ASYMMETRIC HETERO DIELS-ALDER REACTION CATALYZED BY CHIRAL CATIONIC PALLADIUM(II) AND PLATINUM(II) COMPLEXES

The hetero Diels−Alder reactionof nonactivated conjugated dienes 1 with arylglyoxals 2 and glyoxylateesters 7 proceeded enantioselectively in the presence of a catalytic amount of cationic chiral BINAP−palladium or −platinum complexes and 3 A 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- 1,3-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 BINAPand a dienophile is proposed.