Stereochemical and kinetic studies on the Pummerer reaction of arylsulphinylcyclopropanes and phenylsulphinylmethylcycloalkanes with acetic anhydride

Abstract

The Pummerer reactions of arylsulphinylcyclopropanes and phenylsulphinylmethylcycloalkanes with acetic anhydride containing anhydrous sodium acetate afforded 1-phenylthio-1-acetoxycyclopropanes and [acetoxy-(phenylthio)methyl]cycloalkanes, respectively, in high yields but no ring opening products. The Pummerer reactions of trans- and cis-1-phenyl-2-phenylsulphinylcyclopropanes and cis,anti- and cis,syn-1,2-dimethyl-3-(phenylsulphinyl)cyclopropanes gave the corresponding stereoselective products. The pseudo-first-order rate constants for the Pummerer reaction were determined using a large excess of acetic anhydride. A Hammett correlation of the rates with σ values gives a ∪ shape curve, whereas the enthalpy and the entropy of activation for the reaction of phenylsulphinylcyclopropane are ΔH^‡ 41.3 kcal mol^–1 and ΔS^‡+ 10.4 cal mol^–1 K^–1, respectively. Hydrogen–deuterium kinetic isotope effects were found to be rather small (k_H/k_D 1.13, 1.24, and 1.49 for p-MeOH-, and m-CF₃-phenylsulphinyl[1-²H]cyclopropanes, respectively). An ¹⁸O tracer study indicated that there was marked oxygen exchange during the reaction but the products were found to retain a substantial proportion of the ¹⁸O of the original sulphoxides. These observations suggest that the reaction proceeds via an ylide–ion pair intermediate, after initial formation of acetoxysulphonium salts.