Stepwise and Concerted Pathways in Photoinduced and Thermal Electron-Transfer/Bond-Breaking Reactions. Experimental Illustration of Similarities and Contrasts

The electrochemical (cyclic voltammetry) and photoinduced (fluorescence quenching, quantum yields) reductive cleavages of four compounds, 4-cyano-alpha-trifluorotoluene (1), dimethylphenyl sulfonium (2), 4-cyanobenzylmethylphenyl sulfonium (3), and 4-cyanobenzyl chloride (4), are investigated and compared in terms of concerted vs stepwise mechanisms. Bearing in mind that an increase of the thermodynamic driving force shifts the mechanism from concerted to stepwise and that the driving force is larger under photochemical than under electrochemical conditions, 1 and 2 are typical examples where a stepwise mechanism is followed with compatible kinetic characteristics under both regimes. 4 undergoes a concerted electrochemical reductive cleavage, and the same mechanism is followed in the photoinduced reaction with consistent kinetic characteristics. The case of 3 is of particular interest, since a trend of passing from a concerted to a stepwise mechanism when going from the electrochemical to the photochemical conditions indeed appears upon analysis of the experimental results. The change of mechanism is, however, not complete since, in the photoinduced reaction, there is a balanced competition between the two pathways. In the same families of compounds, the unsubstituted benzylmethylphenyl sulfonium cations shows such a borderline behavior during the electrochemical reaction. In the photoinduced reaction, it is the 4-cyano derivative which behaves in a borderline manner, in line with the fact that it gives rise more readily to a concerted mechanism than the unsubstituted compound.