The Research On Mechanism Of Oxidative Coupling Of2-Mercaptobenzothiazole Catalyzed By TEMPO

2,2,6,6-tetramethyl-piperidine-N-oxyl,(TEMPO), as one of organocatalysts, was widly used in the catalytic oxidation of alcohols. The mechanism of oxidation catalyzed by TEMPO was based on the generation of oxoammonium salt rather than free redical process. The oxidation reduction cycle of TEMPO and TEMPOH was rarely used in the reactions.2.2’-Dibenzothiazolyl Disulfide (DM) was prepared by the coupling of free radicals of2-mercaptobenzothiazole (M). Although there was a lot of oxidation methods, the waste water were genetated to pollute the environment. The by-product of areobic oxidation of M was only water, which confirm the idea of green chemistry. However, because of the inertness of oxygen the oxidation of M by oxygen could not be occurred directly, and the catalyst was needed. TEMPO was introduced in the oxidation of M to produce DM, which result in the high yield and quality of DM.In this paper, the mechanism of oxidation of M to DM catalyzed by TEMPO is further studied, and major progresses are as follows:1. The NMR diagram of key step--the reaction of M and TEMPO indicated that the addition of TEMPO effect the chemical shift of active hydrogen on M. After mixing TEMPO and M, the hydrogen bond was generated between two of them. The UV spectrum could also comfirm it.2. The redical of M was initiated by Pb(OAc)4, but the absorption peak of M redical could not be seen in the UV spectrum. The absorption peak of M was rapidly declined as soon as introducing Pb(OAc)4in the solution, and the absorption peak of DM was rapidly rised, which illustrated the reactivity of M redical and the easiness of the redical reaction of M to DM.3. The stoichiometric calculation on the monomolecule and dimolecular tautomer of M explained that dimolecular thione M was more stable. The tautomer of M could be transformed at certain temperature via intermolecular hydrogen exchange calculated by the stoichiometric calculation. 4. The stoichiometric calculation was carried on the reaction of M and TEMPO to generate redical of M and TEMPOH. The reaction of thiol M and TEMPO could occurr at room temperature, while the reaction of thione M and TEMPO needed high temperature. So we could get the result that the thiol M was generated by intermolecular hydrogen exchange, and then reacted with TEMPO to produce M redical and TEMPOH.In summary, the NMR diagram, UV spectrum and stoichiometric calculation confirmed the mechanism of oxidation of M to DM catalyzed by TEMPO was firstly the reaction of TEMPO and M to generate redical M and TEMPOH. The redical of M immediately coupled to produce DM, and TEMPOH also rapidly oxidated by oxygen to rebuild TEMPO.