Palladium-catalyzed cross-coupling in aqueous media: Recent progress and current applications

Water has attracted significant attention as an alternative solvent for homogeneous metal-catalyzed reactions because it is a non-toxic, nonflammable solvent that can allow for simplified recovery of homogeneous catalysts. The use of water as a solvent in palladium-catalyzed cross-coupling reactions is of great academic and industrial interest. Palladium-catalyzed coupling reactions are powerful methods to make carbon-carbon and carbon-heteroatom bonds under mild conditions. Separation of the catalyst from the organic product can be very difficult; however, constraining the catalyst to the aqueous-phase of an aqueous-biphasic solvent system can potentially simplify catalyst recovery. This review will focus on recent developments in the design and application of aqueous-phase, palladium-catalyzed coupling reactions over the period from 2000, through late 2004. A variety of new hydrophilic ligand architectures have been introduced recently that provide significantly more active catalysts towards industrially relevant aryl,bromides and chlorides.