Effects of adjacent acceptors and donors on the stabilities of carbon-centered radicals

Article abstract of DOI:10.1021/ja00046a003

The radical stabilization energies (RSEs) for 21 radicals, GCH₂*, where G is an electron donor or acceptor group, were estimated from differences in homolytic C-H bond dissociation energies (ABDEs relative to that of methane). These RSEs were found to agree reasonably well in order with theoretically calculated RSEs that have been reported and with those obtained by averaging RSEs from nine different methods (a literature RRS_x scale). But the RSEs estimated from ΔBDEs were almost without exception larger because the theoretical calculations and RRS_x method greatly underestimate the size of most RSEs. The RSEs for GCH₂* radicals were found to be enhanced to the extent that G is able to delocalize an odd electron on an adjacent carbon atom and were diminished by the presence of electron-withdrawing properties in G. Most groups stabilize the radical, but when the electron-withdrawing effect of the group is large, as in F₃C^? and Me₃N⁺CH₂^? radicals, the net effect is to destabilize the radical relative to the methyl radical. The RSEs of 12 radicals of the type RSC^?HG or PhSC^?HG, relative to that of the GCH₂^? radical, were found to increase progressively as the acceptor group G was changed along the series Ph, fluorenyl (Fl), CO₂Et, CN, COMe, and COPh. The RSEs of these donor-acceptor radicals were all smaller than the sum of the RSEs of the singly-substituted radicals, GCH₂^? and PhSCH₂^? (or RSCH₂^?). The effects of a second PhS or like donor and of a second acceptor on the RSEs were also determined. Examination of the RSEs of GC^?(Ph)CN radicals indicates that the interactions of G and CN in the GC^?CN moieties, with G = MeO, EtS, or c-C₅H₁₀N, are not synergistic as has been claimed from ESR studies.