Vinyl ether hydrolysis. XVIII. The two-stage reaction of 2,3-dimethoxy-1,3-butadiene

Abstract

A. JERRY KRESGE and YA YIN. Can. J. Chem. 65, 1753Chem. 65, (1987.The kinetics of acid-catalyzed hydrolysis of 2,3-dimethoxy-l,3-butadiene (1) to 3-methoxy-3-buten-2-one (2) and the subsequent lo4 times slower conversion of the latter to biacetyl (3):O M e O M e 0 were studied in aqueous solution at 25OC. Both stages of this process give substantial hydrogen ion isotope effects, kH+/kD+ = 2.95 for Stage 1 and kH+/k,+ = 1.83 for Stage 11, and Stage I shows general acid catalysis in formic and acetic acid buffers; both stages are therefore assigned the conventional mechanism for vinyl ether hydrolysis involving rate-determining proton transfer from catalyzing acid to substrate. The second vinyl ether group of the initial substrate (1) is found to have only a slight (3-fold)accelerative effect on the reactivity of the first group, but the acetyl substituent present in the intermediate 2 decreases its reactivity by a factor of lo4; the latter appears to be due largely to the electron-withdrawing inductive effect of acetyl, with little or no influence from a countervailing electron-supplying resonance effect.A. JERRY KRESGE et YA YIN. Can. J. Chem. 65, 1753Chem. 65, (1987.Optrant a 25°C et dans des solutions aqueuses, on a dCtexmint la cinktique de l'hydrolyse acido-catalyske du dimtthoxy-2,3 butadikne-1,3 (I) en mtthoxy-3 buthe-3 one-2 (2) et de la transformation subsCquente de ce dernier, qui est lo4 fois plus lente que la rCaction prtckdente, en biacCtyle (3). Pour chacune des ttapes, on observe des effets isotopiques cinitiques de l'ion O M e be expected to occur in separate stages proceeding at rather different rates. This follows from the fact that the conventional mechanism for vinyl ether hydrolysis involves rate-determining proton transfer from a catalyzing acid to the substrate in a step that generates a carbocation intermediate (I), and a-acyl sub-We wish to report that we have examined the kinetics of hydrolysis of 2,3-dimethoxy-1,3-butadiene and have found this to be the case. Our results provide a quantitative estimate of this a-acetyl effect on carbocation stability, a subject of some current interest (2), and also give information on the effect of one vinyl ether group in 2,3-dimethoxy-1,3-butadiene on the reactivity of the other. Results Reaction identijication2,3-Dimethoxy-l,3-butadiene has a strong uv absorption band For personal use only.

Electrophilic Additions toanti-Sesquinorbornene. Br2in Methanol and the Addition of Acetic Acid

“…The primary DKIE for a proton in flight between oxygens, or oxygen and carbon, is based on a transition state fractionation factor of ∼0.4 (with values of 0.3−0.5 used as limits). Such a value for an O ..... L ..... C primary DKIE is reasonable considering that proton transfer from H 3 O + is the rate-limiting step for such reactions as acid-catalyzed alkene hydration and vinyl ether hydrolysis, these processes exhibiting DKIE values ranging from 2−6.

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Mechanistic Investigations of Unsaturated Glucuronyl Hydrolase from Clostridium perfringens

“…Beyond suggesting that proton transfer events are involved in both of the kinetically important steps, few conclusions can be drawn from this. Notably, solvent KIEs for the few cases of hydrolysis of vinyl ether acetals that have been determined to proceed by initial hydrolysis of the acetal moiety are around 0.37 to 0.55 (26,31,32), but values for hydration of vinyl ethers range from 1.56 to 7.1 (26,(31)(32)(33)(34). The values determined for UGL clearly fall within the latter range rather than the former, and so are more consistent with a hydration mechanism than a more traditional glycoside hydrolase mechanism, which would act at the acetal first.…”