Catalytic Activity of Phosphine-Copper Complexes for Hydroboration of Styrene with Pinacolborane: Experiment and Theory

We have carried out density functional theory calculations as well as experiments to rationalize the catalytic activity of various phosphine-copper complexes for the hydroboration of styrene with pinacolborane. The experimentally obtained catalytic efficiency was explained on the basis of activation barriers for consecutive reaction mechanism steps as well as by molecular orbitals and charges in the transition state. Bidentate ligands were found to be more efficient than monodentate ligands for catalytic activity. Bidentate ligands make the reactant complexes less stable than monodentate ligands due to steric hindrance. This information could be usefully utilized for new catalysts design. The calculated kinetic data were consistent with the experimental conversion efficiency in a process that was hypothesizd to undergo the addition of Cu-H to styrene as the rate-limiting step. From the electronic distribution of the HOMO and the charge of the copper atom in the transition state, it was found that styrenes substituted with electron withdrawing groups would give higher conversions, and the catalytic efficiency could be increased with properly designed electron-donating ligands for the copper catalyst complex.