α-(Acyloxy)dialkylnitrosamines:  Effects of Structure on the Formation of N-Nitrosiminium Ions and a Predicted Change in Mechanism

The decay of α-(acyloxy)dialkylnitrosamines in aqueous solutions has been studied with a view toward elucidating mechanistic details and effects of structure on mechanism and reactivity. Rate constants (k₁) for the pH-independent decay of 43 α-(acyloxy)dialkylnitrosamines have been determined. Observations from these and other experiments rule out decomposition via an anchimeric assistance mechanism involving the Z isomer that had previously been suggested. All of the reported data for most of the compounds is consistent with a mechanism involving the formation of N-nitrosiminium ions in or before the rate-limiting step. Structure−reactivity correlations indicate that the stability of α-(acyloxy)dialkylnitrosamines is determined by electronic properties of substituents at R_N and R_C as well as by the ability of substituents R_C to engage in hyperconjugative interactions of C−H bonds with the developing cationic center in the transition state for nitrosiminium ion formation. Attachment of substituents of sufficient electron-withdrawing power at R_N and R_C results in a predicted change in mechanism to what appears to be an acyl group attack mechanism.