Chiral Oxazaborolidine−Aluminum Bromide Complexes Are Unusually Powerful and Effective Catalysts for Enantioselective Diels−Alder Reactions

Treatment of the chiral oxazaborolidine 1 with AlBr₃ generates the 1:1 complex 3, which is an even more potent Lewis acid catalyst than protonated 1 (i.e., 2) for enantioselective Diels−Alder reactions. Only 4 mol % of catalyst 3 is required to achieve yields and enantiomeric purities of 90% over a broad range of achiral dienes and dienophiles. The ligand from which 3 is derived can be recovered easily and with high efficiency. The method is illustrated by 22 examples.