Pyridinium-Fused 1,3-Selenazoles via Cyclizations of 2-Pyridylselenyl Chloride with Alkynes: Synthesis, Structures, and Antifungal Properties

We report a straightforward and versatile synthetic route to pyridinium-fused 1,3-selenazoles via the electrophilic cyclization of 2-pyridylselenyl chloride with alkynes. The reaction proceeds efficiently under mild conditions with representative terminal and internal alkynes. While the cyclization exhibits high regioselectivity favoring the 3-substituted isomer for most substrates, reactions with 2-pyridyl- and 2-quinolylacetylenes yield regioisomeric mixtures. DFT calculations rationalize this divergence, revealing a competition between kinetic and thermodynamic control; the 3-isomer is kinetically favored, while the 2-isomer is thermodynamically stabilized by an ancillary chalcogen bond between the selenium atom and the pyridine nitrogen of the alkyne substituent. Molecular structures were confirmed by single-crystal X-ray diffraction, and the non-covalent interactions governing supramolecular assembly in the solid state were rigorously analyzed using MEP surfaces, the QTAIM, and NBO analysis. Antifungal evaluation identified several compounds with notable activity against phytopathogenic fungi, highlighting the potential of this novel heterocyclic scaffold in agrochemical applications.