Radicals masquerading as electrophiles: a computational study of the intramolecular addition reactions of acyl radicals to imines

Abstract

Ab initio calculations using 6-311G**, cc-pVDZ, and aug-cc-pVDZ, with (MP2, QCISD, CCSD(T)) and without (UHF) electron correlation, and density functional methods (BHandHLYP and B3LYP) predict that cyclization of the 5-aza-5-hexenoyl and (E)-6-aza-5-hexenoyl radicals proceed to afford the 5-exo products. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVDZ level of theory, energy barriers (ΔE^‡) of 36.1 and 47.0 kJ mol⁻¹ were calculated for the 5-exo and 6-endo pathways for the cyclization of the 5-aza-5-hexenoyl radical. On the other hand, at the same level of theory, ΔE^‡ of 38.9 and 45.4 kJ mol⁻¹ were obtained for the 5-exo and 6-endo cyclization modes of (E)-6-aza-5-hexenoyl radical, with exothermicities of about 27 and 110 kJ mol⁻¹ calculated for the exo and endo modes, respectively. Under suitable experimental conditions, the 6-endo cyclization product is likely to dominate. Analysis of the molecular orbitals involved in these ring-closure reactions indicate that both reactions at nitrogen are assisted by dual orbital interactions involving simultaneous SOMO–π* and LP–π* overlap in the transitions states. Interestingly, the (Z)-6-aza-5-hexenoyl radical, that cannot benefit from these dual orbital effects is predicted to ring-close exclusively in the 5-exo fashion.