Enantioselective Catalysis Based on Cationic Oxazaborolidines

Abstract

Chiral oxazaborolidines can be activated by N‐protonation using strong protic acids or by N‐coordination with AlBr₃ to form very strong chiral Lewis acids. The resulting chiral boron electrophiles (see structure) are powerful chiral catalysts that effectively promote [4+2], [3+2], and [2+2]‐cycloaddition reactions with high enantioselectivity. Over the past several decades a revolution has occurred in chemistry that has essentially been unnoticed by those outside the field, even in other sciences. In brief, this includes the following: 1) our understanding of how chemical reactions occur, 2) our ability to invent new reactions, 3) our ability to utilize reactions that construct a vast assortment of useful or complicated molecules, and 4) our ability to apply chemical principles and knowledge to understand biological and medical problems. Within synthetic chemistry, a new science has been set in place beside the old, especially in terms of the control of absolute and relative stereochemistry and the creation of new types of useful catalysts that function in ways that were hitherto unimaginable. This Review deals with one aspect of such catalysis which has emerged only in the past six years: the generation and application of super‐Lewis acidic chiral oxazaborolidinium ions for enantioselective catalysis. Progress in this area has encompassed the formation of such catalysts, the detailed pathways of the reactions that they control and accelerate, the reactions that they can promote, and the ways in which they can be applied to advantage.