The effect of a hydrogen bond acceptor catalyst on the dimer mechanism in aromatic nucleophilic substitution

Abstract

The reaction of 2,6-dinitroanisole with cyclohexylamine in toluene–dimethyl sulphoxide (DMSO) mixtures was studied at several amine concentrations. In pure toluene and toluene–DMSO mixtures of low DMSO content (⩽ 1%) third-order kinetics with respect to the amine are observed. These results are interpreted as a consequence of the aggregation of the nucleophile (dimer) in low-polarity solvents. Association of DMSO with the amine (‘mixed dimer’) in a solute-solvent hydrogen-bond interaction can be responsible for the observed catalytic effect of DMSO. At higher DMSO contents behaviour typical for amine catalysis was found which is related to considerable additional stabilization of the intermediate complex through solvation by DMSO.