N-Substituted pyridinium salts constitute one of the most valuable reagent classes in organic synthesis, due to their versatility and ease of use. Herein we report a preliminary synthesis and detailed structural analysis of several N-(1-ethoxyvinyl)pyridinium triflates, an unusual class of pyridinium salts with potentially broad use as a reagent in organic synthesis. Treatment of pyridines with trifluoromethane sulfonic acid and ethoxyacetylene generates stable, isolable adducts which have been extensively characterized, due to their novelty. Three-dimensional structural stability is perpetuated by an array of C-H•••O hydrogen bonds involving oxygen atoms from the -SO₃ groups of the triflate anion, and hydrogen atoms from the aromatic ring and vinyl group of the pyridinium cation. Predictions from density functional theory calculations of the energy landscape for rotation about the exocyclic C-N bond of 2-chloro-1-(1-ethoxyvinyl)pyridine-1-ium trifluoromethanesulfonate (7) and 1-(1-ethoxyvinyl)pyridine-1-ium trifluoromethanesulfonate (16) are also reported. Notably, the predicted global energy minimum of 7 was nearly identical to that found within the crystal structure.
Keywords: N,O-ketene acetal; N-(1-alkoxyvinyl); X-ray diffractions; crystal structure; ethoxyacetylene; hydrogen bonding; pyridinium salt; quaternary ammonium compound; triflic acid.