Addition of tert-Butyl Radical to Substituted Alkenes: A Theoretical Study of the Reaction Mechanism

High-level ab initio calculations at the QCISD/6-311G** + ZPVE level have been carried out to study the addition reactions of ieri-butyl radical to a set of substituted alkenes, CH₂═CHX (X = H, NH₂, F, Cl, CHO, and CN), and the results analyzed with the aid of the curve-crossing model. The reactivity of the tert-butyl radical is found to be governed by a combination of enthalpy and polar factors. The polar factor leads to tert-butyl radical displaying strong nucleophilic character which stabilizes the transition states by 20-25 kJ mol^-1 compared with those for the relatively nonpolar reactions of methyl radical. Consequently, barrier heights are significantly lower for radical addition reactions of tert-butyl (1.9-21.6 kJ mol^-1) than for methyl (24.3-39.8 kJ mol^-1). A transition state structure-enthalpy correlation is found for the addition reactions of tert-butyl radical but is shifted slightly from the correlation line previously found for the CH₃^., CH₂OH^., and Ch₂CN^. radicals, reflecting the increasing importance of polar contributions.