Retardation of hydrolysis of aryl arenesulphonate esters by quinuclidine through complex formation

Abstract

The hydrolysis of aryl arenesulphonate esters is considerably retarded upon the addition of quinuclidine (Quin). Kinetic and spectral data indicate that the protonated form of Quin forms non-covalent complexes with the sulphonate esters and that the complexation partially protects the esters from hydrolysis. Formation constants have been measured with substrates containing various substituents on the benzene ring of either the phenol or the benzenesulphonate portion of the substrate. However, Quin does not affect the hydrolysis of p-nitrophenyl methanesulphonate. In addition, other amines such as benzylamine, 4-N,N-dimethylaminopyridine, triethylamine, and aniline do not affect the hydrolysis of the p-nitrophenyl p-nitrobenzenesulphonate. Based on these data, a structure has been assigned to the complex formed between the protonated form of Quin and the aryl arenesulphonate esters. In this structure, the polar interaction between the ammonium portion of the protonated Quin and the sulphonyl oxygen of the substrate is involved, together with the hydrophobic interaction between the bicyclic ring of Quin and the benzene ring of the benzenesulphonate moiety of the substrate.