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Mechanism and proton activating factors in the base‐induced β ‐elimination reaction of N ‐[2‐(2‐quinolyl)ethyl]quinuclidinium salt
Author(s) -
Alunni S.,
Orazi C.
Publication year - 2001
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.450
Subject(s) - chemistry , carbanion , deprotonation , protonation , conjugate acid , medicinal chemistry , reactivity (psychology) , quinoline , kinetic isotope effect , pyridine , reaction rate constant , acid catalysis , ring (chemistry) , catalysis , solvent , salt (chemistry) , kinetics , organic chemistry , deuterium , medicine , ion , physics , alternative medicine , pathology , quantum mechanics
N ‐[2‐(2‐Quinolyl)ethyl]quinuclidinium salt in OH − –H 2 O, 50 °C, µ = 1 M KCl undergoes an elimination reaction with formation of 2‐vinylquinoline; the second‐order rate constant is k N OH = 12.8 × 10 −3 dm 3 mol −1 s −1 . In acetohydroxamic acid–acetohydroxamate buffers at pH 8–9 the β‐elimination reaction occurs by a reversible E 1 cb mechanism, ( E 1 cb ) 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 p K a = 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.