PASSAGE FROM CONCERTED TO STEPWISE DISSOCIATIVE ELECTRON-TRANSFER AS A FUNCTION OF THE MOLECULAR-STRUCTURE AND OF THE ENERGY OF THE INCOMING ELECTRON - ELECTROCHEMICAL REDUCTION OF ARYLDIALKYL SULFONIUM CATIONS

The electrochemical reductive cleavage of the carbon-sulfur bond in th e title compounds offers the example of a reaction where the concerted or stepwise character of the electron-transfer-bond-breaking process is a function of molecular structure. As with the reductive cleavage o f the carbon-halogen bond in benzyl halides and of the nitrogen-haloge n bond in aromatic N-halosultams, the two main factors governing the n ature of the mechanism are the LUMO energy and the bond strength in th e starting molecule: the higher the former and the weaker the latter, the greater the tendency for the concerted mechanism to prevail over t he stepwise mechanism and vice versa. Consistently with the effect of these two mechanism-governing factors, two borderline cases were ident ified where the reaction passes from the concerted pathway to the step wise pathway upon increasing the driving force by raising the scan rat e and thus shifting the reduction potential toward negative values. Th e reasons for possible variations of the concerted or stepwise charact er of the mechanism of reductive cleavages upon changing the mode of e lectron injection are discussed.