Intramolecular reactions. Part VII. Cyclisation of aryl 3-chloropropyl sulphones

Abstract

Aryl 3-chloropropyl sulphones react with potassium t-butoxide in t-butyl alcohol to give aryl cyclopropyl sulphones in high yields. The effects of substituents in the aromatic nucleus upon the rate of the reaction have been determined, and a rectilinear Hammett plot with ρ=+2·32 is obtained. Hydrogen atoms of the methylene group adjacent to the sulphonyl group are rapidly exchanged under basic conditions in deuteriated solvents, and the rate of cyclisation of 3-chloro-1,1-dideuteriopropyl p-tolyl sulphone in t-butyl [2H] alcohol is twice that of the isotopically normal sulphone in t-butyl [1H]alcohol. These results suggest that an intermediate carbanion is formed and that the ρ-value obtained for the reaction is a composite one which incorporates opposing effects upon the equilibrium concentration of the carbanion and its nucleophilicity in the subsequent ring-closure step. This finding is briefly discussed in relation to current views on cyclisation.