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

Triazoles are an important class of compounds with widespread applications and functionalization of triazole backbone is of significant interest. In comparison to 1,2,3‐triazoles, C–H activation‐functionalization of the congeners 1,2,4‐triazoles is surprisingly underdeveloped. In fact, no such C–H activation‐functionalization is ever reported for 4‐substituted 1,2,4‐triazole core. Furthermore, although denitrogenative (release of N2 via N–N bond cleavage) ring‐opening of 1,2,3‐triazoles is well‐explored, 1,2,4‐triazole/triazolium substrates have not been known so far for exhibiting N–N bond‐cleaving ring‐opening reactivity. In this work, we unveiled an unusual hidden reactivity of 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 of 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 e.g., large Stokes shift and suitable hydrophobicity arising from optimal alkyl chain length at the fully‐substituted pyridine‐ring, new unsymmetrical boron‐aza‐dipyridylmethene dyes were applied for intracellular selective lipid‐droplet‐imaging studies, which provided useful information toward designing suitable lipid‐droplet‐imaging probes for biomedical applications.