Proton Nuclear Magnetic Resonance Study of the Addition of Methoxide Ion to 2,4,6-Triphenylpyrylium, 2,4,6-Triphenylthiopyrylium, 1,2,4,6-Tetraphenylpyridinium, and 1-(p-Nitrophenyl)-2,4,6-triphenylpyridinium Cations

Article abstract of DOI:10.1021/jo01313a027

The reactions of 2,4,6-triphenylpyrylium (1a) and 2,4,6-triphenylthiopyrylium (1b) with methoxide ion are shifted toward the formation of adducts, as shown by 1H NMR.The former yields practically only a 2H-pyran adduct in MeCN, Me2SO, or MeOH.In MeOH, 1b yields an adduct with a 2H-thiopyran structure, whereas in MeCN this adduct is obtained together with the isomeric 4H-thiopyran adduct.In Me2SO or MeCN, the 1,2,4,6-tetraphenylpyridinium cation yields a 1,2-dihydropyridine.In contrast, in MeOH this equilibrium is shifted toward the reagents.The lower degree of delocalization of the positive charge in the pyridinium cation seems to be the main cause of its lower tendency to undergo nucleophilic addition.