Achiral Zeolites as Reaction Media for Chiral Photochemistry

Abstract

Obtaining enantiomerically-enriched photoproducts from achiral reactants has been a long-sought goal. The various methods developed to achieve chiral induction in photoproducts during the last fifty years still suffer from a lack of predictability, generality, and simplicity. With the current emphasis on green chemistry, obtaining enantiomerically enriched products via photochemistry is a likely viable alternative for the future. Of the various approaches developed during the last three decades, the one pioneered in the author’s laboratory involved the use of commercially-available and inexpensive achiral zeolites as the media. This approach does not use any solvent for the reaction. Examples from these studies are highlighted in this article. Since no chiral zeolites were available, when the work was initiated in the author’s laboratory, commercially-available zeolites X and Y were modified with chiral inductors so that the reaction space becomes chiral. The results obtained established the value of chirally-modified, commercial zeolites as media for achieving chiral induction in photochemical reactions. A recent report of the synthesis of a chiral zeolite is likely to stimulate zeolite-based chiral photochemistry in synthesizing enantiomerically-pure organic molecules. The availability of chiral zeolites in future is likely to energize research in this area. Our earlier observations on this topic, we believe, would be valuable for progress of the field. Keeping this in mind, I have summarized the work carried out in our laboratory on chiral photochemistry on chirally-modified zeolites. This review does not include examples where high chiral induction has been obtained via a strategy that examines molecules appended with chiral auxiliary within achiral and chirally-modified zeolites. The latter approach yields products with diastereomeric excess >80%.