Hydridorhodium diphosphite catalysts in the asymmetric hydroformylation of styrene

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (2S,5S)-hexane-2,5-diol, (1S,3S)-diphenylpropane-1,3-diol and N-benzyltartarimide as chiral bridges have been used in the rhodium-catalysed asymmetric hydroformylation of styrene. Enantioselectivities up to 76% at 50% conversion have been obtained with stable hydridorhodium diphosphite catalysts. High regioselectivities (>95%) and high conversions (>99%) to 2-phenylpropanal were found under relatively mild reaction conditions [25-40°C, 9 bar of CO-H2 (1:1) pressure]. The solution structures of [RhH(L)(CO)2] complexes (L = bidentate diphosphite) have been studied; NMR and IR spectroscopic data revealed fluxional behaviour. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial co-ordination to the rhodium. The structure and the stability of the catalysts seems to play a fundamental role in the asymmetric induction.