High-pressure IR studies on the asymmetric hydroformylation of styrene catalyzed by Rh(I)-(R,S)-BINAPHOS

Rhodium-catalyzed asymmetric hydroformylation of styrene using (R,S)-BINAPHOS (1a) and its methoxy-substituted derivative 1b as chiral ligands was monitored by in situ high-pressure IR. The rate of aldehyde production is given by r(obs) = k(obs)[Rh](1.0)[styrene](0.6)[1b]P--0.1(CO)-0.9 (k(obs) is a constant under isothermal conditions). The higher catalytic activity of the Rh-1b catalyst compared with that of Rh-1a is attributed to a higher concentration of the active species, which is generated by dissociation of CO from the major resting state RhH(CO)(2)(1) (2). A rhodium carbonyl species 10, the structure of which is unknown, is suggested to be formed from the rhodium hydride RhH(CO)(2)(1) (2), especially when the rhodium. concentration is high. Complex 10 is less active for hydroformylation and is probably responsible for the loss of %ee observed at high rhodium concentrations.