Annulation‐Induced Hidden Reactivity of the 1,2,4‐Triazole Backbone

Triazoles are an important class of compounds with widespread applications. Functionalization of the triazole backbone is thus of significant interest. In comparison to 1,2,3‐triazoles, C−H activation‐functionalization of the congeners 1,2,4‐triazoles is surprisingly underdeveloped. Indeed, no such C−H activation‐functionalization has been reported for 4‐substituted 1,2,4‐triazole cores. Furthermore, although denitrogenative ring‐opening of 1,2,3‐triazoles is well‐explored, 1,2,4‐triazole/triazolium substrates have not been known to exhibit N−N bond‐cleaving ring‐opening reactivity so far. In this work, we unveiled an unusual hidden reactivity of the 1,2,4‐triazole backbone involving the elusive N−N bond‐cleaving ring‐opening reaction. This new reactivity was induced by a Satoh‐Miura‐type C−H activation‐annulation at the 1,2,4‐triazole motif appended with a pyridine directing group. This unique reaction allowed ready access to a novel class of unsymmetrically substituted 2,2′‐dipyridylamines, with one pyridine ring fully‐substituted with alkyl groups. The unsymmetrical 2,2′‐dipyridylamines were utilized to access unsymmetrical boron‐aza‐dipyridylmethene fluorescent dyes. Empowered with desirable optical/physical properties such as large Stokes shifts and suitable hydrophobicity arising from optimal alkyl chain length at the fully‐substituted pyridine‐ring, these dyes were used for intracellular lipid droplet‐selective imaging studies, which provided useful information toward designing suitable lipid droplet‐selective imaging probes for biomedical applications.