Reaction of cyclopropylbenzene and cyclopropyl phenyl ketone with sulfur trioxide

The sulfonation of cyclopropylbenzene (1a) and cyclopropyl phenyl ketone (5) with sulfur trioxide in both nitromethane and 1,4-dioxane as solvent has been studied. Sulfonation of cyclopropylbenzene (1a) with up to 2.0 mol-equiv. of SO3 in dioxane leads to the initial formation of 3-phenyl-1,3-propanesultone (2a) which product subsequently slowly isomerizes to 3-phenyl-2-propene-1-sulfonic acid (4a). Sulfonation of 1a with 4.0 mol-equiv. of SO3 in nitromethane gives the corresponding 4-sulfo derivatives 2b and 4b, of which the former product also slowly isomerizes to give the latter. Reaction of cyclopropyl phenyl ketone (5) with SO3 in nitromethane yields quantitatively 1-benzoyl-1,3-propanesultone (6). Upon aqueous alkaline work-up of the reaction mixtures, the gamma-sultones 2a and 6 are quantitatively converted into the corresponding potassium hydroxypropanesulfonates 3 and 7, respectively. Mechanisms for the formation of the various products from the starting cyclopropyl substrates are proposed.