Internal return of carbon dioxide in decarboxylation: catalysis of separation and 12C/13C kinetic isotope effects.
It has been proposed that the decarboxylation of mandelylthiamin, the adduct of benzoylformate and thiamin, is uniquely catalyzed by protonated pyridines through a preassociation mechanism in which proton transfer competes with the internal return of carbon dioxide. Application of this mechanism suggests that the observed primary (12)C/(13)C kinetic isotope effect in the absence of catalyst is reduced in magnitude because diffusion of carbon dioxide is partially rate-determining. Where proton transfer blocks the internal return of carbon dioxide, the separation of carbon dioxide is facilitated, and the observed isotope effect increases toward the intrinsic value for carbon-carbon bond breaking. Headspace analysis of carbon dioxide formed over the course of the reaction shows that protonated pyridine increases the magnitude of the observed (12)C/(13)C KIE, consistent with the proposed mechanism.
Mundle
Scott O C
SO
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
Rathgeber
Steven
S
Lacrampe-Couloume
Georges
G
Sherwood Lollar
Barbara
B
Kluger
Ronald
R