Mechanism and proton activating factors in the base‐induced β‐elimination reaction of N‐[2‐(2‐quinolyl)ethyl]quinuclidinium salt

Abstract

AbstractN‐[2‐(2‐Quinolyl)ethyl]quinuclidinium salt in OH−–H2O, 50 °C, µ = 1 M KCl undergoes an elimination reaction with formation of 2‐vinylquinoline; the second‐order rate constant is kNOH = 12.8 × 10−3 dm3 mol−1 s−1. In acetohydroxamic acid–acetohydroxamate buffers at pH 8–9 the β‐elimination reaction occurs by a reversible E1cb mechanism, (E1cb)R. In this process, carbon deprotonation occurs from the conjugate acid, protonated at the nitrogen atom of the quinoline ring, NH+; this species is present at a very low concentration with respect to the unprotonated substrate N, with pKa = 3.87 at 50 °C, µ = 1 M KCl. The reason for the high reactivity of NH+ with respect to N is related to the high stability of the intermediate carbanion formed from NH+, which presents an enamine structure. Kinetic parameters from a study of acid–base catalysis can be calculated and compared with those of the related system activated by a pyridine ring. Studies of H/D exchange and solvent isotope effect are in agreement with the proposed mechanism. Copyright © 2001 John Wiley & Sons, Ltd.