On the importance of electronic and steric effects in the migratory CO insertion step of rhodium–diphosphine catalyzed methanol carbonylation†

To provide further insight into electronic and steric factors and to quantify their relative importance, we studied in detail the migratory CO insertion step for RhMe(CO)I2(L–L) systems (L–L = dppms (PPh2CH2P(S)Ph2) or dppe (PPh2CH2CH2PPh2)), Monsanto catalysts and some electronically unsymmetrical diphosphine model systems. The difference in the reaction rates of dppms and dppe has a clear electronic origin that reflects the different properties of sulfide phosphine (π-donor) and phosphine (π-acid) ligands. Molecular orbital calculations clearly show that dppms strongly increases back-bonding to CO and favors the overlap between CO and methyl. Steric effects modulate the barrier, which decreases more for dppe than it does for dppms. For dppms, the electronic contribution that phenyl phosphine substituents make to lower the barrier is greater than that made by purely steric effects. The sulfide phosphine ligand dppms accelerates the carbonyl insertion because of its π-donor capability. For the diphosphine ligands we studied, the energy barrier varied gradually as basicity varied, and the slowest kinetics is shown by the most electron-donating ligand. The basicity dependence is stronger when the phosphine ligand occupies a trans position to CO. On the other hand and in unsymmetrical diphosphine complexes, phosphine basicity affects stability and reactivity in opposite ways.