Selected syntheses of ureas through phosgene substitutes

Ureas have been traditionally synthesised by methodologies mainly based on the use of dangerous reagents such as phosgene and isocyanates. In the last few years, however, these reagents have been increasingly substituted by cleaner alternative and inherently safer compounds since the goal of the modern environmentally friendly synthetic chemistry is the development and optimisation of reaction conditions to reduce or eliminate the use and production of hazardous materials while maximising energy usage. Thus different and sometimes sophisticated phosgene substitutes such as bis(4-nitrophenyl)carbonate, triphosgene, di-tert-butyl dicarbonate, 1,1-carbonylbisimidazole, 1,1-carbonylbisbenzotriazole, S,S-dimethyldithiocarbonate and trihaloacetylchlorides have been utilised as safer reagents which can be stored and handled without special precautions. Moreover, the large scale production of urea derivatives has been more conveniently performed by using simpler and cheaper raw materials such as CO and CO2 in catalytic processes, avoiding production of large amounts of saline by-products which represent the main constituent of chemical waste.