ENANTIOSELECTIVE HOMOGENEOUS CATALYSIS AND THE 3,5-DIALKYL META-EFFECT - MEO-BIPHEP COMPLEXES RELATED TO HECK ALLYLIC ALKYLATION AND HYDROGENATION CHEMISTRY

The enantioselectivities arising from a Pd-catalyzed Heck reaction (>9 8% eel and an allylic alkylation (>90% ee) using a 3,5-di-tert-butyl-M eO-BIPHEP chiral auxiliary (1) are reported. Higher ee's are observed with the 3,5-dialkyl substituents than with the unsubstituted parent M eO-BIPHEP. It is proposed that the observed dialkyl ''meta-effect'', o n enantioselectivity, is the combined result of a more rigid and sligh tly larger chiral pocket and that this effect will have some generalit y in homogeneous catalysis. Detailed NMR studies on the allyl complex [Pd(PhCHCHCHPh)(1)]PF6 (5), and the model hydrogenation catalyst [RuH( cymene)(1)]BF4 (6), reveal restricted rotation about several of the P- C(ipso) bonds of the phosphorus substituents containing-the 3,5-di-ter t-butyl groups. The X-ray structure of 6 reveals that the cymene ligan d is not symmetrically bound to the Ru atom. This observation is inter preted as an expression of the chiral pocket of 1. MM3 calculations o n 6 support the NMR findings and reproduce the X-ray results.