Theoretical illustration on the catalytic effect of alanine methyl ester nitrate ionic liquid on the Diels–Alder reaction between cyclopentadiene and methacrylate

By performing density functional theory calculations, we systematically studied the Diels–Alder (D–A) reaction between cyclopentadiene and methacrylate catalyzed by alanine methyl ester nitrate ([AME][NO3]), an amino acid-based ionic liquid (AAIL). The uncatalyzed reaction was first calculated in both gas phase and dichloromethane, and then the catalytic effect of [AME][NO3] AAIL on the D–A reaction was mimicked by using one, two, and up to three ion pairs as catalysts. The calculated results show that [AME][NO3] plays a role of Lewis acid to promote the reaction and the catalytic active center is the NH3 group in [AME]+ cation, which forms the NH⋯O H-bond with the carbonyl oxygen atom in methacrylate to effectively polarize the CC double bond in methacrylate. As a result, the energy barrier of reaction is remarkably reduced, and the asynchronicity of reaction is increased. The calculated energy barrier for the reaction with the presence of two ion pairs is lower than those with the presences of one and three ion pairs, implying that the optimal molar ratio among two reactants and the reaction medium/catalyst [AME][NO3] should be 1:1:2. The present results rationalize the early experimental findings, and provide a useful reference for the rational design of usual D–A reactions in AAILs.