Remote steric effects on the disproportionation/recombination ratio of cumyl radicals

The disproportionation/combination ratio in the reaction between two 3,5-di-tert-butylcumyl radicals, generated by thermolysis of the corresponding diazene, is 0.21 ± 0.03 at 60 °C in benzene. This is much larger than the value of 0.06 found for the unsubstituted cumyl radicals. It is suggested that the tert-butyl groups hinder the formation of a parallel association complex, which has been proposed to account for the much smaller amount of disproportionation in the reaction between benzylic radicals as compared with alkyl radicals.