Publication
532
J. Am.
Chem. Soc., 123 (21),
4886 -4895, 2001
DOI: 10.1021/ja004234u S0002-7863(00)04234-7 |
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Stepwise and Concerted Pathways in Photoinduced and Thermal
Electron-Transfer/Bond-Breaking Reactions. Experimental Illustration
of Similarities and Contrasts
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Laurence Pause, Marc Robert, and
Jean-Michel Savéant
Contribution from the Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université-
CNRS No 7591, Université de Paris 7-Denis Diderot, 2 place Jussieu, 75251 Paris Cedex 05, France
The electrochemical (cyclic voltammetry) and photoinduced (fluorescence quenching, quantum yields)
reductive cleavages of four compounds, 4-cyano--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. |