Thermal SRN1 reactions:: How do they work?: Novel evidence that the driving force controls the transition between stepwise and concerted mechanisms in 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 radical resulting from its reductive cleavage. Many S(RN)1 substitutions require an external stimulation involving the injection of a catalytic amount of electrons. In "thermal" S(RN)1 reactions, there is no other source of initiating electrons than the nucleophile which is usually a poor electron donor. Such reactions are unlikely to be initiated by a simple outersphere electron transfer from the nucleophile followed by the cleavage of the substrate anion radical. Rather, initiation follows a mechanism in which electron transfer and bond cleavage are concerted. These conclusions are based on a full analysis of a model system involving 4-nitrocumyl chloride as the substrate 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 reactions reported earlier. These results provide new and unambiguous evidence that a decrease in driving force is able to change the mechanism of homogeneous reductive cleavage reactions from stepwise to concerted. The observation of this mechanism change was made possible by the kinetic amplification offered by the chain character of the S(RN)1 process, which allows the investigation of very slow electron transfers resulting from very low driving forces, that would have otherwise escaped characterization.