Studies On Selenium-catalyzed Synthesis Of Ureas Under Solvent-free And Atmospheric Pressure Conditions

Ureas are very important organic chemicals. The traditional synthetic methods aswell as some new ways developed in recent years are insufficient in one way oranother. In this paper, we aim to develop a new synthetic method which is greener,simpler, milder, more convenient and more efficient.In this paper, using the nitro-compounds as raw materials, carbon monoxide ascarbonyl source, selenium as catalyst, and water as the source of hydrogen, wemanaged to synthesize the ureas under atmospheric pressure and solvent-freeconditions in one pot by making the substrate into membrane phase. Compared withthe high pressure reaction, this method is more accessible and safer. With not solvent,this method not only improves the utilization rate of the capacity, but also reduces thedifficulty of post-processing by simplifying the separation and purification of theproduct. All of above are in line with the principle of green chemistry. Meanwhile, wemake this method more efficient and convenient by using the nitro-compounds as rawmaterial, because it need not to be reduced to amine first, which is often possessed inindustry.Initially, the reaction conditions were optimized using the selenium-catalyzedreductive carbonylation of the nitrobenzene as a model reaction, by investigating theeffect of the reaction time, the amount and type of co-catalyst, the amount of water,selenium powder and substrate, reaction temperature, the size of reactor on thereaction. We concluded a relatively good reaction conditions: nitrobenzene10mmol,reaction time6h, DBU3.5mmol, water5mmol, selenium0.15mmol, reactiontemperature115℃，the size of flask500ml. Under this condition, the yield ofdiphenylurea can reach74.25%. When the substrate was transferred into other nitrocompounds, a series of symmetrical ureas were got. At the end of this part, we alsospeculated a proposed pathway of the reaction, and the products were characterizedby1H NMR spectra, IR spectra and melting point.With optimized reaction conditions in hand, we applied the Se/CO system intothe carbonylation of nitrobenzene with primary amines at atmospheric pressure and under solvent-free conditions. It gives a mixture of PhNHCONHPh, PhNHCONHR,RNHCONHR, in general, the product is given priority to asymmetrical ureas, butwhen the primary amines are connected with strong electron-donating groups, theyield of RNHCONHR will be a little higher than the other two, on the other hand,primary amines with electron-withdrawing groups will give a low yield ofRNHCONHR, thus, the main products are the PhNHCONHPh and PhNHCONHR.The proposed pathway of the selenium-catalyzed carbonylation of nitrobenzene withprimary amine was also concluded.At the end of this paper, we summarized the work we had done.