Nucleophilic attacks on carbon-carbon double bonds. Part XIV. Low-element effects and amine catalysis in the substitution of 1,1-dicyano-2-p- dimethylaminophenyl-2-halogenoethylenes by anilines in alcohols

The substitution of 1,1-dicyano-2-p-dimethylaminophenyl-2-chloro- and -2-fluoro-ethylenes by substituted anilines in methanol, propan-2-ol, and t-butyl alcohol is of the first order in the amine for the chloride, and of higher than first order in the amine for the fluoride. The reactions have low ΔH‡ and high negative ΔS‡ and (especially for the chloride), high negative Hammett's ρ values. A mechanism is suggested in which a reversibly formed zwitterion Ar(ArN⁺H₂)CX- C̄(CN)₂ reacts further by either (a) solvent-assisted expulsion of the halide ion followed by N-H bond cleavage, or by (b) amine-catalysed N-H bond cleavage followed by expulsion of the halide ion. The chloro-ethylene reacts via (a) and the fluoro-ethylene via both (a) and (b). The element effects (k_F/k_Cl) for route (a) are low (3.05-0.71) and decrease with increasing temperature, on increasing the basicity of the amine, and on decreasing the dielectric constant of the solvent.