Nitrosation of Amines in Nonaqueous Solvents. 1. Evidence of a Stepwise Mechanism

We studied the nitrosation of piperidine, morpholine, pyrrolidine, N -methylpiperazine, N, N ‘-dimethylethylenediamine and diethylamine by 2-bromoethyl nitrite, 2,2-dichloroethyl nitrite, 2,2,2-trichloroethyl nitrite, or N -methyl- N -nitroso- p -toluenesulfonamide (MNTS) in cyclohexane, isooctane, dichloromethane, 1,4-dioxane, or tetrahydrofuran. The dependence of the first-order pseudoconstant k 0 on the amine concentration (always in excess) was sigmoid for nitrosation by alkyl nitrites and linear or quadratic for nitrosation by MNTS. The effects on k 0 of isotopic substitution, temperature, and base catalysis by a less reactive amine were also determined. The experimental data are in keeping with a reaction mechanism involving a zwitterionic tetrahedral intermediate T ± analogous to intermediates postulated for the aminolysis of carboxylic esters in similar solvents: according to this mechanism, T ± is formed either directly from the amine and nitrosating agent (in the case of MNTS) or indirectly via a hydrogen-bonded complex between the amine and nitrosating agent (in the case of alkyl nitrites) and decomposes either spontaneously or with the catalytic assistance of a second amine molecule. For alkyl nitrites, the rate-controlling step is the formation of T ± at high amine concentrations and its decomposition at low amine concentrations; for MNTS, the rate-controlling step is the formation of T ± in more polar solvents and its decomposition in less polar solvents. An alternative mechanism, involving the formation of T ± from both monomers and dimers of the amine, is ruled out.