"Thermal" S(RN)1 reactions: How do they work? Novel evidence that the driving force controls the transition between stepwise and concerted mechanismsin dissociative electron transfers

In S(RN)1 reactions, unlike in conventional nucleophilic substitutions, the nucleophile does not react directly with the electrophile but with a radic al resulting from its reductive cleavage. Many S(RN)1 substitutions require an external stimulation involving the injection of a catalytic amount of e lectrons. In "thermal" S(RN)1 reactions, there is no other source of initia ting electrons than the nucleophile which is usually a poor electron donor. Such reactions are unlikely to be initiated by a simple outersphere electr on transfer from the nucleophile followed by the cleavage of the substrate anion radical. Rather, initiation follows a mechanism in which electron tra nsfer and bond cleavage are concerted. These conclusions are based on a ful l analysis of a model system involving 4-nitrocumyl chloride as the substra te and the 2-nitropropanate ion as the nucleophile where all the pertinent thermodynamic and kinetic parameters were determined by direct or indirect electrochemical methods. They extend to other examples of thermal S(RN)1 re actions reported earlier. These results provide new and unambiguous evidenc e that a decrease in driving force is able to change the mechanism of homog eneous reductive cleavage reactions from stepwise to concerted. The observa tion of this mechanism change was made possible by the kinetic amplificatio n offered by the chain character of the S(RN)1 process, which allows the in vestigation of very slow electron transfers resulting from very low driving forces, that would have otherwise escaped characterization.