Synthesis of quinoline derivatives by a Doebner-von Miller reaction using a Ag(I)-exchanged Montmorillonite K10 catalyst.

Abstract

Quinolines play an important role in organic chemistry, as exemplified by their extensive application as biologically and pharmacologically active compounds. However, current methods available to access quinoline compounds employ harsh reaction conditions and expensive starting materials with varying product yields. The Döebner-von Miller reaction is a valuable asset in the synthesis of various natural and biologically active quinoline derivatives; however, there are various challenges associated with this methodology such as harsh reaction conditions, hazardous reagents (10 M hydrochloric acid), tedious isolation procedures, side products and low yields. Consequently, the need to develop a simple and environmentally friendly route to synthesize quinoline derivatives via the Döebner-von Miller reaction is essential. Silver(I)-exchanged Montmorillonite K10 was evaluated as a potential solid acid catalyst towards the synthesis of quinoline derivatives via the Döebner-von Miller reaction. Using this approach, the Döebner-von Miller reaction was evaluated under various reaction conditions with solvent-free, conventional heating conditions affording the best results. Using the optimized reaction conditions, a series of substituted quinoline derivatives were synthesized in moderate to excellent yields (42-89%) in 3 hours. The scope of our methodology towards both aromatic and aliphatic α, β-unsaturated aldehydes was also evaluated and the system was found to be equally efficient on both the substrates mentioned above. A recycle and reuse study was conducted in order to gain an accurate assessment of the activity of our catalyst and it was shown that it can be utilized several times without any appreciable loss in activity, thus making this procedure more environmentally benign. A literature comparison study was conducted and the yields obtained using the silver(I)-exchanged Montmorillnoite K10 approach were found to be superior to most previously reported approaches via the Döebner-von Miller reaction. In addition, silver(I)-exchanged Montmorillonite K10, was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersion X-Ray (EDX) analysis. The results of this study indicated a uniform distribution of silver(I) ions on the surface of Montmorillonite K10 with a total silver(I) content of 3.67 weight %.