Sticky dissociative electron transfer to a series of substituted arene sulfenyl chlorides
The electrochemical reduction of a series of substituted arene sulfenyl chlorides was studied in acetonitrile using cyclic voltammetry and theoretical calculations. The electron transfer (ET) for the studied series was investigated in order to expand the current knowledge of ET chemistry to organic sulfides. This work is an extension of previous studies performed in our group on the electrochemical reduction of aryl and benzyl thiocyanates. The aryl thiocyanates showed a unique autocatalytic process and a transition between a concerted and a stepwise mechanism during their reduction, whereas the mechanism for the benzyl thiocyanates showed a clear-cut example of an electrochemically initiated regioselectivity. In this study, 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 sulfenyl 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-nitrophenyl sulfenyl chloride and o,p-dinitrophenyl sulfenyl chloride a stepwise mechanism is observed. In both cases, the corresponding 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 interaction between the produced fragments, along with gas phase chemical quantum calculations results helped rationalize both the observed change in the ET mechanism and the occurrence of a "sticky dissociative" ET mechanism. This study shows that despite the low magnitude of in-cage interactions in acetonitrile compared to 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. This investigation also allows us to investigate the dynamics of the electron transfer to the aforementioned compounds through the determination of the kinetic, thermodynamic and mechanistic aspects of the electron transfer processes.