Photocatalytic Aerobic Thiol Oxidation with a Self-Sensitized Tellurorhodamine Chromophore

Aerobic oxidation of thiols to disulfides was achieved photocatalytically
using a tellurorhodamine chromophore (9-mesityl-3,6-bis(dimethylamino)telluroxanthylium
hexafluorophosphate) as both the sensitizer and catalyst. The proposed
mechanism, supported experimentally and computationally with DFT,
involves the formation of a tellurorhodamine telluroxide from reaction
with water and singlet oxygen generated by irradiation of the tellurorhodamine.
The oxidation to the telluroxide is accompanied by the formation of
hydrogen peroxide. The telluroxide oxidizes thiols to regenerate the
tellurorhodamine and the disulfide plus water. Mechanistically, DFT
suggests adding two thiols to the telluroxide with the loss of H2O to give a trigonal-bipyramidal Te(IV), which undergoes concerted
loss of disulfide to regenerate 1. Oxidation of thiophenol
and 2-naphthalenethiol was complete after 2 h of irradiation with
visible light under atmospheric conditions. Oxidation of the electron-poor
2,6-dichlorothiophenol, the sterically bulky tert-butylmercaptan, and aliphatic dodecanethiol was slower. The two
aliphatic thiols displayed competing catalyst degradation. The corresponding
selenorhodamine chromophore (9-mesityl-3,6-bis(dimethylamino)selenoxanthylium
hexafluorophosphate) does not form the corresponding selenoxide under
similar conditions and photooxidizes thiophenol and 2-naphthalenethiol
much more slowly (≤6% conversion after 2–3 h).