Theoretical study of CO migratory insertion reactions with group 10 metal-alkyl and -alkoxide bonds

The results of density functional calculations on the alternative migratory insertion reactions of CO with the M-OMe and M-Me bonds of group 10 M(Me)(OMe)(PH₃)₂ model systems are reported. For all three metals insertion into the M-OMe bond to form methoxycarbonyl products is thermodynamically favored over insertion into the M-Me bond to give acyls. This preference is small when M = Ni (ΔΔE_R = 3 kcal/mol) but increases down the triad and becomes significant for M = Pt (ΔΔE_R = 12 kcal/mol). Both associative five-coordinate and phosphine displacement four-coordinate mechanisms for migratory insertion were considered. For Ni associative mechanisms are more accessible and the lowest energy pathway is for reaction with the Ni-Me bond. With Pd and Pt the five- and four-coordinate pathways are close in energy, and for Pd there is a small kinetic preference for insertion into the Pd-OMe bond. For Pt however there is a clear kinetic preference for reaction with the Pt-OMe bond. During migratory insertion into M-OMe bonds the methoxide ligand rotates in the transition state to allow the participation of an oxygen lone pair in C-O bond formation while maintaining some residual M⋯OMe interaction. This M⋯OMe interaction is retained to some extent in the three-coordinate methoxycarbonyl species formed along the four-coordinate pathways. For an isostructural series of reactive species the trend in activation energy is always Ni < Pd ≪ Pt for reaction with the M-Me bond and Ni > Pd < Pt with Pt > Ni) for reaction with the M-OMe bond. Trends in the computed thermodynamic and kinetic data of the alternative migratory insertions can be understood in terms of metal-ligand homolytic bond strengths. All M-C bonds studied show a marked increase down the group 10 triad, whereas much less variation is seen in the M-OMe bonds, which results in reaction with the M-OMe bonds being generally favored. A key additional driving force, however, is the stronger C-O bond formed in the methoxycarbonyl product compared to the C-C bond of the alternative acyl species.