Synthesis of Quinoline Derivatives by Friedlander Reaction Catalyzed by Ruthenium Complexes of Substituted 8-Hydroxyquinoline

The Friedlander quinoline synthesis method is a reaction of o-aminoaryl aldehyde or ketone with methyl ketone to obtain quinoline. In this paper, a method for synthesizing quinoline catalyzed by ruthenium complexes of quinoline was reported. Using 8-hydroxyquinoline ruthenium complex as catalyst, the reaction conditions were optimized. The effects of different substituents of 8-hydroxyquinoline ruthenium complexes on the reaction yield were comparatively studied. Among them, 5-methyl-8-hydroxyquinoline (1e) ruthenium complex catalyzed the synthesis of 2-phenylquinoline from o-aminobenzyl alcohol and acetophenone with the highest yield of 73%. The relationship between ligand structure and catalytic performance was discussed by combining IR, UV and density functional theory (DFT) calculations. A possible mechanism was proposed, which included the formation of aldehyde transition state through beta-H elimination, cross aldol reaction, imination cyclization and finally dehydration to produce the target product. Using (1e)(3)Ru as catalyst, 32 quinoline derivatives with different substitutions were synthesized with 69%similar to 94% yields under the optimized reaction conditions, which confirmed the generality of this method.