Synthesis of quinoxalines in the presence of heteropoly acids

Article history: Received October 25, 2012 Received in Revised form December 6, 2012 Accepted 19 January 2013 Available online 19 January 2013 Efficient synthesis of quinoxaline derivatives from the reaction of α-diketones and ophenylenediamines in the presence of Keggin-type heteropolyacids (HPA) such as H3PMo12O40, H4SiW12O40, K7PMo2W9O40, H3PW12O40.SiO2 and H3PW12O40 in high yields and short reaction times, and at room temperature is introduced. © 2013 Growing Science Ltd. All rights reserved.


Introduction
Heteropoly acids as solid acid catalysts are green catalysts with respect to their non-corrosive nature, safety, low quantity of waste and easy separation.One of the unique features that make solid heteropoly acids economically and environmentally attractive is their stability and high acidity.

Result and Discussions
In continuation of our investigations on the applications of solid acids in organic synthesis, we have investigated the synthesis of quinoxalines in the presence of heteropoly acid at room temperature.Herein, we report that heteropoly acids are efficient catalysts for the synthesis of quinoxaline derivatives comparable with some other applied catalysts.The reaction of 1,2phenylenediamine with benzil was investigated for optimization of the reaction conditions.Reaction at different temperatures and various molar ratios of substrates in the presence of heteropoly acid revealed that the best results were obtained under solvent-free conditions at room temperature and a molar ratio of 1,2-phenylenediamine: benzil: heteropoly acid equal to 1:1:0.01.Various 1,2-phenylenediamines and 1,2-diketones were used as substrates for the synthesis of quinoxalines under solvent free at room temperature (Scheme 1 and Table 1).

Conclusions
Herein, we have reported a mild, easy applicable and efficient method for the preparation of quinoxalines from benzils and o-phenylenediamines using small amount of heteropolyacids as highly efficient solid catalysts.These reactions are characterized by good yields and short reaction times.

Chemicals and apparatus
All chemicals were purchased from commercial suppliers and were used as received.All products were identified by their spectra and physical data.Melting points were measured by using the capillary tube method with an electrothermal 9100 apparatus.Polyoxometalates were prepared according to literature procedures 16 .The IR spectra were recorded on a Shimadzu DT-40 model 883 IR Spectrophotometer (KBr pellets, Nujol mulls, 4000-400 cm −1 ). 1 H NMR spectra were recorded on a Bruker-Avance DRX 400 spectrometer using TMS as an internal standard.Elemental analyses were done by Costech ECS 4010 CHNS-O analyzer.

Preparation of quinoxalines catalyzed using heteropoly acids:
A mixture of o-phenylenediamine (1 mmol), 1,2-dicarbonyl (1 mmol) and catalyst (1 mol %) was grounded in a mortar for 2-4 minute.The progress of the reaction was monitored by TLC.After completion of the reaction, the mixture was dissolved in CH 2 Cl 2 , filtered and washed with diethyl ether (5 mL) to isolate the catalyst.The solvent was evaporated under reduced pressure and the pure product was obtained.