Are weak interactions responsible for kinetic catalytic behaviour inSNAr reactions?

In solvents of low permittivity, non-covalent interactions between nitroaromatic derivatives and amines are detected and quantitatively evaluated by common spectroscopic methods. The nature of these complexes may be discussed. Mainly, these interactions are donor–acceptor and hydrogen bonding interactions. Electron-donor solvents compete with the amine in complexing the substrate. In SNAr reactions carried out with neutral nucleophiles (amines), in poorly polar solvents (such as aliphatic and aromatic hydrocarbons, tetrahydrofuran, chloroform, carbon tetrachloride, etc.), the experimental rate constant (kobs in s−1 mol−1 dm3) increases on increasing the initial concentration value of the amine. The intervention of the amine (or other catalysts) on the leaving group/proton departure from the zwitterionic intermediate is the more usual explanation of the autocatalytic behaviour. This mechanism conflicts with the usual nucleophugality order (in apolar solvents also the reactivity order is F > Cl) observed in SNAr reactions. An alternative interpretation of the ‘anomalous’ kinetic behaviour involves the presence of molecular complexes on the reaction pathway. In agreement with this hypothesis, the evaluation of the stability of complexes from kinetic data agree well with the evaluation from independent spectroscopic data. The main points supporting the presence of the molecular complex on the reaction pathway of SNAr reactions are: (i) the electronic effects of substituents on aniline; (ii) the kinetic behaviour of systems without a leaving group and NH protons; (iii) the effect of change of the temperature; (iv) the absence of self-catalysis which corresponds to the absence of complexes; and (v) the kinetic behaviour of neutral oxygenated nucleophiles which parallels the behaviour of amines.