Strong Zn<sup>2+</sup> and Co<sup>2+</sup> catalysis of the methanolysis of acetyl imidazole and acetyl pyrazole

Abstract

The kinetics of methanolysis of acetyl imidazole (1) and acetyl pyrazole (2) have been investigated under anhydrous conditions in the presence of Zn(ClO4)2, Co(ClO4)2, and HClO4 at 25°C. In all cases, the plots of the pseudo-first-order rate constant for methanolysis (kobs) vs. [metal ion] or [HClO4] show saturation behavior indicative of equilibrium binding of the M2+ or H+ to the amide. Relative to the spontaneous methanolysis rate constant (ko), the catalytic rate constant obtained at saturation, kcat, is larger for metal-ion catalysis than for H + catalysis. The (kcat /ko) ratio is 10.7 and 1.25 for 1 and 2, respectively, while the (kcat /ko) for these divalent metals varies from 150-fold for 1 to between 700 and 5700-fold for 2. By contrast, in water, proton is far more effective at promoting the hydrolysis of 1 than are metals, the aqueous (kcat /ko) ratio being 560, while the (kcat Zn2+ /ko) and (kcat /ko) ratios are 15 and 3.2, respectively.