Aerobic oxidative radical coupling catalyzed by TEMPO for the preparation of 2,2′‐dibenzothiazole disulfide

BACKGROUND: 2,2′‐disbenzothiazole disulfide is widely used as a vulcanization accelerator in rubber production and as an intermediate in the drug industry. Its current industrial process, the oxidation of 2‐mercaptobenzothiazole by sodium nitrite in acidic solution, produces large quantities of liquid waste water. A novel and green synthetic method was developed, which used dioxygen as the oxidant and 2,2,6,6‐tetramethylpiperidyl‐1‐oxyl (TEMPO) as the catalyst without any metallic compounds. RESULTS: The conditions, including temperature, solvents, amount of catalyst, dioxygen pressure and time, were optimized. Thus 94% yield of 2,2′‐disbenzothiazole disulfide was obtained at 60 °C in acetonitrile under 0.2 MPa oxygen pressure for 3 h. Theoretical calculations and UV spectra showed that hydrogen‐transfer reaction between 2‐mercaptobenzothiazole and TEMPO was the key step, and 2,2′‐disbenzothiazole disulfide was generated by the coupling of the formed thiyl radical of 2‐mercaptobenzothiazole. CONCLUSION: 2,2′‐disbenzothiazole disulfide was prepared efficiently by aerobic oxidative coupling of 2‐mercaptobenzothiazole with TEMPO as the catalyst. This ‘environmentally friendly’ approach with easy handling, mild reaction conditions and simple separation represents a viable means of producing 2,2′‐disbenzothiazole disulfide. Copyright © 2011 Society of Chemical Industry