A density functional study of the hydrogenation of ketones catalysed by neutral rhodium-diphosphane complexes

The potential energy profile of Rh-1-catalysed hydrogenation of ketones has been computed for the simple model system [Rh{H3POCH2CH2N(H)PH3}(Cl)] using DFT calculations. The general sequence of the catalytic cycle involves coordination of the carbonyl derivative to the neutral Rh-1 complex followed by oxidative addition of molecular hydrogen providing rhodium dihydride intermediates. The latter are converted into alkoxy hydrides by a migratory insertion reaction. Reductive elimination of the alcohol and substitution of the latter by the incoming substrate completes the catalytic cycle. Intermediates and transition states of all catalytic steps have been located. Two isomeric derivatives bearing the model substrate have been found for the [Rh{H3POCH2CH2N(H)PH3)(Cl)(H2CO)] complex. Eight diastereomeric pathways have been followed for the cis addition of molecular hydrogen to [Rh{H3POCH2CH2N(H)PH3)(Cl)-(H2CO)] leading to eight distinct isomeric dihydride intermediates. Four dihydride complexes can be considered as the more accessible compounds. The site preference for migratory insertion and transition states discriminates the main path of the catalytic reaction. Migratory insertion to form the alkoxy hydride constitute the turn over limiting step of the process. The potential energy profile has been found to be smooth without excessive activation barriers. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)