13C Kinetic Isotope Effects and the Mechanism of the Uncatalyzed Decarboxylation of Orotic Acid

A complete set of 13C kinetic isotope effects were determined for the thermal decarboxylation of 1,3-dimethylorotic acid and compared with theoretically predicted isotope effects for decarboxylation via either O-2 or O-4 protonated pathways. The best correspondence of experimental and calculated isotope effects is found for the O-4 protonated mechanism. This observation and the calculated reaction barriers support the previously predicted preference for this pathway. The preference for the O-4 protonated mechanism is found to result from a general predilection for O-4 protonation over O-2 protonation in the orotate/uracil series, and no significant extra stability appears associated with the formation of a formal carbene in the O-4 protonated decarboxylation. The carboxylate isotope effect for the uncatalyzed reaction is much smaller than the enzyme-catalyzed isotope effect recently reported, suggesting some divergence between uncatalyzed and enzyme-catalyzed mechanisms.