MECHANISTIC ASPECTS OF ASYMMETRIC HYDROFORMYLATION OF OLEFINS CATALYZED BY CHIRAL PHOSPHINE-PHOSPHITE-RHODIUM(I) COMPLEXES

Effects of CO and H-2 partial pressures on the reaction rate and selec tivity of asymmetric hydroformylation of 1-hexene and styrene were exa mined using an (R,S)-BINAPHOS-Rh-I complex as a catalyst ((R,S)-BINAPH OS = (R)-2-(diphenylphosphino)-1,1'-binaphthalen-2'-yl (S)-1,1'-binaph thalene-2,2'-diyl phosphite). For both substrates, the higher CO parti al pressure inhibited the reaction, and the partial pressure of H-2 ha rdly affects the reaction rate (P-H2, P-CO = 5-50 atm). In most cases, no serious change in the regio- and enantioselectivities was observed with variation of the H-2 and CO pressure. As an exception, the regio selectivity and enantioselectivity dropped to some extent for the reac tion of styrene at 1 atm (H-2/CO = 1/1). Deuterioformylation experimen ts clearly demonstrate the irreversibility of the olefin-insertion ste p sit total pressures of 20-100 atm (D-2/CO = 1/1). This fact proves t hat the regio- and enantioselectivity of the present hydroformylation should be controlled by the olefin-insertion step. This is the first e xample of highly enantioselective deuterioformylation under actual pro cess conditions (30-60 degrees C, 1-100 atm). For the reaction of styr ene at 1 atm, the olefin insertion becomes reversible to an extent, an d the lower. regioselectivity under these conditions can be rationaliz ed by the degree of reversibility of olefin insertion to give the bran ched alkylrhodium intermediate, [Rh]CH(CH3)Ph, being much greater than that to give the linear one, [Rh]CH2CH2Ph.