Organocatalytic regio- and stereoselective cyclopropanation of olefins

Abstract

As reactive intermediates and substructures of natural products and bioactive molecules, the smallest cyclic alkanes—cyclopropanes—are an attractive class of molecules for chemists. Arguably, the most general approach to their chemical synthesis involves the addition of metal carbenes to olefins. Whereas catalytic asymmetric cyclopropanations of electronically unbiased olefins with carbenoids have been reported using chiral metal complexes and engineered metalloenzymes, we now report a complementary, metal-free and highly enantioselective cyclopropanation of olefins with diazoalkanes, applying asymmetric counteranion-directed photoredox organocatalysis. We identify an ion pair featuring a thioxanthylium photoredox cation and a chiral imidodiphosphorimidate counteranion that catalyses highly enantioselective cyclopropanations of styrenes and aliphatic dienes with diazo compounds. Mechanistic investigations reveal a wavelength dependence of the enantioselectivity and suggest that the main catalytic pathway proceeds via olefin-derived radical cation intermediates. This metal-free, highly enantioselective organocatalytic approach complements previously reported methods for alkene manipulations.