Kinetics and mechanism for CO(2) scrambling in a N-carboxyimidazolidone analogue for N(1)-carboxybiotin.
The N-carboxyimidazolidone anion, 2(-), was prepared as an analogue for N(1)-carboxybiotin, and the kinetics of its CO(2)-dependent chemistry were studied in polar aprotic media. The objective was to assess the viability of unimolecular CO(2) elimination from N(1)-carboxybiotin as a microscopic step in biotin-dependent carboxyl transfer enzymes. The anionic 2(-) was prepared as its lithium salt by first deprotonating 2-imidazolidone with phenyllithium, followed by direct reaction with carbon dioxide. This procedure also permitted isolation of the (13)C enriched derivative 2(-)[(13)C] by reaction with (13)CO(2). Proton and (13)C NMR and isotope-dependent FTIR measurements confirmed that carboxylation had occurred at the nitrogen atom of 2-imidazolidone to give 2(-). Time-dependent FTIR spectroscopy showed that Li2 undergoes carboxyl exchange with free carbon dioxide, with kinetics indicative of rate-limiting unimolecular dissociation of the N-CO(2) bond. Under these conditions, the weak Lewis acid Mg(2+) catalyses the exchange of 2(-) with free CO(2), which appears to be related to the ability of the metal ion to coordinate to 2(-). Reaction of Li2 with carboxylic acids in DMSO results in acid-dependent decarboxylation of 2(-) with a rate that is dependent on the concentration of the acid and its pK(a). A common mechanistic framework is presented for both Lewis acid catalyzed carboxyl exchange and acid-dependent decarboxylation that involves initial interaction at the carbonyl oxygen and which has the effect of polarizing the nitrogen lone pair toward the imidazolidone ring rather than the carboxyl group. Lewis acid interaction with the carbonyl oxygen thus weakens the N-CO(2)(-) bond and functions as a trigger for dissociation of CO(2). In the context of biotin-dependent enzymes, this suggests a means by which the kinetically stable N(1)-carboxybiotin cofactor intermediate might be triggered for dissociation of CO(2).[1]References
- Kinetics and mechanism for CO(2) scrambling in a N-carboxyimidazolidone analogue for N(1)-carboxybiotin. Lihs, F.J., Caudle, M.T. J. Am. Chem. Soc. (2002) [Pubmed]
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