Isotopic investigation of hydrogen transfer related to cobalt-catalyzed free-radical chain transfer

The process of cobalt porphyrin catalyzed chain transfer in methacrylate free radical polymerizations shows a deuterium kinetic isotope effect greater than 3, indicating that hydrogen atom transfer occurs in the rate-limiting step of the catalytic cycle. This step involves formation of a transient hydridocobalt(III) complex by a reversible hydrogen atom abstraction from an incoming radical; the other product is an olefin. In some cases, the resulting radical can be trapped as the alkylcobalt(III) product. The use of labeled cyclic olefins indicates that the formation of the alkylcobalt(III) species can be at least partially stereoselective in some cases, and when it is, the addition is cis. Deuterated vinylidene olefins such as methyl methacrylate-d(8) were found to exchange the hydrogen of a cobalt porphyrin hydride to deuteride under free radical conditions. This exchange reaction can be used for deuterium labeling experiments-the transient cobalt deuterides are similarly trapped by addition of protioolefins. Thermal isomerization of the labeled product with time results in gradual loss of stereoselectivity, but observation of the initial stereoselectivity of this reaction indicates an underlying limit to the reaction selectivity. The ready availability of radical homolysis products will always limit the selectivity of the products. Similar experiments with acetylenes usually result in trans-vinyl products except when steric effects dominate. This shift from cis products with olefins to trans products for acetylenes is unexplained but may be attributed to radical intermediates or may indicate a role for a second cobalt complex in the transition state.