Kinetics of the thiazolium ion-catalyzed benzoin condensation

The formation of benzoin (Ph-CHOH-CO-Ph) from two molecules of benzaldehyde, catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide in methanol buffered with Et3N/Et3NH+-Cl- has been studied. Initial-rate studies at various concentrations of PhCHO (0.1-1.7 M) showed that the reaction is close to being first order in PhCHO. Following the reaction in deuteriomethanol, H-1 NMR spectroscopy allowed rate constants for all three kinetically significant steps to be determined. These show that all three steps are partially rate-determining. A normal deuterium kinetic isotope effect for the overall reaction (k(H)/k(D) approximate to 3.4) is observed using PhCDO, and a large inverse solvent isotope effect (k(D)/k(H) approximate to 5.9) is observed using deuteriomethanol, consistent with the kinetic scheme presented here.