A novel class of 1,2,4-diselenazoles obtained via cationic polar cycloaddition between ambiphilic 2-pyridylselenyl reagents and isoselenocyanates exhibiting high antifungal activity

We report the synthesis and structural characterization of a novel class of pyridinium-fused 1,2,4-diselenazoles, obtained via a regioselective cationic polar cycloaddition between ambiphilic 2-pyridylselenyl reagents and isoselenocyanates. All new compounds were isolated in high yields and fully characterized by NMR, HRMS, and single-crystal X-ray diffraction. Density functional theory calculations reveal a concerted cycloaddition mechanism with a low activation barrier and no detectable intermediates. Molecular electrostatic potential analysis of representative diselenazolium cations highlights the presence of two prominent σ-holes per molecule, favoring strong directional chalcogen bonding with halide anions. QTAIM/NCIplot and NBO analyses confirm the noncovalent and orbital-driven nature of the Cl⋯Se–Se interactions, supported by significant second-order stabilization energies and short Se⋯Cl contacts. The antifungal activity of novel diselenazoles was evaluated against six phytopathogenic fungi. Most compounds demonstrated strong inhibition at 30 mg L−1, with several derivatives exhibiting superior efficacy compared to the commercial fungicide tebuconazole at reduced concentrations (15 and 3 mg L−1). Notably, certain diselenazoles showed exceptional activity against Venturia inaequalis and Rhizoctonia solani, surpassing tebuconazole's performance. The best-in-class compounds were evaluated for their effects on the seeds of higher plants. Root growth was found to be stimulated at the lowest exposure concentration of 3 mg L−1 for wheat, barley and cress. This journal is © the Partner Organisations, 2025