Reaction of Methoxide Ion with Dibenzo-1,2-dithiin 1,1-Dioxide: Surprising Behavior in the Reaction of an Aryl Thiolsulfonate with an Alkoxide

Article abstract of DOI:10.1021/jo00155a013

Although the six membered cyclic thiosulfonate dibenzo-1,2-dithiin 1,1-dioxide (1) reacts with such nucleophiles as RS^-, CN^-, or SO3^2- at a rate only slightly slower than they react with an acyclic aryl thiosulfonate (PhSSO2Ph), it reacts with methoxide ion over 1E4 times slower than does PhSSO2Ph, and in methanol the equilibrium constant for the reaction of 1 with CH3O^- to form the ring-opened sulfenate ester 2a (eq 3) is so small that at equilibrium only a few percent of 1 is converted to 2a, even at quite high methoxide concentrations.The equilibrium constant for the conversion of 1 to 2a is, as might be expected, much larger in dimethyl sulfoxide (Me2SO)-methanol, and the rate constants for both the forward and reverse steps of the equilibrium can be determined in 70-90 percent Me2SO by stopped-flow spectrophotometry.The rate for the conversion of 1 to 2a is found to increase markedly with an increase in the Me2SO content of the medium, but the rate of the reverse reaction (2a->1+MeO^-) is not significantly dependent on solvent composition.It is shown that all these results can seemingly be satisfactorily explained only if the reaction of 1 with methoxide to form 2a is assumed to take place by a stepwise mechanism (eq 8) in which a hypervalent sulfur species (6a) is present on the reaction coordinate as an intermediate which lies in a potential well of sufficient depth that there is a substantial ΔG^* for collapse of 6a to 2a.Because of the six-membered ring in 6a, ΔG^* for 6a going to 2a is 5-6 kcal/mol larger than ΔG^* for the collapse of the equivalent intermediate (6o) to PhSOCH3 plus PhSO2^- in the reaction of PhSSO2Ph with methoxide.In the reactions of RS^-, CN^- or SO3^2- with 1, the free energy of 6b, the intermediate analogous to 6a, is substantially higher, so much so that ΔG^* for collapse of 6b to ring-opened products is in no case more than 1-2 kcal/mol.For this reason there is little difference for these nucleophiles in the rates for 1 vs.PhSSO2Ph.The preceding explanations are all in accord with expectations based on the findings of Martin and co-workers^10,11 regarding the relative stability of isolable hypervalent sulfur species containing apical ligands of differing electronegativity and the influence of a ring on the stability of such species.