Biphasic Catalysis—Homogeneous

AbstractHomogeneous catalysis offers advantages over heterogeneous catalytic methods in that all catalyst molecules are available for the substrate, and the catalyst can be tailored to the desired chemical transformation by choosing (designing) the proper ligand environment. In most cases, however, industry uses heterogeneous catalysts because (in addition to their high activity and selectivity) catalyst–product separation poses no serious problems. A successful way of heterogenization of homogeneous catalysts is the separation of the reactants (products) and the catalyst into immiscible liquid phases (liquid multiphase catalysis).This article reviewsbiphasic catalysiswith homogeneously (molecularly) dispersed catalysts. Attention is focused on liquid–liquid biphasic systems, containing all the reactants and the catalyst(s) in dissolved state. Such biphasic systems are built up of two or more solvents with limited mutual solubility.Aqueous–organicandorganic–organic(including alsofluorous) biphasic systems have been studied for long; however, the use of biphasic reaction mixtures designed by usingfluorous solvents,ionic liquids, andsupercritical gasesin combination with each other or with appropriate aqueous or organic solvents has been rapidly developing recently. The properties of such systems together with the need of special ligand design or modifications are discussed, and their use is illustrated by describing typical reactions (hydrogenation, hydrogen transfer, hydroformylation, hydrocarboxylation, carbonylation, alkene metathesis, dimerization, polymerization, and other CC coupling reactions).