Palladium-catalyzed synthesis of symmetrical urea derivatives by oxidative carbonylation of primary amines in carbon dioxide medium

An efficient palladium-catalyzed synthesis of symmetrically disubstituted ureas via oxidative carbonylation of primary amines is described. The reactions are carried out in the presence of a large excess of carbon dioxide as reaction medium or under solvent-free conditions. The adopted catalyst such as potassium tetraiodopalladate, stable and easy to prepare, allows the use of air as a cheap oxidizing agent. The reactions yield urea and water as the only by-product and proceed with high efficiency with aliphatic and aromatic amines as well. While with primary aliphatic amines, no significant improvement on reactivity is observed when carbon dioxide is used as a solvent, in comparison with the conventional ones, a remarkable high efficiency is obtained with aromatic amines, which shows a dramatic increase in the performance of the catalyst, in terms of turnover number (TON), the highest known so far for this kind of process. Reactions take place in two-phase systems consisting of a homogeneous liquid phase formed by the CO2 expanded amine solution containing the catalyst and a supercritical phase of CO2, CO, O2, and N2.

Graphical abstractPrimary aliphatic and aromatic amines and carbon monoxide react effectively in solvent-free conditions and/or in carbon dioxide medium in the presence of tetraiodopalladate and oxygen leading to symmetrical urea derivatives. The use of carbon dioxide as the solvent medium allows in some cases high performance of the catalyst in terms of turnover number.Figure optionsDownload full-size imageDownload high-quality image (56 K)Download as PowerPoint slideHighlights► Efficient catalytic synthesis of symmetrical ureas in CO2 is described. ► We have examined the behavior of aliphatic and aromatic amines. ► The catalyst adopted, K2PdI4, is stable and easy to prepare. ► A cheap oxidizing agent such as air is used. ► A remarkable high efficiency was obtained with aromatic amines.