An Economical, Green Pathway to Biaryls: Palladium Nanoparticles Catalyzed Ullmann Reaction in Ionic Liquid/Supercritical Carbon Dioxide System

In this paper, an economical, green pathway involving the palladium nanoparticles (Pd NPs) catalyzed reductive Ullmann reaction of an aryl chloride to afford a biaryl with high conversion and selectivity in an ionic liquid (IL)/supercritical carbon dioxide (ScCO2) system was developed. The combination of IL and ScCO2 provides superior advantages in product separation, catalyst recycling and reuse of reaction media over traditional organic solvents. Further investigations showed that the Bronsted-acidic imidazolium IL {e.g., (1-butyl-3-(sulfobutyl)-imidazolium) hydrogen sulfate, [bmim(HSO3C4)][HSO4]}, can replace the traditional active hydrogen donor readily with much enhanced product separation efficiency, the use of IL also led to an obvious improved stability of the Pd NPs, which was very helpful for catalyst recycling. Carbon dioxide, a naturally abundant, non-flammable, relatively non-toxic, economical and recyclable "greenhouse" gas, was found to significantly promote the selectivity of the Pd NPs-catalyzed aluminium-induced reductive Ullmann reaction of aryl chlorides. Investigations showed that the Pd NPs catalyst and IL can be recycled for at least five runs, indicating the economic viability of this process.