Nitrosation of amines in nonaqueous solvents .2. Solvent-induced mechanistic changes

We studied the nitrosation of amines (pyrrolidine, piperidine, diethylamine, N-methylpiperazine, N,N'-dimethylethylenediamine, and morpholine) by alkyl nitrites (2-bromoethyl nitrite or 2,2-dichloroethyl nitrite) or by N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the solvents chloroform, acetonitrile, and dimethyl sulfoxide (DMSO). The mechanism of nitrosation by alkyl nitrites depends on the solvent: in chloroform, all the results were in keeping with formation of a hydrogen-bonded complex between the amine and alkyl nitrite being followed by rate-controlling formation of a tetrahedral intermediate T+/- that rapidly decomposes to afford the final products; in acetonitrile, a situation intermediate between those obtaining in chloroform and cyclohexane results in the [amine] dependence of the first-order pseudoconstant k(0) being qualitatively influenced by temperature and by the identities of both the amine and the alkyl nitrite; in DMSO, the results suggest a mechanism close to the mechanism acting in water. For nitrosation by MNTS, k(0) depended linearly on [amine] in all three solvents. The Grunwald-Winstein coefficients correlating the rate constants k for nitrosation by MNTS in the chloroform, acetonitrile, DMSO, dioxane, dichloromethane, and water were l = 0.12 and m = 0.29. Correlation with the Kamlet-Abboud-Taft equation confirmed that k depends largely on the dipolarity of the solvent and, to a lesser extent, its capacity for hydrogen bonding.