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    Estudo químico-computacional sobre o mecanismo de formação da ligação amídica catalisada por ZrCl4
    (Universidade Federal de São Paulo, 2016-12-13) Assad, Felipe Vieira Zauith [UNIFESP]; Sensato, Fabricio Ronil [UNIFESP]; Universidade Federal de São Paulo (UNIFESP)
    The ZrCl4-catalyzed direct amide bond formation between non-activated carboxylic acids and amines has been theoretically investigated at DFT/B3LYP level of theory. In addition we have also characterized the uncatalyzed mechanism as well as the amide bond formation promoted by the use of carbodiimides. In particular, we have characterized two non-catalyzed mechanism (channels 1-2), six ZrCl4-catalyzed pathways (channels 3-8) and two process promoted by carbodiimides. Solvent effect of toluene is assessed using SMD solvation model.. The favored non-catalyzed process is stepwise with two steps. The corresponding rate-determining step requires a free energy of activation of 11.6 kcal/mol in gas phase and 8.7 kcal/mol in toluene for the N-methylformamide formation. The most favored pathway for zirconium-catalyzed amidation is predicted to be stepwise via four steps. The rate-determining process is related to a free energy of activation of 6.3 kcal/mol in gas phase and 7.3 kcal/mol in toluene. The charge decomposition analysis (CDA) of the calculated transition structures revealed that the C?N bond formation is related to the nucleophilic attack of the amine toward the ?*(C=O) orbital of the carboxylic acid. It has been found that ZrCl4 activates the carboxylic acid by lowering the ?*(C=O) energy level. For the set of amines and carboxylic acids investigated, our results reveal that ?G?, for the rate-determining process, correlates linearly with the pKb of the amines and with the Fukui function for nucleophilic attack on the carbonyl carbon. It decreases with both decreasing pKb and increasing fk+. Zirconium carboxylate generated in situ is also predicted to be an active species in the zirconium-catalyzed direct amide bond formation. The assistance of an additional molecule of carboxylic acid was found to lower the activation energy in both non-catalyzed and ZrCl4-catalyzed process. The intermediate aminodiol plays a very key role in the non-catalyzed as well as in the ZrCl4-catalyzed mechanism. The rate-determining step of amide bond formation mediated by DIC is the carboxylic acid activation, which demands a free energy of activation of 18.9 kcal/mol in both gas phase and toluene. The molecular mechanism of the formation of N-acyl urea, a very pervasive byproduct, has been also elucidated.