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

Abstract

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.