Structures and reaction pathways in rhodium(I)-catalyzed hydrogenation of enamides: A model DFT study

The potential energy profile of Rh(I)-catalyzed hydrogenation of enamides h as been studied for the simple model system [Rh(PH3)(2)(alpha-acetamidoacry lonitrile)](+) using a nonlocal density functional method (B3LYP). Intermed iates and transition states along four isomeric pathways for dihydrogen act ivation have been located, and pathways for interconversion between isomeri c reaction pathways have been explored. The general sequence of the catalyt ic cycle involves coordination of H-2 to [Rh(PH3)(2)(alpha-acetamidoacrylon itrile)](+) to form a five-coordinate molecular H-2 complex, followed by ox idative addition of the coordinated molecular hydrogen to form a dihydride complex, [RhH2(PH3)(2)(alpha-acetamidoacrylonitrile)](+). This dihydride is converted into an alkyl hydride by a migratory insertion reaction. Reducti ve elimination of the hydrogenated acetamidoacrylonitrile completes the cat alytic cycle. No computational support for alternate H-2 activation pathway s, such as direct conversion of H-2 and [Rh(PH3)(2)(alpha-acetamidoacryloni trile)](+) to an alkyl hydride, was found. Four isomeric pathways for hydro genation are followed, corresponding to the four distinct dihydride isomers resulting from cis addition of H-2 to [Rh(PH3)(2)(alpha-acetamidoacrylonit rile)](+). Two of these pathways are excluded from further consideration by virtue of their surprisingly high activation barriers for formation of mol ecular H-2 complexes. Of the two pathways with low barriers to formation of dihydride complexes, only one has a sufficiently low barrier for migratory insertion to contribute significantly to catalytic product formation. Over all, we find that formation of a dihydride is endergonic, rapid, and revers ible. Migratory insertion to form an alkyl hydride constitutes the turnover -limiting step in the catalytic cycle. This conclusion is supported by comp arison of computed and experimental isotope effects in catalytic enamide hy drogenation.