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% ee) 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 )]PF 6 ( 5 ), and the model hydrogenation catalyst [RuH(cymene)( 1 )]BF 4 ( 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.