Addition of carbon-centered radicals to double bonds: influence of the alkene structure

The radical addition reaction to the double bond is well-recognized in organic chemistry as a powerful tool for C-C bond formation. The reactivity of three selected carbon centered radicals (aminoalkyl, methyl, and cyanomethyl) toward five double bonds, also representative of widespread monomers (vinyl ether, vinyl acetate, acrylonitrile, methyl acrylate, and ethylene), was examined in detail by using molecular orbital calculations. The observed reactivity is strongly influenced by the reaction exothermicity demonstrating that the energy barrier is governed in large part by the enthalpy term. The polar effect, as computed by molecular orbital calculations from the transition state structures, can drastically enhance the reactivity. A clear separation and quantification of the relative role of the polar and enthalpy effects in the different radical/double bond systems are obtained and the observed trend of reactivity is discussed. In addition to the effect of the charge-transfer configurations on the barrier, a large influence on the transition state geometry was evidenced.