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| - Theoretical Study of CO Migratory Insertion Reactionswith Group 10 Metal−Alkyl and −Alkoxide Bonds
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| - The results of density functional calculations on the alternative migratory insertionreactions of CO with the M−OMe and M−Me bonds of group 10 M(Me)(OMe)(PH3)2 modelsystems are reported. For all three metals insertion into the M−OMe bond to formmethoxycarbonyl products is thermodynamically favored over insertion into the M−Me bondto give acyls. This preference is small when M = Ni (ΔΔER = 3 kcal/mol) but increases downthe triad and becomes significant for M = Pt (ΔΔER = 12 kcal/mol). Both associative five-coordinate and phosphine displacement four-coordinate mechanisms for migratory insertionwere considered. For Ni associative mechanisms are more accessible and the lowest energypathway is for reaction with the Ni−Me bond. With Pd and Pt the five- and four-coordinatepathways are close in energy, and for Pd there is a small kinetic preference for insertioninto the Pd−OMe bond. For Pt however there is a clear kinetic preference for reaction withthe Pt−OMe bond. During migratory insertion into M−OMe bonds the methoxide ligandrotates in the transition state to allow the participation of an oxygen lone pair in C−O bondformation while maintaining some residual M···OMe interaction. This M···OMe interactionis retained to some extent in the three-coordinate methoxycarbonyl species formed alongthe four-coordinate pathways. For an isostructural series of reactive species the trend inactivation 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 thermodynamicand 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 thegroup 10 triad, whereas much less variation is seen in the M−OMe bonds, which results inreaction 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 tothe C−C bond of the alternative acyl species.
- Density functional calculations on competing CO migratory insertion into the M−OMe and M−Me bonds of group 10 M(Me)(OMe)(PH3)2 model systems show that methoxycarbonyl formation is increasingly favored both thermodynamically and kinetically down the group 10 triad. These results can be understood in terms of trends in M−ligand bond strengths, the stronger C−O bond formed in the methoxycarbonyl and methoxide lone pair participation that lowers the energy of M−OMe migratory insertion transition states.
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