Dendrimers as support for recoverable catalysts and reagents.

The development of well-defined catalysts that enable rapid and selective chemical transformations and can be separated completely from the product is still a paramount challenge.1 The recent success of homogeneous catalysis is reflected in the number of applications that is known today both in the laboratory and in the industrial practice, but so far there is not a single solution to the catalyst-product separation problem. In fact, all “unit operations” for separation, including distillation, liquid-liquid separation or extraction, stripping, catalyst destruction, and crystallization, are being applied in industry. While several methods are being applied commercially, the search for new approaches continues. These new methodologies include fluorous phase catalysis,2 the use of ionic liquids,3 the use of supercritical fluids,4 and supported aqueous phase catalysis.5 A widely studied approach to facilitate catalystproduct separation is the attachment of homogeneous catalysts to insoluble organic, inorganic, or hybrid supports.6 Inorganic materials such as silica are particularly suited as heterogeneous catalyst support because of their high physical strength and chemical inertness. The main problems related to this approach are the nonuniform and partly unknown structures of the heterogenized catalysts, mass transport limitations due to slow diffusion, the generally lower activity compared to the homogeneous analogue, and metal leaching. Metal leaching can be suppressed by using properly chosen ligands 3717 Chem. Rev. 2002, 102, 3717−3756