). 1H-13C Cross Polarization Magic Angle Spinning (CP-MAS) experiments, with a model ketone (perdeuterated acetophenone) and chiral amine (pseudoephedrine) included within MY zeolites, suggested that the cation brings the reactant and the chiral amine closer. The role of the cation in such a process is also revealed by the computation results. The results presented here highlight the importance of a supramolecular structure in forcing a closer interaction between a reactant and a chiral inductor that could be used to achieve asymmetric induction in photoproducts.     

Enantioselective photoreduction of arylalkyl ketones via restricting the reaction to chirally modified zeolite cages

Obtaining a high enantiomeric excess during a photoreaction within a zeolite is hampered by the statistical distribution of reactant and chiral inductor molecules within the cages of a zeolite. By restricting the photoreactions to only those cages that contain both the reactant and a chiral inductor, one should be able to avoid reactions that yield racemic products. This approach is illustrated with the photoreduction of an arylalkyl ketone by a chiral inductor with an amino group.