Computational study of the aminolysis of anhydrides: effect of the catalysis to the reaction of succinic anhydride with methylamine in gas phase and nonpolar solution.

Different possible pathways of the aminolysis reaction of succinic anhydride were investigated by applying high level electronic structure theory, examining the general base catalysis by amine and the general acid catalysis by acetic acid, and studying the effect of solvent. The density functional theory at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels was employed to investigate the reaction pathways for the aminolysis reaction between succinic anhydride and methylamine. The single point ab initio calculations were based on the second-order Møller-Plesset perturbation theory (MP2) with 6-31G(d) and 6-311++G(d,p) basis sets and CCSD(T)/6-31G(d) level calculations for geometries optimized at the B3LYP/6-311++G(d,p) level of theory. A detailed analysis of the atomic movements during the process of concerted aminolysis was further obtained by intrinsic reaction coordinate calculations. Solvent effects were assessed by the polarized continuum model method. The results show that the concerted mechanism of noncatalyzed aminolysis has distinctly lower activation energy compared with the addition/elimination stepwise mechanism. In the case of the process catalyzed by a second methylamine molecule, asynchronous proton transfer takes place, while the transition vectors of the acid-catalyzed transition states correspond to the simultaneous motion of protons. The most favorable pathway of the reaction was found through the bifunctional acid catalyzed stepwise mechanism that involves formation of eight-membered rings in the transition state structures. The difference between the activation barriers for the two mechanisms averages 2 kcal/mol at various levels of theory.