Chiral 1,3,2-Oxazaborolidine Catalysts for Enantioselective Photochemical Reactions
Daniel P. Schwinger, Thorsten Bach
Abstract
photocycloaddition, the latter an oxadi-π-methane rearrangement. Several new 1,3,2-oxazaborolidines were designed, and the products were obtained in high enantioselectivity with only 10 mol % of catalyst. Recently, an iridium-based triplet sensitizer was employed to facilitate enantioselective [2 + 2] photocycloadditions of cinnamates with 25 mol % of chiral 1,3,2-oxazaborolidine. In this case, the relatively low catalyst loading was possible because the oxazaborolidine-substrate complex exhibits a lower triplet energy and an improved electronic coupling compared to the uncomplexed substrate, allowing for a selective energy transfer.By synthetic and theoretical studies, it has become evident that chiral 1,3,2-oxazaborolidines are multifaceted catalysts: they change absorption behavior, alter energetic states, and induce chirality. While a diverse set of substrates has been shown to undergo enantioselective photochemical transformations in the presence of chiral 1,3,2-oxazaborolidines either through direct excitation or through triplet sensitization, these catalysts took on different roles for different substrates. Based on the studies presented in this Account, it can be assumed that there are still more photochemical reactions and substrate classes that could profit from chiral 1,3,2-oxazaborolidines.