Oxathiaborolium: A Type of Chiral Lewis Acid Catalyst and Its Application in Catalytic and Highly Enantioselective Diels–Alder Reactions

The first reported sulfur-stabilized borenium cations were synthesized through halide abstraction of a haloborane intermediate by halophilic reagents. Different from the well-known cationic oxazaborolidines, a sulfide instead of an amine was used to not only simplify the preparation of the catalysts but also increase Lewis acidity of the boron atom. The in situ generated borenium salts showed exceptional Lewis acidity and successfully catalyzed asymmetric Diels–Alder reactions of cyclopentadiene and dienophiles in excellent yields and enantioselectivities. The NMR studies of these oxathiaborolium structures were reported as well.