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 (>98% eel and an allylic alkylation (>90% ee) using a 3,5-di-tert-butyl-MeO-BIPHEP chiral auxiliary (1) are reported. Higher ee's are observed with the 3,5-dialkyl substituents than with the unsubstituted parent MeO-BIPHEP. It is proposed that the observed dialkyl ''meta-effect'', on enantioselectivity, is the combined result of a more rigid and slightly larger chiral pocket and that this effect will have some generality 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-tert-butyl groups. The X-ray structure of 6 reveals that the cymene ligand is not symmetrically bound to the Ru atom. This observation is interpreted as an expression of the chiral pocket of 1. MM3* calculations on 6 support the NMR findings and reproduce the X-ray results.