Homolytic ring fission reactions of cycloalkylmethyl and bicycloalkyl radicals

The geometries and enthalpies of formation of cyclopropyl, cyclopropylmethyl, cyclobut-2-enylmethyl, cyclobutylmethyl, the isomeric bicyclo[2.1.0]pentyl, bicyclo[1.1.1]pentyl, and spiropentyl radicals have been studied by the semi-empirical MINDO/3 method and compared with the calculated and experimental data for the corresponding cyclo- or bicyclo-alkanes. The ring fission reactions of the radicals have been investigated by calculating the energies for successive increments of a chosen reaction co-ordinate. For the cycloalkylmethyl radicals the calculated enthalpies of activation show the same trend as the experimental activation energies for β-scission. The MINDO/3 calculations for the bicycloalkyl radicals are also in agreement with such experimental evidence as is available. For the bicycloalkyl radicals which have not been examined by experiment MINDO/3 suggests that ring fission should be favoured for bicyclo[2.1.0]-pent-5-yl radicals. The calculations indicate that overlap of the SOMO with the bond to be broken is usually the most important factor in controlling which bond breaks and the rate of the process. Release of ring strain may, however, oppose and overrule the stereoelectronic effect, e.g. with bicyclo[2.1.0]pent-2-yl radicals. If overlap of the SOMO with the β,γ bond is prevented then ring fission is predicted to have a high energy barrier, e.g. in cyclopropyl and spiropentyl radicals.