Lithium-stabilized nucleophilic addition of thiamin to a ketone provides an efficient route to mandelylthiamin, a critical pre-decarboxylation intermediate

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

Aromatic π-interactions have been recognized as enhancing enzymatic catalytic processes, providing an efficient route to overcome entropic barriers. A nonenzymic analogue, a complex of protonated pyridine and a phenyl substituent in a thiamin conjugate, facilitates the departure of CO2 by protonation of a vicinal carbanion in a reactive complex. To evaluate the efficiency of the catalytic pathway from the π-associated proton donor, a system was assessed that produced measurable competition through the rates of formation of alternative products resulting from the same thiamin-derived carbanion. The barriers to competing pathways from the decarboxylation of p-(bromomethyl)-mandelylthiamin in the presence and absence of protonated pyridine were determined, establishing the efficiency of the vicinal proton transfer between π-associated species. The formation of the complex of CO2 and the co-formed carbanion also addresses the mechanism of the uncatalyzed exchange of 13CO2 into carboxyl groups discovered by Lundgren. Finally, microscopic reversibility implicates pyridine as a vicinal Brønsted base in thiamin-aldehyde adducts, producing carbanions that could incorporate dissolved CO2 into carboxyl groups.

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

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.

Charge Dispersion and Its Effects on the Reactivity of Thiamin-Derived Breslow Intermediates

The enzymic decarboxylation of 2-ketoacids proceeds via their C2-thiazolium adducts of thiamin diphosphate (ThDP). Loss of CO₂ from these adducts leads to reactive species that are known as Breslow intermediates. The protein-bound adducts of the 2-ketoacids and ThDP are several orders of magnitude more reactive than the synthetic analogues in solution. Studies of enzymes are consistent with formulation of protein-bound Breslow intermediates with localized carbanionic character at the reactive C2α position, reflecting the charge-stabilized transition state that leads to this form. Our study reveals that nonenzymic decarboxylation of the related thiamin adducts proceeds to the alternative charge-dispersed enol form of the Breslow intermediate. These differences suggest that the greatly enhanced rate of decarboxylation of the precursors to Breslow intermediates in enzymes arises from maintenance of the carbanionic character at the position from which the carboxyl group departs, avoiding charge dispersion by stabilizing electrostatic interactions with the protein as formulated by Warshel. Applying Guthrie's "no-barrier" addition to Marcus theory also leads to the conclusion that maintaining the tetrahedral carbanion at C2α of the resulting adduct minimizes associated kinetic barriers by avoiding rehybridization as part of steps to and from the intermediate. Finally, maintenance of the reactive energetic carbanion agrees with the concepts of Albery and Knowles as the outcome of evolved enzymic processes.

The Need for an Alternative to Radicals as the Cause of Fragmentation of a Thiamin-Derived Breslow Intermediate

Mandelylthiamin (1) is a conjugate of benzoylformate and thiamin that loses CO₂ to form the classic Breslow intermediate (2), whose expected fate is formation of the thiamin conjugate of benzaldehyde (3). Surprisingly, it was observed that 2 decomposes to 4 and 5 and rearranges to 6 in competition with the expected protonation to give 3. Recent reports propose that the alternatives to protonation arise from homolysis followed by radical-centered processes. It is now found, instead, that the spectroscopic observations cited in support of the proposed radical pathways are likely to be the result of other events. An alternative explanation is that ionization of the enolic hydroxy group of 2 and resultant electronic reorganization leads to C-C bond cleavage and non-radical intermediates that readily form 4, 5, and 6.