Nucleophilic Attacks on Carbon-Nitrogen Double Bonds. 3. Diversity of Mechanisms for the Substitution of Diarylimidoyl Chlorides by Amines in Acetonitrile

The reactions of diarylimidoyl chlorides XC₆H₄(Cl)=NC₆H₄Y with amines in acetonitrile show multiplicity of kinetic behavior, (i) When X =H, Y = m-Cl or p-Cl, the first-order rate constant (K_obsd) for the reaction with piperidine increases nonlinearly with the [amine], and 1/k_obsd is linear in 1/[R₂NH]. (ii) For several systems, e.g., when X =. Y = p-NO₂, with several amines k_obsd= k° + k'[amine], (iii) When X = Y = p-NO₂, the reaction with piperidine is of overall second order, (iv) When X =H, Y = H or p-NO₂ and R₂NH = i-Pr₂NH, K_obsd decreases during the reaction or by addition of external chloride ion. These results are explained by a unified reaction scheme with three different substitution routes: (a) a bimolecular, probably a nucleophilic addition-elimination reaction which predominates with strong nucleophiles and when X and Y are strong electron-attracting groups; (b) reaction via the ion pair 3 which is formed in the initial heterolysis; this route occurs simultaneously with route a when either X or Y is strongly electron attracting and becomes dominant when X =H, Y = p-Cl or m-C1; (c) reaction via a free nitrilium ion 4 which is formed from the dissociation of 3 is the main substitution route by the sterically hindered diisopropylamine.