Electronic ligand effects on the regioselectivity of the rhodium-diphosphine-catalyzed hydroformylation of propene

Electronic effects induced by diphosphine bidentate ligands on the regioselectivity of the rhodium-catalyzed hydroformylation of propene were investigated using density functional theory based calculations (B3LYP). To this end, the key hydride migration step was evaluated for HRh(propene)(CO)L₂ (L₂ = PF₃, PF₃; PH₃, PH₃; PMe₃, PMe₃; PH₃, PF₃; PH₃, PMe₃) incorporating either two identical or two electronically distinct phosphorus moieties. The phosphorus moieties span a wide range of ligand basicities. While the electronic properties of the ligands do not influence the regioselectivity of the hydride migration reaction directly, they do govern the amount of back-donation from the metal to the alkene substrate. As a result, important differences in transition-state geometries are obtained for different ligand systems. For electron-withdrawing ligands low activation energies and trigonal-bipyramidal transition-state geometries are observed. Increasing the basicity of the diphosphine ligand leads to higher activation energies and distortion of the transition-state structures toward square-pyramidal geometries. In systems containing two electronically distinct phosphorus ligands, this geometric distortion leads to a preference for the formation of the new rhodium-alkyl sigma-bond trans to the least donating phosphorus moiety, generating the most stable rhodium-alkyl isomer. In all cases, bis-equatorial coordination of the two phosphorus ligands yields considerably lower transition-state energies than equatorial-axial coordination of the same ligands. The resulting rhodium-alkyl products are stabilized relative to the reactant by electron-donating ligands. On the basis of these observations it is argued that, for electron-withdrawing and/or wide-bite-angle ligands, beta-hydride elimination plays an important role in determining the overall regioselectivity of the hydroformylation reaction, while for equatorial-axial coordinating ligands, the regioselectivity is determined exclusively by the relative energies of the hydride migration transition states.