Rate and Equilibrium Constants for the Reaction of Thiolate Ions with Dibenzo-1,2-dithiin and Naphtho-1,2-dithiole 1,1-Dioxides

Article abstract of DOI:10.1021/jo00132a016

In aqueous dioxane the cyclic thiosulfonate dibenzo-1,2-dithiin 1,1-dioxide (1) reacts rapidly with thiolate ions and undergoes opening of the thiosulfonate ring (eq 2), forming disulfide 3a.Acidification of solutions of 3a with carboxylic acid buffers of appropiate pH leads to facile reversal of ring-opening reaction and the quantitative regeneration of 1.Since this reversal of ring opening is not acid-catalyzed, it must take place via a simple intramolecular displacement of RS^- by the sulfinate (SO2^-) group present in 3a and is therefore the microscopic reverse of the ring-opening reaction.Rate constants have been determined for both ring opening (k_RS) and reversal of ring opening (k_-RS) for a series of alkanethiolates of varying pK_a.From these data one may also calculate the equilibrium constant, K_eq(=k_RS/k_-RS), for reaction of each thiolate with 1.From comparison of the log K_eq's with previously determined equilibrium constants for reaction of cyanide and sulfite ions with 1 one obtains quantitative information on the thermodynamics of reactions of the type ArSSR + CN^- = ArSCN + RS^- and ArSSR + SO3^2- = ArSSO3^- + RS^- that should be of considerable value for predicting the magnitude of equilibrium constants for cyanide-disulfide and sulfite-disulfide equilibria.Plots of log K_eq, log K_RS, and log k_-RS vs. the pK_a of RSH reveal that β_eq=1.25, β_RS=0.26, and β_-RS=-0.99.These β values show that the transition state for eq 2 is quite unsymmetrical, with a structure Δ-...S-SO2^δ-> where the RS-S bond is only ca. 20percent formed.The β_RS and β_-RS values are compared with the β values for several other previously studied displacements involving disulfides.The reaction of naphthol<1,8-cd>-1,2-dithiole 1,1-dioxide (2) with thiolates behaves in a fashion analogous to that of the reaction of RS^- with 1.Comparison of K_eq, k_RS, and k_-RS for an equilibrium involving 2 and a thiolate with those for the corresponding thiolate reacting with 1 allows one to assess how a change from a six- to a five-membered thiosulfonate ring influences K_eq, k_RS, and k_-RS.The major effects are that k_-RS is much larger and K_eq is considerably smaller.