Oxidative dimerization -1,2,4-thiadiazole

Oxidative cyclization of dithiobiuret under basic conditions provides bis(5-amino-l,2,4-thiadiazolyl)-3,3 -disulfide 92 via oxidative dimerization of the intermediate 5-amino-3-mercapto-l,2,4-thiadiazole 91. However, alkylation of disulfide 91 under basic conditions gives the thioalkyl-l,2,4-thiadiazole 93 (Scheme 9) <2003H(60)1401>. 

Cheng, D., and Chen, Z. (2002). Hypervalent iodine in synthesis. 84. Facile synthesis of 3,5-disubstituted 1,2,4-thiadiazoles by oxidative dimerization of thioamides using polymer-supported iodobenzene diacetate. Synth. Commun., 32, 2155-2159. 

Another convenient strategy for the synthesis of 3,5-diaryl-1,2,4-thiadiazoles is the oxidative dimerization of arylthioamides by using 2,4,6-trichloro-l,3,5-triazine and dimethylsulfoxide in polyethylene glycol 400 (PEG-400) as solvent at ambient temperature. This methodology can be applied to various fluoroarylated systems (ArF=mono-, poly-, or perfluorophenyl). The reaction give rise to 4-fluoro substituted derivatives 147 during 8 min in yields of 96 % (Schane 38) [69], The same reaction has been recently reported by using l-butyl-3-methylimidazolium tetrafluoroborate as eco-friendly reaction medium at room temperature [70],... 

Scheme 38 Synthesis of 3,5-diaryl-1,2,4-thiadiazoles 147 by oxidative dimerization of arylthioamides...

Khosropour AR, Noei J (2010) A convenient strategy for the synthesis of 3,5-diaryl-l,2,4-thiadiazoles oxidative dimerization of arylthioamides using CC-DMSO in PEG-400. MonatshChem 141 649-651... 

Azirines are dimerized under various conditions to dihydropyrazines or their dehydrogenated products, namely pyrazines (Section 8.03.9.5). Quinoxalines are oxidized with potassium permanganate to afford 2,3-pyrazinedicarboxylic acids, and pteridines are hydrolyzed to give 3-amino-2-pyrazinecarboxylic acid derivatives. Condensation of 3,4-diamino-l,2,5-thiadiazole 176 with a-diketones produces l,2,5-thiadiazolo[3,4- Jpyrazines 177, which are reduced to provide 2,3-diaminopyrazines 178 (Scheme 51) <1997JCM250>. 

Several methods are known to form 1,2,4-thiadiazoles via dimerization of thioureas. A novel method has been reported this year that involves oxidative azacyclization of 1-monosubstituted thioureas. In this reaction, thiourea 286 reacts with [bis(acyloxy)iodo]-arenes (BIA) to form 3,5-bis-phenylamino-l,2,4-thiadiazole 287. A reaction mechanism is proposed wherein the polyvalent iodo intermediate 288 undergoes elimination of iodobenzene to form 289. BIA-initiated oxidative azacyclization of 289 yields 1,2,4-thiadiazole 287 <03T7521>. 

Dithiins and It4-Di elenins.—1,4-Dithiins (36) are among the photolysis products of biaryl- and benzo-l,2,3-thiadiazoles. The postulated intermediate biradicals (35) dimerize in the direction shown. Thermolysis of cycloalkeno-l,2,3-selenadiazoles produces 1,4-diselenins (37) together with cycloalkynes. A structure determination of 2,5-diphenyl-l,4-dithiin 1-oxide reveals a boat-shaped configuration (Figure) with the sulphur... 

The mechanism of the oxidation of 2-mercapto-5-methyl-l,3,4-thiadiazole (McMT) to its disulfide dimer and its subsequent reduction was examined with a combined approach employing experimental data and digital simulation. To elucidate the influence of proton transfers on these redox processes, special attention was paid to the influence of various bases and proton donors on both the oxidation and reduction reactions. In particular, McMT oxidation is facilitated by a rapid bimolecular proton transfer from McMT to weak bases such as pyridine that produces McMT , the thiol-ate form, which is then oxidized. There is no such facilitation in the presence of the sterically hindered base 2,6-di-r-butylpyridine, suggesting that the facilitation occurs through the formation of a discrete hydrogen-bonded complex/ ... 

---