5.6- Diphenyl-1,2,4-triazine 4-oxide formation

Diphenyl-1,2,4-triazine 4-oxide 139 was obtained by reaction of diphenyl-glyoxal mono-2-ethoxymethylenehydrazone 140 with hydroxylamine. The reaction proceeds via formation of an isonitroso intermediate, followed by cyclization to the 1,2,4-triazine 4-oxide 139 (71LA12). 

When the dihydro-1,2,4-triazinone (194) is heated at 180 °C under reducing conditions there is rupture of the 1,2-bond followed by ring closure, producing 4,5-diphenyl-imidazolinone (195). Such reductive ring closures of benzotriazine 1-oxides (196) can give benzimidazoles, in particular 2-(4-thiazolyl)benzimidazoles (197) ( thiabendazole ). Similar reactions take place with 1,3,5-triazines. In fact, primary amines cleave the compounds completely with evolution of ammonia and formation of iV.iV -disubstituted formamidines. With suitable primary amines, though, this reaction can be designed to produce imidazolines or benzimidazoles (Scheme 113). 

The electrophilic character of 1,2,4-triazines allows their introduction into different other systems. Thus, calix[4]arenes 12 were modified at the meso-carbon atom with a 1,2,4-triazine ring by nucleophilic addition of a calrx-arene lithium derivative to 3,6-diphenyl-l,2,4-triazine 13 with formation of o-adduct 14 which was then oxidized with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) giving an aromatic product 15 (13MHC308). 

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