Competing Protonation and Halide Elimination as a Probe of the Character of Thiamin-Derived Reactive Intermediates

Abstract

Decarboxylation reactions from comparable thiamin diphosphate- and thiamin-derived adducts of p-(halomethyl)­benzoylformic acids in enzymic and non-enzymic reactions, respectively, reveal critical distinctions in otherwise similar Breslow intermediates. The ratio of protonation to chloride elimination from the Breslow intermediate is 102-fold greater in the enzymic process. This is consistent with a lower intrinsic barrier to proton transfer on the enzyme, implicating formation of a localized tetrahedral (sp3) carbanion that is formed as CO2 is produced. In contrast, slower protonation in solution of the decarboxylated intermediate is consistent with formation of a delocalized planar carbanionic enol/enamine. The proposed structural and reactive character of the enzymic Breslow intermediate is consistent with Warshel’s general theory of enzymic catalysis, structural characterization of related intermediates, and the lower kinetic barrier in reactions that occur without changes in hybridization.

Proton Transfer via π-Interactions from Pyridine Provides a Facilitated Route for Transfer of CO2 in Its Complex with a Carbanion

“…That step is followed independently by rapid elimination of the bromide ion. These results require that reversal through addition of the carbanion to CO 2 has a lower barrier than the unimolecular, irreversible rapid elimination of bromide from BMMTh (with a rate constant of about 10 8 s –1 ) …”

“…The formation of an initial complex of a residual carbanion and CO 2 from BMMTh leads to rapid release of the bromide ion from the remote bromomethyl group (rate constant >10 8 s −1 = k 3 in Scheme 3). 11 The mechanism is an extension of the E1 CB mechanism, with an electron source at the carbanionic center created upon formation of CO 2 , leading to an extended conjugated system (4 in Scheme 3), whose formation requires elimination of the bromide ion from the carbanionic form of the intermediate. 11 Catalysis by Pyridine and Insights from Isotopic CO 2 Exchange.…”

“…We noted that the Oka process does not occur where bromide elimination is possible, indicating that bromide elimination has a considerably lower free energy barrier than fragmentation. For comparison, elimination of the bromide ion is still much slower than the internal return of CO 2 from the same carbanion or protonation of the initial carbanion by π-associated pyridine.…”

“…The ester and TBDPS group were cleaved with concentrated hydrobromic acid to produce p-(bromomethyl)mandelylthiamin (BMMTh). 11 The decarboxylation of BMMTh was measured by changes in the intensity of the UV absorbance at 290 nm in buffer solutions at 30 °C with a computer-interfaced UV−vis spectrometer. Data for the decrease in absorbance at 290 nm correlate with the concentration of the carboxyl group of the remaining reactant.…”

“…Our examination of the mechanism of biomimetic decarboxylation of adducts of thiamin and 2-ketoacids has provided accurate comparisons of their intrinsic reactivity with those of analogous enzymic intermediates. ,− To observe the reactivity patterns of the carbanion that results from loss of CO 2 , we examined the effects of protonated pyridine on the decarboxylation of p -(bromomethyl)-mandelylthiamin (BMMTh), a compound that provides the possibility of bromide ion elimination from the proposed complex in addition to the typical products of decarboxylation. In that case, the carbanion formed upon loss of CO 2 undergoes rapid elimination of a bromide ion, a process that competes directly with protonation of the carbanion . This implies that the specific association of the proton donor prior to the formation of the carbanion enhances the rate of protonation of the carbanion relative to the reversion reaction of the carbanion with CO 2 in the same complex.…”