The mechanistic origin of regiochemical changes in the nitrosative N-dealkylation of N,N-dialkyl aromatic amines

Abstract

The regioselectivity of the nitrous acid mediated dealkylation of 4-substituted-N-ethyl-N-methylanilines is a function of the acidity of the reaction mixture. At high acidity deethylation predominates, whereas demethylation is the predominant reaction in nitrosamine formation at pH 2 and above. In some cases the regioselectivity of nitrosative dealkylation changes as the run proceeds. Through the use of the corresponding 4-nitroaniline as the primary substrate, CIDNP, kinetics, kinetic deuterium isotope effects and other transformations involving nitrosations with NO₂ or NOBF₄ in aprotic solvents, a new mechanism of tertiary amine nitrosation has been deduced and proposed to explain regioselective deethylation. The mechanism involves the oxidation of the substrate to the amine radical cation by NO⁺. This is followed by the abstraction of a hydrogen atom from the carbon adjacent to the amine nitrogen by NO₂ to produce an iminium ion which reacts further to produce the corresponding aldehyde and the nitrosamine. Depending upon the acidity, this process competes with three other mechanistic pathways, two of which give the nitrosamine through the iminium ion, and one leads to the formation of C-nitro compounds. The competing pathways to nitrosamine formation involve NOH elimination from a nitrosammonium ion and deprotonation of the radical cation to give an α-amino radical which rapidly oxidized to the iminium ion. Predominant, but not highly regioselective demethylation occurs by these pathways. Nitro compound formation principally arises from the reaction of NO₂ with the radical cation followed by deprotonation, but also occurs by para C-nitrosation followed by oxidation.