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.

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