Nucleophilic attacks on carbon–carbon double bonds. Part XVI. Some relative reactivities of the leaving groups (kBr/kCl, kOMs/kCl, and kOBs/kOTs) in vinylic substitutions

Abstract

Displacement of the halogen of 1-chloro- and 1-bromo-2,2-dimethoxycarbonylethylenes (II) and (III) by several substituted anilines in acetonitrile is of the second order; Hammett's ρ values are between –2·24 and –2·40 and k_II/k_III is ca. 1. Reaction of p-cyanoaniline with 1-chloro- and 1-mesyloxy-2,2-dicyanoethylenes gives k_OMs/k_Cl= 1. Substitution of 1-p-nitrophenyl-1-tosyloxy-(and 1-brosyloxy-)2,2-diethoxycarbonylethylenes by piperidine and morpholine gives k_OBs/k_OTs= 1·63–1·90, and k_pip/k_morp of 13·7–16·2. Two α-cyanogroups are 100-fold more activating than two α-ethoxycarbonyl groups in nucleophilic vinylic substitution. A rate-determining nucleophilic addition of amines is suggested for these reactions. The relative reactivities concerning the first step in nucleophilic vinylic substitutions of the addition–elimination type (Ad_N–E) are discussed, as well as criteria for distinguishing between the Ad_N–E and the vinylic S_N1 mechanisms.