Die direkte katalytische Carboxylierung einfacher Arene mit CO$_{2}$ : Detaillierte Analyse eines postulierten Katalysezyklus

• The direct catalytic carboxylation of simple arenes with CO$_{2}$ : detailed analysis of a postulated catalytic cycle

Voit, Gregor; Leitner, Walter (Thesis advisor); Okuda, Jun (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

In this thesis a catalytic cycle, postulated for the direct carboxylation of simple arenes with carbon dioxide, was investigated in detail. Therefore, at first palladium and platinum complexes bearing phosphine sulfonamido ligands were identified as a suitable complex system, applying experimental and computational methods. In contrast to the well-studied phosphine sulfonato ligand system, these show a monomeric structure of the carboxylato complexes, involved in the catalytic cycle as most stable intermediates. So, they reveal energy barriers small enough to enable an analysis of the catalytic cycle. For this reason, a portfolio of palladium and platinum complexes bearing different ligands of this type were synthesized and analyzed regarding to their structures and their dynamic behavior in solution. In this course, conformational and configurational differences were identified, which are related to the ligands fine structure and enable to effectively influence the energies of the intermediates and transition states, involved in the postulated catalytic cycle (chapter 3). Complexes of this kind can catalytically decarboxylate bis-methoxy substituted aromatic carboxylic acids at room temperature without the need of an external proton source. This is the back reaction of the analyzed carboxylation and proceeds via metal aryl intermediates. This could be shown by successful decarboxylative coupling with olefins at room temperature. As a consequence of the spatial structure of the utilized complexes, this results in selective formation of 1,1-disubstituted olefins (chapter 4).The activation and cleavage of an aromatic C-H-bond, which is a necessary partial step of the postulated cycle, was successfully shown by H/D-exchange experiments with anisole (chapter 6). The second partial step of the postulated cycle, migratory insertion of CO2, was successfully verified on a phosphine sulfonamido palladium complex with a para-anisyl ligand, that was synthesized therefor. It was shown that this reaction proceeds via a migratory insertion mechanism with pre-coordination of the CO2 molecule to the metal center (chapter 5).First experiments regarding the catalytic carboxylation of non-preactivated arenes suggest, that this reaction suffers under a thermodynamic hinderance, despite the addition of an amine base for product stabilization. First proposals to overcome this problem were given (chapter 7).