One-Pot Sequential Synthesis of Fused Isoquinolines via Intramolecular Cyclization/Annulation and their Photophysical Investigation

One of the cyano group of gamma-keto malononitrile gets hydrolyzed selectively to an amide in the presence of copper(II) acetate monohydrate. The in situ generated amide undergo an intramolecular dehydrative cyclization to a 1,2-dihydropyridone intermediate. Further annulation of the 1,2-dihydropyridone with an internal alkyne in the same pot produce a fused isoquinolone, 4-oxo-2,6,7-triaryl-4H-pyrido[2,1-a]isoquinoline-3-carbonitrile. This one-pot process is associated with the formation of one C-C, two C-N, two C=C and a C=O bonds. The final synthesis is a four-step process consisting of selective hydrolysis of a cyano group to an amide, dehydrative cyclization of the amide to a cyclic amide, aromatization of the cyclic amide (2-oxo-1,2,3,4-tetrahydropyridine moiety) to a 2-oxo-1,2-dihydropyridine and finally, the C-H/N-H annulation with an alkyne. Density functional theory calculation reveals that the highest occupied molecular orbital (HOMO) is localized at the central core extending to the nitrile group and a negligible contribution from the two phenyl rings of the diphenylacetylene. On the other hand, the lowest unoccupied molecular orbital (LUMO) is also localized at the identical central core and extended up to the other phenyl ring. The calculated Delta E(LUMO-HOMO) is in the range of 2.88 to 3.45 eV and the compounds display emission in the green region (502-560) nm and absorption (lambda(max)) in the range of (454-490) nm. Therefore, these molecules may find application in bio-imaging, theranostics and various applications in material science.