Theoretical studies of asymmetric hydroformylation using the Rh-(R,S)-BINAPHOS catalyst--origin of coordination preferences and stereoinduction.

ABSTRACT

We have undertaken theoretical investigations of the asymmetric hydroformylation of styrene by the [Rh{(R,S)-BINAPHOS}(CO)(2)H] catalyst, focusing on the origin of the ligand coordination preferences and stereoinduction. We evaluated the different factors governing the preference of the BINAPHOS ligand to coordinate with the phosphane moiety at the equatorial site and the phosphite moiety at the apical site. The donor-acceptor interactions, obtained using a modified version of energy decomposition analysis (EDA) based on orbital deletion, favour the phosphite moiety at the equatorial site. However, the electronic distortion and the steric effects inverse this tendency. Calculations also suggest that the coordination preference was transferred to the selectivity-determining transition state. We propose a stereochemical model based on quantitative quadrant maps obtained from a new molecular descriptor, the distance-weighted volume (V(W)), which is easily computed from ground-state structures. Repulsive interactions between the substrate and the apical phosphite were responsible for the enantiodifferentiation. The axial chirality of the phosphite discriminated one of the competitive equatorial-apical paths, whereas the axial chirality of the backbone discriminated one of the two enantiomers. Transition-state calculations revealed that the placement of phosphane at the apical site would lower enantioselectivity, explaining the poor performance of other phosphane-phosphite ligands. Finally, comparison with previous studies allowed the definition of several prerequisites for diphosphane ligands for high stereoselectivity 1) specific equatorial-apical coordination bringing chirality to the apical site, 2) combination of two stereogenic centres and 3) rigid structures.