The kinetics of carbonyl radical ring closures

Abstract

Intramolecular homolytic addition reactions of acyl and oxyacyl radicals 1, 7–11 have been investigated by computational techniques. Acyl radical (X = CH₂) cyclizations appear to proceed through Beckwith–Houk transition states that adopt pseudo-chair like structures, while the oxyacyl radicals (X = O) appear to behave more like sp²-hybridized radicals in which the 6-endo transition states are predicted to adopt boat-like structures. G3(MP2)-RAD calculations provide activation energies (E_a) that lie in the range: 24–37 kJ mol⁻¹ (acyl, 5-exo); 17–29 kJ mol⁻¹ (oxyacyl, 5-exo); 32–41 kJ mol⁻¹ (acyl, 6-endo); and 40–50 kJ mol⁻¹ (oxyacyl, 6-endo), with associated log(A/s⁻¹) values in the expected range: 10.5–12.5. These data provide rate constants for 5-exo cyclization (k_c) of ∼10⁴ to 10⁷ s⁻¹ for the acyl radicals, and ∼10⁶ to 10⁹ s⁻¹ for the oxyacyl radicals (296 K) and are in excellent agreement with the few available kinetic data. This study provides valuable new information for practitioners wishing to employ this chemistry in synthesis.