Breslow Intermediates from Aromatic N‐Heterocyclic Carbenes (Benzimidazolin‐2‐ylidenes, Thiazolin‐2‐ylidenes)

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.