E1cB eliminations. Part V. Two types of E1cB reaction in the amine-catalysed elimination of HCN from 2,6-dimethyl-4-(ααββ-tetracyanoethyl)aniline in chloroform

Abstract

Amine-catalysed elimination of HCN from 2,6-dimethyl-4-(ααββ-tetracyanoethyl)aniline (I) in chloroform goes to completion even if [I] > [Amine]. With Et₃N and Buⁿ₃N the reaction is of zero order in both (I) and the amine within a run, and of the first order in the amine for different runs when [Amine] < [I]. For eight other amines with lower pK_b′s the reaction is of the second order. The k_H/k_D values for all the amines are 1·0 ± 0·06. The reaction of Et₃N and Buⁿ₃N is discussed in terms of ‘E1cB of the second type’ through ion-pairs, where almost complete neutralisation of (I) is followed by a rate-determining trialkylammonium ion-assisted elimination of CN^– from the intermediate carbanion–ammonium ion pair. The reactions with the other amines give less neutralisation of (I) and are discussed in terms of the ‘pre-equilibrium’E1cB mechanism. The expulsion of CN^– is also assisted by the ammonium ion, probably within the ion pairs.