Benzylic Stabilization as a Mechanistic Tool for Studying Radical Rearrangements

The kinetic parameters for the rearrangement of, and the relative energies of radicals involved in the equilibria of, a bicyclic cyclopropylmethyl/homoallyl system were determined. Benzylic stabilization of the cyclopropylmethyl radical facilitates characterization of all four ring opening and closing processes. Kinetically disfavored endocyclic ring opening produces the thermodynamically favored homoallyl radical (10). Cyclohexenyl radical 10 is approximately 1.7 kcal/mol lower in energy than 9. The regioselectivity of ring opening of 8 is independent of any assumptions involving trapping rate constants of radicals. The regioselectivity for ring opening of 8 (k1/₂ = 30.2 at 298 K) is similar to that of bicyclo[3.1.0]hexan-l-yl (1). These experiments suggest that benzylic stabilization does not significantly perturb the position of the transition state for ring opening. Therefore, phenyl substitution of a bicyclic cyclopropylmethyl radical should be a useful tool for analyzing the effects of substituents elsewhere in the system.