Dendritic catalysts for compartmentalized tandem catalysis

Abstract

Homogeneous catalysts are the catalysts of choice for many synthetic procedures in academia and industry. The choice to use a soluble organometallic catalyst is often related to its high activity, the possibility for selective catalysis, its high tunability, the mild reaction conditions, and the well-described reaction mechanisms that characterize homogeneous catalysts. An obvious disadvantage of the use of such catalysts is the difficulty in separating them from reaction mixtures, which hampers the subsequent reuse of the organometallic species and which may lead to the loss of the precious metal and the ligand. A common approach for the removal and recycling of homogeneous catalysts is the immobilization of multiple of these catalysts on a single macromolecular support. This can take place in a homogeneous or heterogeneous way, depending on the chosen support. Dendrimers, i.e. tree-like polymers with repeating branches and multiple peripheral end groups, are widely explored for the immobilization of homogeneous catalysts because of their low polydispersity and superior solubility with respect to, e.g., linear polymers. The end groups of these dendrimers can be connected to (modified) monomeric homogeneous catalysts to create peripherally functionalized dendrimers that contain a distinct number of organometallic catalytic sites. By means of the resulting molecular weight enlargement, these dendritic catalysts can be separated from reaction mixtures via nano- or ultrafiltration, or by dialysis techniques. In this way, the dendritic catalyst can be made available for reuse in either a batch-like or a continuous manner. The use of dendritic catalysts in tandem catalysis, i.e. a type of catalysis where various, chemically different reactions are performed in the same vessel in a consecutive manner, in view of process intensification, would lead to a further improvement in organic synthesis according to the principles of green chemistry. Shorter synthetic procedures lead to less reaction workup procedures in multi-step synthesis, less waste products, and as a result to lower production costs. Although hinted at for some time, no examples of dendritic tandem catalysis have been reported so far. In this thesis, the use of dendritic catalysts in tandem catalysis was investigated both in homogeneous solutions as well as in compartmentalized settings in order to arrive at tandem reaction setups with optimized catalyst performance and recyclability. The synthesis and catalytic performance of various dendritic Pd and Ru complexes is described for a number of tandem and single-step reactions and is benchmarked against the properties of their monomeric analogues.