Mild Catalytic Mechanism of the Mannich Reaction for Synthesizing Methylacrolein by sec-Amine Short-Chain Aliphatic Acid Ionic Liquid Catalysts

By density functional theory (DFT), we report a detailed mechanistic study on the synthesis of methylacrolein (MAL) by a mild Mannich reaction of formaldehyde (FA) and propionaldehyde (PA) catalyzed by sec-amine short-chain aliphatic acid ionic liquids (ILs). ILs exhibit excellent catalytic activity and create mild reaction conditions (45 °C) by dramatically decreasing the reaction energy barrier (24.43 kcal mol–1) in the decomposition step of Mannich bases (MBs) comapred to without ILs. Three key intermediates observed by DFT calcutations were identified by electron spray ionization mass spectrometry (ESI-MS) analysis. We systematically investigated the catalytic effect of ILs with different sec-amines (HNR2, R = CH3, C2H5, etc.) on the activation energy and different short-chain aliphatic acids (RCOOH, R= H, CH3 et al.) on the decomposition step of MBs. This work is intended to provide a thorough explanation of the synthesis mechanism of the mild Mannich reaction of MAL catalyzed by ILs from the theoretical aspect, which may give a favorable guidance for the practical application.