Mechanism and Rate-Determining Factors of Amide Bond Formation through Acyl Transfer of Mixed Carboxylic-Carbamic Anhydrides: A Computational Study.

Acyl transfer of in situ-generated mixed anhydrides is an important method for amide bond formation from short linkages with the easily removed byproduct CO. To improve our understanding of the inherently difficult acyl transfer hindered by the large ring strain, a density functional theory study was performed. The calculations indicate that the amidation of activated α-aminoesters and N-protected amino acids is more likely to proceed via the self-catalytic nucleophilic substitution of the two substrates and the subsequent 1,3-acyl transfer. By comparison, the mechanism involving 1,5-acyl transfer is less kinetically favored because of the slow homocoupling of activated α-aminoesters. Furthermore, we found that the detailed mechanism of 1,3-acyl transfer on the mixed carboxylic-carbamic anhydrides depends on the catalysts. Strong acidic catalysts and bifunctional catalysts both lead to stepwise pathways, but their elementary steps are different. Basic catalysts cause a concerted C-N bond formation/decarboxylation pathway. The calculations successfully explain the reported performances of different Brønsted-type catalysts and substrates, which validates the proposed mechanism and reveals the dependence of the reaction rates on the acid-base property of catalysts and the acidity of substrates.