Abstract
The semiempirical MNDO SCF-MO method has been used to calculate kinetic isotope effects for the reactions of singlet and triplet oxygen with methylamine, methanol, and ethane. These reactions were chosen as models for cytochrome P-450 catalyzed N-dealkylation, O-dealkylation, and carbon oxidation. The reactant-like transition states were qualitatively similar for each substrate. The singlet reactions led to oxygen insertion via an attachment-rearrangement mechanism, while the triplet reactions proceeded via a hydrogen radical abstraction-recombination mechanism. For the singlet reactions with methylamine, methanol, and ethane, calculated deuterium isotope effects of 1.63, 1.68, and 1.65, respectively, were obtained. For the triplet reactions, the corresponding values were 8.19, 8.35, and 8.75. Comparison with experimental isotope effects suggests that P-450-mediated O-dealkylation and carbon oxidation, but not N-dealkylation, proceed via a mechanism involving H. abstraction by a triplet ''oxene-like'' species.
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