Entwicklung neuer elektrophiler Platin-Katalysatoren für die C-H-Aktivierung und -Funktionalisierung von Methan
Final Report Abstract
During the funding period of the Forschungsstipendium I was able to synthesize and characterize new cationic mono- and bidentate ligands 2a, 4a/b, 6. 4a and 4b were transmetallated to afford Pd and Pt complexes Pt-5a/b and Pd-5a that are highly active catalyst systems for C-H activation and show the highest reported TONs in H/D exchange between benzene and AcOH-D4. The cationic ligands enhance both stability and activity of the Pd and Pt catalysts even upon prolonged heating for several days. Furthermore, 4a is the only known ligand that enhances the rate of the C-H acetoxylation of benzene when employed in a 1:1 ratio with Pd(OAc)2. Detailed exploration of the Pd catalyzed C-H acetoxylation of benzene evaluating various ligands led us to the general conclusion that this reaction (as an example of undirected C-X bond formation) is exquisitely sensitive to the metal to ligand ratio. Unprecedented highly active catalyst systems can be formed at “ligation site to Pd” ratios of <2:1. Further mechanistic considerations led us to discover a highly active catalyst system, employing simple commercial pyridine as ligand. With this new catalyst system, the non-directed efficient C-H oxygenation of electron deficient arenes is possible for the first time; moreover, the highest reported TON for a homogeneous catalyzed C-H oxygenation can be achieved, and the catalyst is stable for days under the reported conditions. Selectivity studies using substituted arenes as substrates and Pd/pyr catalysts show that the site selectivity of C-X bond formation can be influenced through the use of ligands. Furthermore, it could be demonstrated that sterically differentiated oxidants have a strong effect on the site selectivity of acetoxylation. Both concepts (catalyst and reagent control) can potentially be used to overcome substrate controlled selectivity in non-directed C-H functionalization reactions. Finally, the Fujiwara-Moritani reaction of benzene with ethylacrylate was studied in detail and revealed a similar sensitivity to the Pd:pyr ratio as seen previously in C-H acetoxylation. These studies highlight the importance of exploring the ligand to metal ratio, along with the structure of spectator ligands, during the optimization of reaction conditions for catalytic C–H functionalization.
Publications
- "Cationic Pt and Pd Complexes for C-H Bond Functionalization," ACS National Meeting, Boston, MA, 240: 245-INOR, 23. August 2010
M. H. Emmert, M. S. Sanford
- "Electrophilic Pt and Pd Catalysts for C-H Activation and Functionalization," Research Seminar, Emory University, 8. Dezember 2010
M. H. Emmert
- Angew. Chem. 2010, 122, 6020- 6022; Angew. Chem. Int. Ed. 2010, 49, 5884-5886
M. H. Emmert, J. B. Gary, J. M. Villalobos, M. S. Sanford
