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 ] - 0.1 P 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.