Anchored homogeneous catalysts: high turnover number applications

Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as "boomerang" type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.

Development of Robust Heterogeneous Chiral Rhodium Catalysts Utilizing Acid-Base and Electrostatic Interactions for Efficient Continuous-Flow Asymmetric Hydrogenations

Heterogeneous chiral Rh catalysts based on acid-base and electrostatic interactions have been developed. The robust catalysts demonstrate high activity and selectivity in the continuous-flow asymmetric hydrogenation of a wide variety of enamides and dehydroamino acids, providing optically active amides without leaching of metal species. The chiral environments can be easily tuned by changing the chiral ligands, demonstrating the high versatility of the heterogeneous catalysts. By applying these efficient catalysts, continuous synthesis of several active pharmaceutical ingredient intermediates was achieved.

Chiral Amine−Polyoxometalate Hybrids as Highly Efficient and Recoverable Asymmetric Enamine Catalysts

Chiral amine and polyoxometalate (POM) acids were combined to give highly efficient, stereoselective, and recoverable enamine-based catalysts. Less than 1 mol % of chiral amine loading was sufficient for good catalytic activity, and the catalyst could be recovered and reused 6 times with unchanged enantioselectivity and slightly decreased activity.