Mechanistic Investigation of DBU-Based Ionic Liquids for Aza-Michael Reaction: Mass Spectrometry and DFT Studies of Catalyst Role

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)-based ionic liquids (ILs) has exhibited a high catalytic activity in the aza-Michael reactions compared to conventional catalysts and with imidazole-based ILs. In the present work DBU-based ILs showed high catalytic potential for aza-Michael addition of aromatic amines to 2-cyclohexen-1-one under solvent-free condition. Electrospray ionization-mass spectrometry (ESI-MS) and density functional theory studies have been carried out to provide an effective activation mode of DBU-based ILs in aza-Michael addition. Our results show that both the presence of the acid hydrogen in the IL and the ability of the anion to carry out a hydrogen bond with the −NH2 group of the arylamine are fundamental for the reaction catalysis. The catalytic model proposed can be used for the rational development of new ILs with excellent catalytic properties.

以1,8-二氮杂双环[5.4.0]十一碳-7-烯（1,8-Diazabicyclo[5.4.0]undec-7-ene，DBU）为骨架的离子液体（ionic liquids, ILs）相较传统催化剂以及咪唑类离子液体，在氮杂迈克尔（aza-Michael）反应中展现出更为优异的催化活性。本研究中，该类DBU基离子液体在无溶剂条件下，用于芳胺与2-环己烯-1-酮的氮杂迈克尔加成反应时，展现出较高的催化应用潜力。本研究通过电喷雾电离质谱（Electrospray ionization-mass spectrometry, ESI-MS）与密度泛函理论（density functional theory）分析，阐明了DBU基离子液体在氮杂迈克尔加成反应中的有效活化模式。研究结果表明，离子液体中的酸性氢组分，以及阴离子与芳胺的-NH₂基团形成氢键的能力，均为该催化反应的核心必要条件。本次提出的催化模型可用于理性设计开发具备优异催化性能的新型离子液体。