This project focuses on a plan to develop building blocks for a ligand type that has been shown to provide metal complexes possessing unique reactivity, N-heterocyclic carbene (NHC) ligands. NHCs are being exploited as ligands for a variety of transition metal catalysts, in a number of cases possessing better reactivity than ligands investigated previously. They have been shown to be instrumental in a number of important reactions the most important of which may be olefin metathesis. Additionally, NHCs have demonstrated activity in the area of nucleophilic catalysis. This work will develop approaches to provide building blocks that facilitate the synthesis of a variety of different NHC ligands. The methods developed in this work will facilitate the synthesis of a wide variety of N-heterocyclic carbenes and an even larger number of transition metal complexes.
With this award, the Chemical Synthesis program is supporting the research of Professor Scott R. Gilbertson in the Department of Chemistry at the University of Houston. Professor Gilbertson's research efforts revolve around the development of new ligands for catalysis. The discovery of new catalysts is often limited by the availability of new ligands. Consequently, the development of chemistry that provides access to new ligand types is important. New and efficient catalysts are critical in the manufacturing of many products, including therapeutic agents, agrochemicals, and commodity chemicals.
Over the last 15 years small peptides have been used to catalyze nonbiological reactions ranging from acyl, phosphoryl and sulfinyl transfers to Baylis-Hillman reactions, aldol reactions and epoxidations. Metal binding peptides containing either natural or unnatural amino acids have been used to control or mimic enzyme activity and perform reactions that are not typically seen in biological systems. The synthesis of a building block, a proline based N-heterocyclic carbene ligand, is reported. An approach to a variety of proline containing imidazolium NHC precursors is presented. The proline amino acids are shown to be compatible with peptide synthetic methods by incorporation into tripeptides. Following peptide synthesis the carbene is generated and bound to rhodium. This system allows for the synthesis and testing of new catalysts for a variety of different chemical reactions. Such catalysts have potential application in areas ranging from the synthesis of new drugs to the new catalytic processes for the petroleum. The molecules synthesized can be combined with a mixture of other molecules to form ligands for transition metals. The resulting complexes will constitute new catalysts. Additionally, the molecules synthesized in this project can also be used as building blocks for new organic catalysts that can catalyze reactions that are very similar to those of enzymes.
