THE MECHANISM OF ACID-CATALYZED DECARBOXYLATION OF PYRROLE-2-CARBOXYLIC ACID: INSIGHTS FROM CLUSTER-CONTINUUM MODEL CALCULATIONS

The decarboxylation of pyrrole-2-carboxylic acid comprises the addition of water to the carboxyl group and the C-C bond cleavage leading to the protonated carbonic acid. Herein possible concerted and stepwise mechanisms for the C-protonated and O-protonated pathways were extensively investigated by using the cluster-continuum model. The calculated results indicate that the initial hydration or the nucleophilic attack of water at the carbonyl group of both C-and O-protonated derivatives is the rate-determining step for the overall reaction, and the O-protonated pathway will dominate the whole reaction. The predicted activation Gibbs energies for the overall reaction initialized by the O-protonated species fall in the range of 83.3 similar to 123.0 kJ/mol, showing good agreement with experimental values of 91.6 similar to 101.3 kJ/mol. On the basis of extensive calculations, the remarkable dependence of the predicted mechanisms and thermodynamic values on the number of explicit water molecules in the cluster-continuum model was discussed.