Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base

Abstract

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH < 5.16 (slope = −0.30), and a change in rate-determining step to bromide ion departure for pKBH > 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.