Radical/Ion Pair Formation in the Electrochemical Reduction of Arene Sulfenyl Chlorides

Abstract

Important aspects of the electrochemical reduction of a series of substituted arene sulfenyl chlorides are investigated. A striking change is observed in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the arene sulfenyl chloride. With p-substituted phenyl chlorides a "sticky" dissociative ET mechanism takes place where a concerted ET mechanism leads to the formation of a radical/anion cluster before decomposition. With o-nitropheyl sulfenyl substituted chlorides a stepwise mechanism is observed where through space S...O interactions play an important role stabilizing both the neutral molecules and their reduced forms. Disulfides are generated through a nucleophilic reaction of the two-electron reduction produced anion (arenethiolate) on the parent molecule. The dissociative electron transfer theory, as well as its extension to the case of strong in-cage interactions between the produced fragments, along with the gas phase chemical quantum calculations results helped rationalize both the observed change in the ET mechanism and the occurrence of the "sticky dissociative" ET mechanism. The radical/anion pair interactions have been determined both in solution as well as in gas phase. This study shows that despite the low magnitude of in-cage interactions in acetonitrile as compared to in the gas phase, their existence strongly affects the kinetics of the involved reactions. It also shows that, as expected, these interactions are reinforced by the existence of strong electron-withdrawing substituents.

Application of the dissociative electron transfer theory and its extension to the case of in-cage interactions in the electrochemical reduction of arenesulfonyl chlorides

“…We previously reported the electrochemical reduction of arene sulfenyl chlorides (ArSCl) and showed that their reduction results in the cleavage of the S-Cl bond. 34,63 Our investigation showed that the electrochemical reduction of 4-substituted sulfenyl chlorides follows a sticky dissociative ET mechanism involving the formation of a radical/anion (4-substituted phenyl sulfenyl radical/chloride anion) pair. 34 This manuscript describes the full investigation of an extended series of 4-substituted arene sulfonyl chlorides 1a-g (Chart 1).…”

“…The cyano-and nitrosubstituted compounds (1f,g) provide only the corresponding aryl sulfinates and no side products, similar to what has been seen for the nitro substituted arene sulfenyl chlorides. 34 Based on the electrochemical data, and considering, for now, a two electron reductive dissociation of the S-Cl chemical bond, the reduction mechanism for these compounds can be written as per Scheme 1. The sulfinate anion generated through the two electron reduction process transfers an electron to the parent sulfonyl chloride (reaction B).…”

“…3 shows the LUMOs of compounds 1a-g and the SOMOs of their reduced forms (radical anions or radical/anion pairs). [28][29][30][31][32][33][34]36 For compounds 1a-e, the LUMOs are more located on the S-Cl group with a lower factor on the aryl moiety, indicating that the extra electron is most likely injected directly into the s S-Cl bond leading to or concomitant with its dissociation. The SOMOs corresponding to their reduced forms are also more localized on the S-Cl group (s radical anion or radical/ion pair) and provide thus more support to the injection of the extra electron into the s S-Cl bond.…”

“…The factors controlling the ET mechanism variation as well as the extend of the in-cage interactions between the reduction fragments are discussed on the basis of the dissociative ET theory and its extension to the case of in-cage interactions (''sticky'' dissociative ET). [28][29][30][31][32][33][34]64 Theoretical calculations help rationalize both the difference in the ET mechanism as well as the cluster formation. Electrochemical measurements were conducted in three electrode glass cells, thermostatted at 25 1C, and under dry nitrogen.…”

Electrochemical Reduction of 4-Iodophenyl Sulfonyl Chloride

“…Some electrochemical data are also supported by theoretical calculations. This also allows a comparison with arene sulfenyl chlorides that we reported previously (27,28).…”