An ab initio study of β-fragmentation reactions in some alkoxyacyl (alkoxycarbonyl) and related radicals†

Abstract

Ab initio molecular orbital calculations using the 6-311G**, cc-pVDZ and aug-cc-pVDZ basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation indicate that decarboxylation reactions of alkoxyacyl (alkoxycarbonyl) radicals are significantly exothermic. Transition states (16) for these decarboxylation reactions are calculated to have C_TS–O_TS separations in the range: 1.813–1.892 Å; these distances appear to be affected somewhat by steric compression. At the CCSD(T)/6-311G**//MP2/6-311G** level of theory, energy barriers of 75.9, 72.8, 67.0 and 60.3 kJ mol^–1 are calculated for the decarboxylation reactions involving the methoxyacyl, ethoxyacyl, isopropoxyacyl and tert-butoxyacyl radicals (2) respectively, while the reverse reactions are calculated to require energies in excess of 130.9 kJ mol^–1. By comparison, the decarbonylation reaction of the acetyl radical (8) is predicted to be significantly endothermic; methyl radicals are calculated to prefer to add to carbon monoxide with an energy barrier of only 24.0 kJ mol^–1 at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level of theory, in good agreement with available experimental data.

Similar calculations for reactions involving (methoxy)thioacyl, (methylthio)acyl and (methylthio)thioacyl radicals (12–14, R = Me) suggest that only (alkoxy)thioacyl radicals (12) provide synthetically useful β-fragmentation reactions, the remaining systems (13, 14) are unlikely to be useful as alkyl radical precursors in synthesis; the reverse reactions are calculated to be competitive with the β-fragmentation process in these cases.