Hydrogen-Atom Transfer in Reactions of Organic Radicals with [Co-II(por)](center dot) (por = Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins

The mechanisms for hydrogen-atom transfer from the cyanoisopropyl radical C-center dot(CH3)(2)CN to [Co-II(por)](center dot) (yielding [Co-III(H)(por)] and CH2=C(CH3)(CN); por = porphyrinato) and the insertion of vinyl acetate (CH2=CHOAc) into the Co-H bond of [Co(H)(por)] (giving [Co-III{CH-(OAc)CH3}(por)]) were investigated by DFT calculations. The results are compared with experimental data. These reactions are relevant to catalytic chain transfer (CCT) in radical polymerization of olefins mediated by [Co-II(por)](center dot), the formation and homolysis of organo-cobalt complexes that mediate living radical polymerization of vinyl acetate, and cobalt-mediated hydrogenation of olefins. Hydrogen transfer from C-center dot(CH3)(2)CN to [Co-II(por)](center dot) proceeds via a single transition state that has structural features resembling the products [Co(H)(por)] and CH2=C(CH3)CN. The separated radicals approach to form a close-contact radical pair and then pass through the transition state for hydrogen-atom transfer to form [Co-III(H)(por)] and CH2=C(CH3)CN. This process provides a very low overall barrier for the hydrogen-atom transfer reaction (Delta G((sic)) = + 3.8 kcal mol(-1)). The reverse reaction corresponding to the addition of [Co(H)(por)] to CH2=C(CH3)CN has a low barrier (Delta G((sic)) = + 8.9 kcal mol(-1)) as well. Insertion of vinyl acetate into the Co-H bond of [Co-III(H)(por)] also proceeds over a low barrier (Delta G((sic)) = + 11.4 kcal mol(-1)) hydrogen-transfer step from [Co-III(H)(por)] to a carbon atom of the alkene to produce a close-contact radical pair. Dissociation of the radical pair, reorientation, and radical-radical coupling to form an organo-cobalt complex are the culminating steps in the net insertion of an olefin into the Co-H bond. The computed energies obtained for the hydrogen-atom transfer reactions from C-center dot(CH3)(2)CN to [Co-II(por)](center dot) and from [Co(H)(por)] to olefins, as well as the organo-cobalt bond homolysis energies correspond well with the experimental observations. The mechanism of alkene insertion into the Co-H bond of [Co-III(H)(por)] is of general interest, because the species does not contain any cis-vacant sites to the hydride and the usual migratory insertion pathway is not available. The low barrier predicted here for the multistep insertion process suggests that (depending on the bond strengths) even for systems that do have a cis-vacant site, the radical-type insertion might compete with classical migratory insertion.