Asymmetric hydrosilylation of styrenes catalyzed by palladium-MOP complexes: Ligand modification and mechanistic studies

In the palladium-catalyzed asymmetric hydrosilylation of styrene (3a) with trichlorosilane, several chiral monophosphine ligands, (R)-2-diarylphosphin o-1,1'-binaphthyls(2a-g), were examined for their enantioselectivity. The h ighest enantioselectivity was observed in the reaction with (R)-2-bis [3,5- bis(trifluoromethyl)phenyl] phosphino-l,1'-binaphthyl (2g), which gave (S)- 1-phenylethanol (5a) of 98% ee after oxidation of the hydrosilylation produ ct, 1-phenyl-1-(trichlorosilyl)ethane (4a). The palladium complex of 2g als o efficiently catalyzed the asymmetric hydrosilylation of substituted styre nes on the phenyl ring or at the beta position to give the corresponding ch iral benzylic alcohols of over 96% ee. Deuterium-labeling studies on the hy drosilylation of regiospecifically deuterated styrene revealed that P-hydro gen elimination from l-phenylethyl(silyl)palladium intermediate is very fas t compared with reductive elimination giving hydrosilylation product when l igand 2g is used. The reaction of o-allylstyrene (9) with trichlorosilane c atalyzed by (R)-2g/Pd gave (1S,2R)-1-methyl-2-(trichlorosilylmethyl)indan ( 10) (91% ee) and (S)-1-(2-(propenyl)phenyl)-1-trichlorosilylethanes (11a an d 11b) (95% ee). On the basis of their opposite configurations at the benzy lic position, a rationale for the high enantioselectivity of ligand 2g is p roposed.