Solid-phase synthesis of recyclable phosphorus donor ligands for the development of immobilized transition-metal catalyst libraries

Abstract

Phosphorus-based ligands play a key role in a plethora of transition-metal catalyzed transformations. To date, only a few privileged ligand motifs have been developed for high performance application in a wide range of reactions. Despite the advances in rational design of highly selective phosphorus-based ligands in (asymmetric) homogeneous catalysis, synthetic approaches through trial-and-error remain the most common methodologies for the discovery of new powerful catalysts. High throughput experimentation has been embraced by both academia and industry to accelerate catalyst optimization requiring accesses to large and diverse ligand libraries. There is, however, still a lack of efficient combinatorial techniques enabling the synthesis and screening of vast phosphorus-based ligand libraries. Solid-phase synthesis (SPS) offers an useful tool towards the parallel synthesis of large multidentate ligand libraries. While being covalently bound to an insoluble polymeric support, a stepwise preparation of modular ligands can be realized via systematic variation of various building blocks. Moreover, purification procedures can be greatly simplified when employing this SPS approach, often requiring only easy filtration steps. Another advantage offered by immobilization of homogeneous catalysts on insoluble supports is the facilitated catalyst recovery and recycling as catalyst separation remains one of the major problems in applied homogenous catalysis. Consequently, resin-bound catalysts represent promising candidates for application in continuously operated processes. This thesis presents the efficient preparation of multidentate phosphorus ligand libraries using as solid-phase synthesis approach. Chapter 2 describes the modular access to a large and highly diverse supported phosphine-phosphite ligand library for application in asymmetric hydrogenation of enamides. The synthesis of a supported PNP pincer ligand library for application in ester reduction underlines the versatility of this SPS approach (chapter 3). Furthermore, the combinatorial ligand synthesis on a solid support has been successfully transferred to chiral PNP-type ligands (chapter 4). In chapter 5, a series of supported tripodal phosphorus ligand-based ruthenium complexes has been employed in nitrile hydrogenation providing tunable product selectivity by a simple change in the catalyst structure. Ultimately, the recovery and reusability of these heterogenized homogeneous catalysts has been investigated under batch and continuous flow conditions.