Photoconversion of N1‐Aryl‐1,2,3‐Triazoles to Indoles

A 1,2,3‐triazole is a five‐membered aromatic heterocycle that contains three contiguous nitrogen atoms (N1 to N3 as conventionally labeled). Photoconversion of 1,2,3‐triazoles to indoles by eliminating two of the three nitrogen atoms in the form of nitrogen gas was discovered in the 1960s. This reaction has since been sparingly used in synthetic chemistry. In this review article, the applications of this reaction, as well as the accumulated mechanistic information and speculations, are summarized. The reactivity of this transformation depends on the initial phototriggered N1–N2 bond scission to afford a diazonium intermediate and likely also on the subsequent elimination of a molecular nitrogen (hence “denitrogenation”) to afford what can be described as either an iminocarbene or a diradical. Wolff rearrangement or indole formation may follow, and the selectivity for either of two possible regioisomeric indole products depends on whether the path to arrive at a 1H‐azirine intermediate is taken. No rearranged indole is expected if the azirine is skipped on the reaction coordinate; but once there, the property of the azirine determines how it partitions into two regioisomeric indoles. The inadequate body of knowledge on the reactivity and regioselectivity hinders the adoption of triazole photoconversion as a major tool for the synthesis of multisubstituted indoles.