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Photosensitized Oxidation of Sulfides: Discriminating between the Singlet‐Oxygen Mechanism and Electron Transfer Involving Superoxide Anion or Molecular Oxygen
Author(s) -
Bonesi Sergio M.,
Manet Ilse,
Freccero Mauro,
Fagi Maurizio,
Albini Angelo
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200501144
Subject(s) - chemistry , photochemistry , flash photolysis , electron transfer , singlet oxygen , sulfide , quenching (fluorescence) , radical ion , intersystem crossing , singlet state , acetonitrile , oxygen , ion , organic chemistry , reaction rate constant , excited state , fluorescence , physics , quantum mechanics , nuclear physics , kinetics
The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N ‐methylquinolinium tetrafluoroborate (NMQ + ), and triphenylpyrylium tetrafluoroborate (TPP + ) has been explored by steady‐state and laser flash photolysis studies in acetonitrile, methanol, and 1,2‐dichloroethane. In the Et 2 S/DCA system sulfide‐enhanced intersystem crossing leads to generation of 1 O 2 , which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph 2 S, though enhanced formation of 3 DCA has been demonstrated. In all other cases an electron‐transfer (ET) mechanism is involved. Electron‐transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph 2 SO cooxidation. Formation of the radical cations R 2 S . + has been assessed by flash photolysis (medium‐dependent yield, dichloroethane≫CH 3 CN>CH 3 OH) and confirmed by quenching with 1,4‐dimethoxybenzene. Electron‐transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA .− , NMQ . ) and when this is not the case (TPP . ). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R 2 S . + and O 2 .− mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS 2 + ⋅⋅⋅OO . adds to a sulfide molecule and gives strongly stabilized R 2 SO . + OSR 2 via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet‐oxygen process.