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).

以碲代罗丹明发色团（tellurorhodamine chromophore）——六氟磷酸9-均三甲苯基-3,6-双(二甲氨基)碲氧杂蒽鎓——作为敏化剂与催化剂，成功实现了硫醇的需氧光催化氧化，将其转化为二硫化物。本研究提出的反应机理经实验与密度泛函理论（Density Functional Theory, DFT）计算验证，其过程为：碲代罗丹明经光照生成单线态氧，随后与水发生反应，生成碲代罗丹明氧化碲中间体；该氧化为氧化碲的过程同时伴随过氧化氢的生成。生成的氧化碲中间体可将硫醇氧化，自身再生为碲代罗丹明，同时得到二硫化物产物。从机理角度来看，DFT计算结果表明，两个硫醇分子与氧化碲中间体结合，脱去一分子水后形成三角双锥构型的四价碲（Te(IV)）物种，该物种经协同过程脱去二硫化物，完成碲代罗丹明的再生。在大气环境、可见光照射2小时后，苯硫酚与2-萘硫酚的氧化反应可完全进行。而缺电子的2,6-二氯苯硫酚、空间位阻较大的叔丁基硫醇以及脂肪族十二硫醇的氧化速率相对较慢；其中两种脂肪族硫醇还会伴随催化剂的竞争性降解。对应的硒代罗丹明发色团（selenorhodamine chromophore）——六氟磷酸9-均三甲苯基-3,6-双(二甲氨基)硒氧杂蒽鎓——在相似条件下无法生成相应的硒氧化物，且其光催化氧化苯硫酚与2-萘硫酚的速率显著更慢（2-3小时后转化率≤6%）。