Friedel−Crafts Sulfonylation in 1-Butyl-3-methylimidazolium Chloroaluminate Ionic Liquids

Abstract

1-Butyl-3-methylimidazolium chloroaluminate ionic liquids have been employed as an unconventional reaction media and as Lewis acid catalyst for Friedel−Crafts sulfonylation reaction of benzene and substituted benzenes with 4-methyl benzenesulfonyl chloride. The substrates exhibited enhanced reactivity, furnishing almost quantitative yields of diaryl sulfones, under ambient conditions. Studies concerning the effect of Lewis acidity of the ionic liquid on the initial extent of conversion of this reaction has been carried out. ²⁷Al NMR spectroscopy has been exploited as a tool to investigate the mechanistic details of the reaction. ²⁷Al NMR spectral studies show the predominance of [Al₂Cl₇]^- species in [bmim]Cl-AlCl₃, N = 0.67, acidic ionic liquid in the presence of 4-methyl benzenesulfonyl chloride, and after the reaction with the aromatic hydrocarbon, [AlCl₄]^- species predominates. This change in speciation of aluminum can be attributed to the interaction of the Lewis acidic species [Al₂Cl₇]^- of the ionic liquid with the formed HCl during the sulfonylation reaction, which is evidenced by the control experiment. Preliminary investigations on Friedel−Crafts acylation further substantiate the argument.