Nucleophilic attacks on carbon–carbon double bonds. Part XXI. Substitution of (E)-α-chloro-β-nitrostyrene by anilines in acetonitrile

The replacement of the chloro-substituent of (E)-α-chloro-β-nitrostyrene (VI) by substituted anilines in acetonitrile, which gives the corresponding nitro-enamines (VII), is first-order in the amine, giving ΔH= 3·7–5·7 kcal mol^–1, ΔS=–50 to –58 cal mol–1 K^–1 and a Hammett ρ value of 3. Replacing the vinylic hydrogen atom of (VI) by deuterium resulted in an isotope effect of ca. 1 with several anilines, and in the reaction of aniline with (VI) in MeCN–D₂O no deuterium is incorporated into the product (VII). A multi-step mechanism, in which the nucleophilic attack step (k₁) to form the zwitterion (I) is followed by its reversal (k_-1) or by expulsion of chloride ion from (I)(k₂), with k_obs=k₁k₂/k_–1, is suggested. The absence of catalysis in the reactions of (VI) and the activating effect of the nitro-group in nucleophilic vinylic substitution are discussed.