The Aza-Michael Addition And Knoevenagel Condensation In Pyrazolium-based Ionic Liquids

Ionic liquids(ILs)are organic salts that exist as liquids below a threshold temperature(usually around 100 ?).The following features are recognized to be typical properties of ILs:less volatile and flammable,generally thermally stable,no toxic et.Because of their negligible vapor pressure and other properties,such as non-flammability and high thermostability,ILs are considered as green solvents and good substitutes for traditional flammable and volatile solvents.Furthermore,ILs are showed poor solubility in hexane or ether,which allows realization of an easy work-up process.ILs have been widely studied in various important reactions.Pyrazolium-basedionic liquids have been applied in solar cells,new high-performance batteries and capacitors due to its good electric conductivities to date.On the other hand,the application of Pyrazolium-based ionic liquids in organic reactions has been seldom reported.In order to expand the application area of pyrazolium-based ionic liquids in organic reactions,we have designed and synthesized 2-substituted pyrazolium-based ionic liquids based on the nucleophilic substitution reaction of 1,3,5-trimethyl-1-H-pyrazole with alkyl halides or benzyl halides.The catalytic performance of the obtained ILs are investigated in the reactions of Aza-Michael and Knoevenagel.1.1,3,5-trimethyl-2-benzyl-1-H-pyrazolechloride ionic liquid [BnTMPz][Cl] and1,3,5-trimethyl-2-n-butyl-1-H-pyrazole bromide ionic liquid [Bu TMPz][Br] are prepared from the nucleophilic substitution of 1,3,5-trimethyl-1-H-pyrazole with benzyl chloride and n-bromobutane,respectively.The catalyst conditions are optimized by single-factor experimental analysis and the experimental results showed that [BnTMPz][Cl] is good catalyst for the Aza-Michael and Knoevenagel condensation reaction.The research results showed that [BnTMPz][Cl] has higher catalytic activity than [Bu TMPz][Br] for the Aza-Michael addition reaction.We have prepared 1,3,5-trimethyl-2-benzyl-1-H-pyrazol bromate [BnTMPz][Br] ionic liquid to validate the cause of catalytic activity.The ionic liquid is applied to the Aza-Michael addition reaction and experimental results showed high catalytic activity,mild conditions,the method convenient,excellent yield and good recycling efficiency.We reached a conclusion that the benzyl group in 2-positin of pyrazole plays a key role in catalytic activity.2.The catalytic screening test showed that the [BnTMPz][Cl] and [BnTMPz][Br] can accelerate the Knoevenagel condensation except the Aza-Michael reaction.We think it is important to increase the alkaline of the ILs by replacement of the anions because of the more nucleophilic the reagent has,the faster the reaction goes.By anion displacement,in which anion [Cl] is substituted with anion [Ac O],the [BnTMPz][Ac O]isobtained and applied to Knoevenagel condensation reaction.The results showed that the [BnTMPz][Ac O]is excellent catalyst for the Knoevenagel condensation reaction and experimental results showed high catalytic activity,mild conditions,the method convenient,excellent yield and good recycling efficiency.It is notable that the reaction can be completely catalyzed in 2 minutes at room temperature with high yield.The [BnTMPz][Br],[BnTMPz][Cl] and [BnTMPz][Ac O] ILs designed and prepared in this paper are excellent catalyst for Aza-Michael addition reactionand Knoevenagel condensation reaction,respectively.The research result has laid a firm foundation for the further development of pyrazolium-based ionic liquids in organic reactions.