Experimental and theoretical evaluation of dimerization mechanism in the synthesis of 7H-6,8-dichloro-7-methylpyrano[3,2-b:3,2-b′]diquinoline

In this work, we report a combined experimental and theoretical study on the Diels Alder dimerization reaction of 2,4-dihydroxyquinoline with vinyl acetate. The structure of one of the dimerized compounds has been reassigned as symmetric linear bisquinoline rather than the reported symmetric angular bisquinoline by using spectroscopic i.e., 1H NMR, 15N NMR and HH COSY experiments and X-ray crystallographic analysis. In order to rationalize the origin of differential dimerization character, four major pathways of mechanism have been proposed for the formation of all the possible structural isomers. The feasibility of the proposed mechanisms was achieved by the DFT method employing B3LYP/6-311+G* and B3LYP/aug-cc-pVDZ levels of theory. The potential energy barrier between the steps of the reaction is calculated and compared with experimental results.