Experimental and DFT mechanistic insights into one-pot synthesis of 1 H -pyrazolo[1,2- b ]phthalazine-5,10-diones under catalysis of DBU-based ionic liquids

• Published in: New journal of chemistry 2020-10, Vol.44 (38), p.16594-16601
• Main Authors: Fallah-Ghasemi Gildeh, Sara, Mehrdad, Morteza, Roohi, Hossein, Ghauri, Khatereh, Fallah-Ghasemi Gildeh, Sahar, Rad-Moghadam, Kurosh
• Format: Article
• Language: English
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/D0NJ03478A

Benzylation of DBU followed by anion exchange of the resulting salt with trifluoroacetate gave nearly quantitatively the ionic liquid [Bn-DBU][TFA]. It is shown here to be an efficient catalyst for the synthesis of 1 H -pyrazolo[1,2- b ]phthalazine-5,10-diones via the three-component reaction of phthalhydrazide, aromatic aldehydes, and active α-methylene nitriles. DFT calculations at the B3LYP/SVP level and the experimental results are in agreement with a three-step mechanism for this reaction. Based on the DFT calculations, the catalytic effect largely arises from the intrinsic ionic properties of the ionic liquid rather than its action as a simple base. These calculations also predict the existence of two close-in-energy activated complexes whose rate determining roles and energies depend on their interaction with the anionic component of the ionic liquid, as [Bn-DBU][TFA] has shown a higher catalytic activity than [Bn-DBU][OAc]. This mechanistic approach opens up new and promising insights into the rational design of ionic liquid catalysts for the synthesis of 1 H -pyrazolo[1,2- b ]phthalazine-5,10-diones. The synthetic method presented here has several prominent advantages.