Abstract
The oxidative condensation of aldehydes catalyzed by thiazolium salts is of interest in relation to the in vivo reaction from pyruvate to acetyl CoA. In order to have information to understand mechanism of the natural reaction and to apply it to the artificial reaction system, we carried out the mechanistic studies of its model reaction systems. The reaction of p-substituted benzaldehyde with 4,4'-disubstituted azobenzene in the presence of octanethiol, 3-butyl-4-methylthiazolium bromide and triethylamine, which were employed as a nucleophile, catalyst, and base, respectively, in dichloromethane gave the thiol ester together with the hydrazide formed from the corresponding benzoic acid and hydrazobenzene as by-product. And it was found that the product distribution was dependent on both substituents of benzaldehyde and azobenzene. Substituent effects on pseudo-first-order rate constants for the reaction from benzaldehydes in deuterium chloroform were investigated by 1H-NMR. The Hammett-plot of the observed rate constants against substituent constants (σ) of benzaldehydes was curved, indicating the presence of plural rate-determining steps. Moreover, the study of the substituent effect of azobenzene derivatives endorsed that the rate-determining step was shifted with the electrophilicities of p-substitueted benzaldehydes.
