The effect of the nature of the amine leaving group on the nature of the E2 transition state for the reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol

Abstract

To investigate the effect of the leaving group on the elimination reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol at 60°C, the reaction of seven different quaternary ammonium salts and their P-deuterated analogues with trimethylamine, N-methylpipendine, N-methyldiethylamine, triethylamine, N,N-dimethylbenzylamine, tripropylamine, and N,N-diethylbenzylamine as leaving groups has been studied. In all cases the elimination, which was shown to proceed via the concerted E2 process, was accompanied by competing substitution reactions. Although a significant dependence of the rate of the elimination process on the nature of the leaving group was noted, there was not any linear correlation with the basicity of the amine leaving group. The primary hydrogen-deuterium kinetic isotope effect for the elimination process, (kH/kD)E, was found to increase initially with an increase of reaction rate, kF, for substrates containing the leaving roups trimethylamine, N-methylpipendine, N-methyldiethylamine, triethylamine, and N,N-dimethylbenzylamine; i.e., (kH/$)E = 5. 03,5.26,5.40, 5.83, and 5.85, respectively. A further increase in rate, using substrates with tripropylamine and N,N-diethylbenzylamine as leaving groups resulted in a decrease of the magnitude of the hydrogen-deuterium isotope effect; i.e., (kH / kDIE = 5.42 and 4.67, respectively. It is concluded that steric effects mainly determine leaving group ability. As well, it is concluded that the leaving group ability of the amine determines the structure of the E2 transition state. For the reaction of the poorer leaving groups, trimethylamine, N-methylpipendine, and N-methyldiethylamine, the proton is more than one-half transferred at the transition state while for reaction involving the two best leaving groups, tripropylamine and N,N-diethylbenzylamine, the Cp-H bond is less than one-half broken at the transition state. The conclusions are considered in the light of the More O'Ferrall -Jencks potential energy surface diagram.Key words: elimination mechanism, transition state, isotope effects, leaving group, quaternary salts.PETER JAMES SMITH et MD. AMIN. Can. J. Chem. 67, 1457 (1989).Dans le but d'Ctudier l'effet du groupement nuclCofuge sur la rtaction d'klimination des ions 1-phCnylCthylammonium sous l'influence de 1'Cthylate de sodium dans l'tthanol a 60°C, on aCtudiC la rkaction de sept sels d'ammonium quaternaire diffirents, ainsi que leurs analogues deutCrCs en position P, portant la trimkthylamine, la N-mCthylpipCridine, la N-mCthyldiCthylamine, la tritthylamine, la N,N-dimCthylbenzylamine, la tripropylamine et N,N-ditthylbenzylamine comme groupements nuclCofuges. Dans tous les cas, 1'Climination qui prockde par un processus concerte E2 est accompagnCe par les reactions de substitution qui sont en compttition. MCme si on a not6 une corrClation importante entre la vitesse du processus d'klimination et la nature du groupement nuclCofuge, il n'existe aucune corrClation linkaire avec la basicit6 de l'amine qui quitte. On a trouvC que, au dkbut d...

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