Vinylogous nucleophilic catalysis. Tertiary amine promoted hydrolysis of 1-alkene-1-sulfonyl chlorides

Abstract

We present evidence that the reactions of ethenesulfonyl chloride (1) and trans-1-propene-1-sulfonyl chloride (3) with water in the presence of pyridine, trimethylamine, and a number of other tertiary amines proceed primarily by way of an initial vinylogous substitution reaction to form the cationic sulfene, [Formula: see text], which subsequently reacts with water either by addition (and deprotonation) to form the betaine [Formula: see text], or by vinylogous substitution (and deprotonation) to give the alkenesulfonate anion, [Formula: see text] (R = H or CH3). Formation of the latter represents the first well-supported example of vinylogous nucleophilic catalysis. These conclusions are drawn from kinetic and product composition observations, including (a) α-monodeuteration in the betaine and lack of deuteration of the ethenesulfonate [Formula: see text] from the reaction in D2O, (b) rate lowerings of up to 2000-fold for 2- (and 6-) substituted pyridines from those expected from Brønsted-type relationships shown by "unhindered" pyridine bases, (c) lack of a kinetic solvent isotope effect in the reaction of 1 with 3-cyanopyridine, (d) a lower rate of reaction of 3 vs. 1 not directly correlated with product composition, and (e) formation of similar product mixtures from either 1 or Pyr+CH2CH2SO2Cl Cl− (18a) with aqueous pyridine. For the initial formation of the sulfene, [Formula: see text], the available evidence does not distinguish between a two-step mechanism via an intermediate zwitterion and a closely related concerted reaction, but for the further reaction of the sulfene a process involving a zwitterionic intermediate common to both products is favoured. For the reaction of 1 or 3 in the absence of tertiary amines evidence is presented for a direct displacement on sulfur mechanism leading to the alkenesulfonate anion, plus a small proportion (up to 15%) of formation of the 2-hydroxy-1-alkanesulfonate anion by way of the sulfene HOCHRCH=SO2 (R = H or CH3).