Evidence for an Anionic Sulfene Intermediate in the Alkaline Hydrolysis of Aryl (Methanesulfonyl)methanesulfonate Esters

Article abstract of DOI:10.1021/jo00350a032

The hydrolysis of aryl (methylsulfonyl)methanesulfonates obeys the kinetic law k_obsd = (k_a + k_b->)/(1 + +>/K_a), where Ka is the ionization constant of the ester.An E1cB mechanism is consistent with the above rate law and the results of studies on Broensted and Hammett selectivities for variation in the leaving group substituents, entropy of activation data on k_a and k_b, trapping with an amine, oxygen-18 incorporation into the acid products, deuterium exchange from D2O into the substrates, and the effect on rate constants of substituting one or both hydrogen atoms adjacent to the sulfonate group with methyls.The k_a term involves unimolecular expulsion of the leaving group from the ionized ester to give a sulfene (I).The k_b term is due to further ionization of the conjugate base of the ester to give a dianion which expels the leaving group to yield the unprecedented anionic sulfene (II).Deuterium exchange studies indicate that the anion ^-CH2SO2CH=SO2 is not involved kinetically.The variation of effective charge on the leaving oxygen is traced throughout the reaction path.