Experimental and theoretical study on one-pot, three-component route to 2H-indazolo[2,1-b]phthalazine-triones catalyzed by nano-alumina sulforic acid

A simple, efficient, and high yielding one-pot protocol for the synthesis of 2H-indazolo[2,1-b]phthalazine-trione derivatives has been developed by three-component coupling of phthalhydrazide, dimedone and some aromatic aldehydes in ecofriendly neat conditions promoted by nano-γ-alumina sulforic acid. This protocol avoids the use of expensive catalysts, toxic solvents and chromatographic separation. Also the proposed protocol was extended to the linear β-diketone (acetylaceton) instead of cyclic diketone (dimedone). The generality and functional tolerance of this convergent and environmentally benign method is demonstrated. The nature of each step of the suggested mechanism for this condensation reaction is also identified through Density Functional Theory (DFT) calculations using B3LYP/6-31G** level of theory. It is shown that the first step is charge controlled; whereas the second one is controlled by frontier molecular orbitals. The 1H and 13C chemical shift values together with the structural parameters of the title compound have been also computed and the scaled values have been compared with the corresponding experimental NMR and X-ray spectra. As a result, the calculated spectroscopic data and optimized geometry show a good agreement with the experimental results.

► 2H-indazol [2,1-b]phthalazine-triones synthesized using nano-alumina sulforic acid.
► The protocol extended for 1H-pyrazolo[1,2-b]phthalazine-diones synthesis.
► The nature of each step of suggested mechanism investigated.
► Theoretical spectra and structural analysis performed for synthesized compounds.