Nucleophilic attacks on carbon–carbon double bonds. Part XXII. Base catalysis, leaving group effects, and solvent effects in several nucleophilic vinylic substitutions by amines

Abstract

Displacements of the leaving groups X from 1-X-2,2-bisethoxycarbonyl-1-p-nitrophenylethylenes [6; X = Cl, OSO₂Me(OMs), OSO₂C₆H₄Me-p(OTs), OSO₂C₆H₄Br-p(OBs)] by piperidine and morpholine in MeCN, tetrahydrofuran (THF), and EtOH and by benzenethiolate ions in EtOH are of the second order (rate constant k_obs). Reactions of anilines with 1-chloro-(and 1-bromo-)2,2-dicyano-1-p-nitrophenylethylenes (7) show very mild catalysis by the amine, while the reactions of 1-chloro-(and 1-bromo-)2-cyano-2-methoxycarbonylethylenes (9) with p-cyanoaniline, and of other electrophilic olefins with piperidine and morpholine show no catalysis. The reactivity ratios of the leaving groups in system (6) are: k_OMs/k_Cl= 10·4–59·7; k_OTs/k_Cl= 10·6–29·9; k_OBs/K_OTs= 1·56–2·85, and for systems (7) and (9)k_Br/k_Cl= 0·67–1·32. For most systems the solvent effect on the reactivity is MeCN > Thf > EtOH. It is suggested that a zwitterion is formed by the nucleophilic attack (k₁) of the amine on the electrophilic olefin. This is followed either by a reversal of this step (k_–1), or by expulsion of X^–(k₂), or by deprotonation of the zwitterion by another amine molecule (k₃). In the catalysed reaction of system (7)k₃/k₂= 0·69–6·55, for system (9)k₁ is rate determining, while for system (6)k_obs=k₁k₂k_–1. The leaving group effect is mainly attributed to k₂ and an early transition state is suggested for this step. The solvent effect is due to the change of the dielectric constant and to the formation of intra- and inter-molecular hydrogen bonds.