Thianthrene cation radical induced reactions of semicarbazones and acylhydrazones



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Texas Tech University


Single-electron transfer (SET) reactions of cation radicals have been widely studied." Recently, cation radical induced oxidative intramolecular cyclization reactions and oxidative cycloaddition reactions involving nitrile solvents have been discovered. It was found that cation radicals are capable of causing the oxidative cyclization of arylhydrazones of chalcones and benzalacetones to pyrazoles. In acetonitrile solvent. On the other hand, reactions of thianthrene cation radical with arylhydrazones and oximes of non-conjugated aldehydes in nitrile solvents can cause oxidative cycloaddition to the nitrile group with the formation of 1,2,4-triazoles.

Shin investigated the reactions of aldehyde semicarbazones with thianthrene cation radical perchlorate in acetonitrile solvent. These reactions caused oxidative intramolecular cyclization, producing 1,3,4-oxadiazoles as the major product. During this investigation some interesting observations were made. Reactions were generally slow and the yield of the product oxadiazole was moderate. However, addition of a small amount of water to the reaction mixture dramatically accelerated the reaction and increased the yield of the oxadiazole.

Therefore, it was of interest to explore the mechanistic aspects of these reactions further and to probe the possible role of added water in enhancing the reaction and product yield.



Hydrazones, Ring formation, Radicals, Cations