Novel Class of 1,2-Diselenazoles 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-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, with several derivatives exhibiting superior efficacy compared to the commercial fungicide Tebuconazole at reduced concentrations (15 and 3 mg/L). Notably, certain diselenazoles showed exceptional activity against Venturia inaequalis and Rhizoctonia solani, surpassing Tebuconazoles’ 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 for wheat, barley and cress.