Oxygen-18 isotope effect in carbon-13 nuclear magnetic resonance spectroscopy. 8. Oxygen exchange of 2,4,6-trimethylpyrylium cation

The rate of the oxygen-exchange reaction in an aqueous solution of a heterocyclic oxygen compound, 2,4,6-trimethylpyrylium perchlorate, was quantitated. The /sup 18/O isotope effect in /sup 13/C NMR spectroscopy affords a direct, continuous, and relatively simple analysis of the reaction and was used to obtain data on the incorporation of /sup 18/O from solvent H/sub 2//sup 18/O into the pyrylium salt. Due to the low solubility of this salt in aqueous solutions, 2,4,6-trimethyl(2-/sup 13/C)pyrylium perchlorate and 2,4,6-trimethyl(2,6-/sup 13/C/sub 2/)pyrylium perchlorate were synthesized to facilitate the acquisition of the data on an NTC-200 spectrometer operating at 50.31 MHz. Upon incorporation of /sup 18/O into the pyrylium salt, the /sup 13/C NMR signal of the directly bonded carbon atom is shifted upfield 0.038 ppm. A /sup 13/C isotope effect is also detected in the doubly /sup 13/C-enriched pyrylium salt where the /sup 13/C NMR signal for /sup 13/C-O-/sup 13/C appears 0.011 ppm downfield from the /sup 13/C NMR signal for /sup 12/C-O-/sup 13/C. The rate of the oxygen-exchange reaction was studied as a function of pH (2.0-4.8), buffer concentration, added electrolyte, and temperature. A positive salt effect is observed. The oxygen-exchange reaction is subject to specific hydroxide ion catalysis and to generalmore » base catalysis by buffers such as acetate. The /sup 18/O-exchange reaction is compared to the other reactions of the pyrylium salt including hydrolysis and /sup 2/H exchange. The apparent energy of activation for the oxygen-exchange reaction at pH 3.1 in the absence of buffer and electrolyte is approximately 25 kcal mol/sup -1/ while at pH 4.7 in 50 mM sodium acetate, ..mu.. = 0.2 M with NaCl, it is 10 kcal mol/sup -1/. 36 references, 2 figures, 1 table.« less