Stoichiometric radical hydrogenations with HCo(CO)4, HMn(CO)5 and their deuterio analogs

Abstract Rate data for the stoichiometric radical hydrogenation of four conjugated aromatic olefins with HCo(CO)4 and DCo(CO)4 were obtained and the activation parameters deterimined. The kinetic isotope effect, kHkD, varied from 0.43 with ethylidenefluorene to 2.02 with bifluorenylidene. These results are explained on the basis of the extent of hydrogen (deuterium) transfer in the transition state of the endothermic reaction leading to radical intermediates. The rates of the reaction of three of these olefins with HMn(CO)5 and DMn(CO)5 were also determined and were found to be uniformly more than two orders of magnitude slower than the corresponding reactions with HCo(CO)4 and DCo(CO)4. The data show that hydrogenation reactions of HMn(CO)5 and HCo(CO)4 proceed by similar mechanisms.