Morpholino Demethoxylation on Pyrylium and Pyridinium Rings: Rate Parameters and Base Catalysis

Article abstract of DOI:10.1021/ja00410a028

The kinetics of morpholino demethoxylation of 4-methoxy-1-methyl-2,6-diphenylpyridinium (1) and 4-methoxy-2,6-diphenylpyrylium (2) ions were studied in methanol, in the presence of morpholine/morpholinium buffers, at 25 deg C.Both reactions display general base catalysis and can be assumed to proceed through tetrahedral intermediates.By steady-state treatment the rate constant for the adduct formation and the several rate ratios in the base catalysis scheme were evaluated.Attack of the nucleophile to pyrylium occurs 3*10⁶ times as fast as that to pyridinium.The detailed mechanism of base catalysis seems to differ in the two reactions.The reaction of cation 1 appears to involve a rate-limiting proton transfer, as generally observed in nucleophilic aromatic substitutions of activated benzene derivatives with amines in protic solvents.In contrast, the more reactive cation 2 seems to prefer the less common specific base-general acid mechanism.