Visible-light-assisted base-catalyzed, one-pot synthesis of highly functionalized cinnolines

Abstract

The synthesis of cinnolines has found great interest due to their diverse biological and industrial potency. Yet, the reported synthetic protocols for their synthesis showed limitations that involve harsh reaction conditions such as strong acidic or basic medium, low reaction yields, and using expensive and high loading catalysts. The C-H functionalization has been recognized as intriguing synthetic approach for the synthesis of aromatic/heteroaromatic scaffolds over the past two decades. Here, we reported a novel metal-catalyzed free photocatalytic synthesis of polyfunctionally substituted cinnolines. When ethyl 1-aryl-5-cyano-4-methyl-6-oxo-1,6-dihydropyridazine-3-carboxylates and nitrostyrene derivatives are irradiated with white light (LED 30 W) in ethanol in the presence of piperidine (30 mol%) in open air for 8 h at room temperature, the corresponding polyfunctionally substituted cinnolines are obtained in excellent yields (90-95%) via C-H activation of pyridazine methyl group and nitrostyrene (-N═O) function. Several merits were achieved, which are as follows: (1) the reaction is metal-free; (2) the reaction proceeds with increasing energy efficiency; (3) diversity of functionally substituted cinnolines; (4) high EcoScale value, which reflects the greens of the reaction; and (5) ease handling either in conducting the reaction or in the isolation of products.