Evaluation of the Reactivity of Cyclohexanone СН Bonds in Reactions with tert-Butylperoxy Radical by Quantum Chemical Methods.

The effective charges on hydrogen and carbon atoms and the Fukui functions for the electrophilic and radical attacks for the hydrogen atoms of the chair and boat conformations of cyclohexanone were calculated using the density functional theory (DFT) with the hybrid functionals: B3LYP in the 6-311g++(d,p) basis set; X3LYP in the 6-311g++(d,p) and D95++(d,p) basis sets; and pbe1pbe in the D95++(d,p) basis set, and by the MP2 method in the 6-311g++(d,p) basis set. For these conformations, the transition states were localized; the length of the broken CH bond in the transition state was determined; and the activation energies, activation entropies, and enthalpies of the elementary reactions of the tert-butylperoxyl ((СН3)3СOO•) radical with all types of CH bonds of cyclohexanone were calculated. The relative reactivity of the CH bonds of cyclohexanone in reactions with ((СН3)3СOO•); the local electrophilicity indices of the carbon-centered radicals formed from cyclohexanone after elimination of the hydrogen atom; and the energy difference (ΔЕSOMO) between the single-occupied molecular orbitals of these carbon-centered radicals and the (СН3)3СOO• radical were calculated. The CH bond dissociation energies of cyclohexanone were calculated by the DFT B3LYP 6-311g++(d,p), CBS-QB3, and G3MP2B3 methods using the isodesmic reactions technique. The factors governing the CH bond reactivity of cyclohexanone in reactions with the (СН3)3СOO• radical were considered. The increased reactivity of CH bonds of cyclohexanone in the 2, 6 (α) positions was shown to be due to the low bond dissociation energy, high electron-donor capacity of hydrogen atoms, smaller length of the broken CH bond in the transition state, and small ΔЕSOMO value. The lower reactivity of CH bonds at positions 3, 5 (β), and 4 (γ) is determined by the higher bond dissociation energies, decreased electron density due to the inductive effect of the carbonyl group, nucleophilicity of the carbon-centered radicals, greater length of the broken CH bonds in the transition state, and larger ΔЕSOMO value. The difference in the reactivity of CH bonds at positions 3, 5 (β), and 4 (γ) is determined by these factors, but not by the difference in the dissociation energies of these bonds. The reactivity of CH bonds in the boat conformation is higher than that in the chair conformation in the reactions with the (СН3)3СOO• radical. [ABSTRACT FROM AUTHOR]