proton transfers of substituted ammonium salts—XIII : N-inversion of piperidines in aqueous acidic solutions

The kinetics of the nitrogen inversion of 1,2-dimethyl-(2); 1,4-dimethyl-(8); 1-trans(2,6)-trimethyl-(4); 1,4-cis(2,6)-tetramethyl-(7) and 1,2,2,6,6-pentamethyl-(5)piperidines have been investigated in aqueous acidic solution (pH = 6.5–8.5) at 33° by dynamic NMR. In all cases, two isomeric cations AH⁺ and BH⁺ are observed in acidic conditions (pH<6?), and the nitrogen inversion is brought to the NMR time scale as a result of a progressive deprotonation of the cations into their conjugate amines on increasing the pH. Low rate constants K_A are obtained for α-substituted or unsubstituted compounds (k_A ?10³ s^?1), except for piperidine 5 where the rate constant k_A = 4.3 × 10⁵ s^?1 is of the same order of magnitude as the one found for tertiary acyclic amines.