Photoinduced Molecular Rearrangements. The Photochemistry of Some 1,2,4-Oxadiazoles in the Presence of Nitrogen Nucleophiles. Formation of 1,2,4-Triazoles, Indazoles, and Benzimidazoles

Abstract

The photochemistry of some 3,5-disubstituted 1,2,4-oxadiazoles in the presence of nitrogen nucleophiles [external, such as added amines or hydrazines, or internal, such as an o-aminophenyl moiety at C(3) of the oxadiazole ring] has been investigated. In the irradiation of 5-amino-(or 5-N-substituted amino) 3-phenyl-1,2,4-oxadiazoles in the presence of aliphatic primary amines (or ammonia), photolytic species arising from heterolytic cleavage of the ring O−N bond capture the nucleophilic reagent to give open-chain intermediates, which develop into 1,2,4-triazolin-5-ones. Similarly, irradiations of 3,5-diphenyl-, 3-methoxy-5-phenyl-, and 5-methyl-3-phenyl-1,2,4-oxadiazoles gave 1,2,4-triazoles. In the same context, irradiations of representative substrates in the presence of hydrazines have been also investigated. In the irradiation of 3-(o-aminophenyl)-5-methyl-, 3-[o-(methylamino)phenyl]-5-methyl-, and 3-(o-aminophenyl)-5-phenyl-1,2,4-oxadiazoles, concomitant formation of indazoles and benzimidazoles, presumably arising from a common photolytic species, has been observed. Some mechanistic aspects have been considered, and possible applications in synthesis have been pointed out.

Five‐Membered Heterocycles with Three Heteroatoms: Triazoles

“…1,3,-Dipolar cyclocondensation of C-acetyl-N-arylnitrilimines 167 with benzoylhydrazones 168 furnished 1,2,4-triazoles 169 (Scheme 12.55) [221] 12.3.4.3 Reactions of Oxadiazoles or Thiadiazoles 1,2,4-Triazoles can be synthesized from oxadiazoles. Photolysis of 1,2,4-oxadiazoles in the presence of nucleophiles led to 1,2,4-triazole products [224] and 1,3,4-oxadiazoles can undergo ring-cleavage with nitrogen nucleophiles followed by recyclization of the intermediates to give 1,2,4-triazoles. For example, the unusual hydrazinolysis of 5-perfluoroalkyl-1,2,4-oxadiazoles 174 provided an expedient route to 5-perfluoroalkyl-1,2,4-triazoles 175 (Scheme 12.57) [225].…”

Five‐Membered Heterocycles: 1,2‐Azoles. Part 1. Pyrazoles

“…The photochemistry of some 3,5-disubstituted-1,2,4-oxadiazoles 355 bearing a nitrogen nucleophilic group, such as an ortho-aminophenyl moiety, at C3 of the oxadiazole ring leads to the concomitant formation of indazoles 357 and benzimidazoles 359 (Scheme 8.110) [307]. The photochemistry of 355 is characterized by 1H-Indazoles 357 can also be obtained from 3,5-disubstituted 1,2,4-oxadiazoles 355 in almost quantitative yield, by heating these compounds, without solvent, at a temperature much higher than their melting points [308].…”

Synthesis of Heteroaromatics via Rearrangement Reactions

“…In this reaction, a photolytic cleavage of the ring OÀN bond, followed by migration of the aryl substituent, leads to the carbodiimide intermediate 69, a precursor of the benzimidazole nucleus (Scheme 12.20) [47].…”