A kinetic and theoretical study of ring-opening of bicyclo[1.1.1 ]pentylmethyl and bicyclo[2.1.1]hexylmethyl radicals

The rearrangement of the bicyclo[1.1.1]pentan-1-ylmethyl radical (9) and the bicyclo[2.1.1]hexan-1-yl-methyl radical (10) to the corresponding methylenecycloalkylmethyl radical (11, 12) has been studied kinetically by EPR spectroscopy and, in the case of 10, by tributyltin hydride reduction. Activation energies of 7.1 ± 1.2 kcal mol^-1 and 9.3 ± 0.5 kcal mol^-1 have been determined for 9 and 10, respectively, while the corresponding pre-exponential factors have been determined to be 12.3 ± 1.5 and 12.0 ± 1.0, respectively. Rearrangement of 9 has an unusually low activation barrier; this has also been investigated by ab initio molecular-orbital calculations. UHF/3-21G level of theory predicts a barrier of 7.8 kcal mol^-1, in excellent agreement with that obtained experimentally, while a somewhat higher value (11.2 kcal mol^-1) is predicted at the UHF/6-31G* level. All of these data indicate that relief of ring strain is a major contributing factor to the rapid rearrangement of these radicals.