Asymmetric hetero Diels-Alder reaction catalyzed by chiral cationic palladium(II) and platinum(II) complexes

The hetero Diels-Alder reaction of nonactivated conjugated dienes 1 with arylglyoxals 2 and glyoxylate esters 7 proceeded enantioselectively in the presence of a catalytic amount of cationic chiral BINAP-palladium or - platinum complexes and 3 Å molecular sieves (MS3A). The addition of MS3A effectively improved the enantioselectivity of the reaction. Excellent ee's were obtained from the reactions of 2,3-dimethyl-l,3-butadiene (1a) and 1,3- cyclohexadiene (1d) with dienophiles 2 and 7. The square-planar structure of [Pd(S-BINAP)(PhCN)₂](PF₆)₂ was determined by X-ray diffraction, and a chiral induction model involving the square-planar palladium complex coordinated with BINAP and a dienophile is proposed.