Quantum mechanical modelling of alkenehydroformylation as catalyzed by xantphos-Rh complexes

Abstract

Fundamental issues concerning the hydroformylation of 1-alkenes as catalyzed by Rh complexes ligated with the xantphos diphosphine ligand are explored using ONIOM calculations. In this study xantphos serves as a prototype of the large bite-angle ligands that are associated with high regioselectivity and rates in catalytic hydroformylation. Computations have been used to explore the thermodynamics of 56 unique isomers (e.g., cis vs. trans isomers of square planar complexes, diequatorial vs. axial-equatorial five-coordinate complexes) and conformers for intermediates along the reaction pathway. More than 20 transition states relevant to the catalyst mechanism have been determined. In terms of realistically modelling experiment, the computational results are mixed. In agreement with experiment, the computations yield a mixture of diequatorial and axial-equatorial isomers of HRh(xantphos)(CO)₂ as the catalyst resting state. Dissociation of CO from these complexes is computed to be barrierless leading to a computed free energy for exchange of CO ligands around 15 kcal mol⁻¹, somewhat lower than the value of ca. 20 kcal mol⁻¹ derived from experimental data. The computed ratios of rates of propene insertion to form n-propyl and i-propyl Rh-alkyl (42 ∶ 1) is in good agreement with experimental ratios of n-nonanal to i-nonanal (52 ∶ 1) for 1-octene hydroformylation. Nonetheless, the computations dramatically overestimate the overall activation free energies for catalytic hydroformylation. Thus, at this stage computations do not provide useful insight into the the kinetics of hydroformylation and detailed mechanistic issues. It appears that much of this discrepancy between computed and experimental activation energies originates from the underestimation of propene bonding energies.