Interpretation of the reactivity of benzyl free radical towards peroxyacids in terms of orbital interactions. Competition between energy gap control and overlap control

Abstract

The factors which control the reactivity of alkyl free radicals R' in reaction (i) are studied. The reactivityof R' in (i) depends on the key orbital interaction between the SOMO of the radical and the LUMO of the peroxyacid. This interaction involves t w o contributions: (i) the energy gap SOMO-LUMO and (ii) the overlap SOMO-LUMO. In reaction (i) the main factor is overlap control which depends on spin delocalisation in the radical R'. This proves that reaction (i) does not involve electron transfer. The energy gap control, which depends on the nucleophilic character of R', is only observed when the first factor is constant along a series of R'.Previous work on peroxyacids has shown a correlation between the ionisation potentials, IP, of alkyl-localised free radicals R' and their reactivity towards the peroxide oxygen of the peroxyacid.' Thus, radicals with a low IP such as 1,ldimethylundec-1 -yl (1) or adamantan-1 -yl (2) easily transfer HO from peroxyacids. On the other hand, radicals with the unpaired electron in a highly hybridised orbital, such as bicyclo-C2.2. llheptan-1-yl (3X2 phenyl (4), or 2-phenylcyclopropyl (S)4 react slowly or not at all upon the peroxide oxygen.These results are well interpreted in terms of orbital interactions, in which the key interaction occurs between the SOMO of the radical and the LUMO of the peroxyacid, i.e. the low lying a antibonding 0-0 orbital. * According to the PMO theory the energy effect, AE(PMO), associated with this interaction is proportional to the square of the overlap integral SOMO-LUMO, (Sij)*, and inversely proportional to the energy gap between these two orbitals before interaction, AE = cic j (Figure). Sj2l(~i -Hijj'Sij)'Ei -E j