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 the 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 of the carbon-halogen bond in benzyl halides and of the nitrogen-halogen bond in aromatic N-halosultams, the two main factors governing the nature of the mechanism are the LUMO energy and the bond strength in the starting molecule: the higher the former and the weaker the latter, the greater the tendency for the concerted mechanism to prevail over the stepwise mechanism and vice versa. Consistently with the effect of these two mechanism-governing factors, two borderline cases were identified where the reaction passes from the concerted pathway to the stepwise pathway upon increasing the driving force by raising the scan rate and thus shifting the reduction potential toward negative values. The reasons for possible variations of the concerted or stepwise character of the mechanism of reductive cleavages upon changing the mode of electron injection are discussed.