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 selectivity of asymmetric hydroformylation of 1-hexene and styrene were examined using an (R,S)-BINAPHOS-Rh-I complex as a catalyst ((R,S)-BINAPHOS = (R)-2-(diphenylphosphino)-1,1'-binaphthalen-2'-yl (S)-1,1'-binaphthalene-2,2'-diyl phosphite). For both substrates, the higher CO partial pressure inhibited the reaction, and the partial pressure of H-2 hardly 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 regioselectivity and enantioselectivity dropped to some extent for the reaction of styrene at 1 atm (H-2/CO = 1/1). Deuterioformylation experiments clearly demonstrate the irreversibility of the olefin-insertion step sit total pressures of 20-100 atm (D-2/CO = 1/1). This fact proves that the regio- and enantioselectivity of the present hydroformylation should be controlled by the olefin-insertion step. This is the first example of highly enantioselective deuterioformylation under actual process conditions (30-60 degrees C, 1-100 atm). For the reaction of styrene at 1 atm, the olefin insertion becomes reversible to an extent, and the lower. regioselectivity under these conditions can be rationalized by the degree of reversibility of olefin insertion to give the branched alkylrhodium intermediate, [Rh]CH(CH3)Ph, being much greater than that to give the linear one, [Rh]CH2CH2Ph.