Dioxirane oxidation of nitrosoamines. An ab initio study

Abstract

Pathways for oxidation of the parent nitrosoamine 2, nitrosodimethylamine 3, and anti- nitrosoethylmethylamine 4 by the parent dioxirane 1 have been explored computationally using the B3LYP hybrid density functional theoretical method in conjunction with the 6-31G* and 6-311+G** basis sets. Oxygen transfer from 1 to the nitrogen of the NO group (NO oxidation), yielding nitroamines 5,7, and 10, has the lowest activation barrier (15, 12.8, and 12 kcal mol-1 for 2, 3, and anti-4, respectively). Oxidation of the amine nitrogen (N oxidation) in 2, 3 leads to nitric oxide and nitroxyl radicals 6, 8 and is characterized by the highest activation energy: 28.5 kcal mol-1 for 2, 22.3 kcal mol-1 for 3. The potential barrier to hydroxylation of the methyl groups in 3 (CH oxidation) is intermediate - ca. 19 kcal mol-1. Introduction of a methyl group to the carbon reaction centre decreases the activation barrier of the anti-CH oxidation by 2-3 kcal mol-1. In 3, a gas-phase small preference for anti-CH oxidation over syn-CH oxidation, 0.1 kcal mol-1, is predicted to increase dramatically to 4.7 kcal mol-1 when the reaction is carried out in a dielectric medium (acetone, IPCM model). In general, polar solvents (CH2Cl2, acetone) lower the activation barriers for the NO, N, and CH oxidations, more so for the species of higher polarity than those of lower polarity (reactive complexes (1+3), (1+anti-4), transition state anti,syn-TSCH(1+4)]. However, the chemoselectivity (EaNO <Eaanti-CH <Easyn-CH <EaN) in the polar solvents is predicted to be the same as in the gas phase.Key words: dioxirane, nitrosoamines, oxidation, ab initio, omputation.