Meisenheimer complexes in aromatic nucleophilic substitution reactions.

When this research was commenced it was known that many aromatic nucleophilic substitution reactions proceeded by a mechanism which involves the formation and the decomposition of a Meisenheimer- type of complex. In some cases the formation, and in others the decomposition of such a complex was the rate-determining step. However, the mechanism of formation and the decomposition of these complexes was not known. This work represents an attempt to establish whether or not the transfer of a hydrogen atom or photon played a major part in the formation and the decomposition of Meisenheimer complexes. The methoxy exchange between 1-methoxy (C-14), 2,4-dinitronaphthalene and methoxide ions where the decomposition of the complex is rate- determining step has been studied in methanol and methanol-o-d and no evidence of a primary kinetic isotope effect has been found. Methoxy exchange reactions are able to proceed in DMSO solution and have been studied in the presence of 0.3% of methanol or methanol-o-d to aid solubility of sodium methoxide. Kinetic data for the reaction with p-nitroanisole where formation of the complex is rate-determining step are independent of whether methanol or methanol-o-d is used and give no evidence of the existence of a primary isotope effect. The enhanced rate of such reactions in DMSO as compared with methanol as solvent have enabled kinetic data to be obtained for this exchange reaction and the chlorine exchange between 1-chloro,2,4-dinitronaphthalene and chloride ions which are too slow to be studied easily in methanol. It is concluded tentatively that proton intercession is less important in the decomposition of a Meisenheimer complex than solvation of the incipient anion (methoxide) and is less important in the formation of such a complex than is base catalysis.