Catalytic and Structural Studies on Complexes of a Binaphthyl-Phosphino-Oxazoline Auxiliary: The Meta Dialkyl Effect on Enantioselectivity

The chiral phosphino-oxazoline ligand (S,R)-2-[4-(isopropyl)oxazol-2-yl]-2′-bis(3,5-dimethylphenylphosphino)-1, 1'-binaphthyl, 6b, its Pd-dichloro complex, 9, the p-cyanoaryl complex [PdBr(p-NC-C₆H₄](6b)], 10, a model for the Heck reaction, the cationic 1,3-diphenylallyl derivative [Pd(η³-PhCHCHCHPh)(6b)]OTf, 11, a model for the allylic allylation, and the Rh- and Ir-1,5-COD compounds [M(1,5-COD)(6b)]BF₄, 12 and 13, respectively, have been prepared. In enantioselective catalytic experiments, the presence of the 3,5-dimethylphenyl groups generally increases the observed ee. The solid-state structure of PdCl₂(6b), 9, has been determined. 2-D and variable-temperature NMR experiments suggest that one of the two 3,5-dimethyl P-aryl rings interacts selectively with the remaining ligands. Consequently, the entire chiral pocket becomes slightly more rigid and the correlation with substrate improves.