This thesis describes examples of the use of quantum-chemical calculations to investigate reaction mechanisms in homogeneous catalysis. Four projects are examined: two illustrate catalysis by main group elements and two focus on transition metal catalysis. In the first project, an organocatalytic isomerization of exo- to endo-vinylene carbonates was investigated. The catalyst system is a mixture of an organic base and phenol. Density functional theory (DFT) calculations revealed a ring-opening mechanism with a ring-opened ketone intermediate, which is experimentally isolable. For substrates bearing an aryl substituent, a ring-retaining pathway is accessible as well, which proceeds without the involvement of phenol. Based on this knowledge, a control experiment was designed, yielding further evidence for the ring-opening mechanism. The second project focused on the mechanistic investigation of a Cu(II)-catalyzed aniline synthesis from aryl chlorides in aqueous ammonia. DFT investigations showed that deprotonation of an initial Cu(II)-ammine complex yields the active form of the catalyst, a Cu(II)-amido complex. The aniline formation proceeds via a nucleophilic aromatic substitution mechanism. Product liberation proceeds by subsequent ligand exchanges. These results led to the design of spectroscopic control experiments, giving indications for the deprotonated Cu(II)-amido complex. In the third project, the reaction of acetylene with formaldehyde was investigated, selectively yielding propargyl alcohol while suppressing the second reaction to butynediol. Optimization studies established a Cu(I) catalyst with a cheap and air-stable phenanthroline ligand. Quantum-chemical investigations on phenylacetylene conducted in this work suggest that the reaction mechanism is preferably mediated by a mononuclear active species. The mechanism was transferred to the acetylene system. Kinetic modeling indicated that the selectivity to propargyl alcohol primarily results from concentration effects. The fourth project describes computational studies for a bismuth-catalyzed C−N coupling. Experimentally, a mixture of a C−N and a C−O coupled product was observed, where the selectivity depends on the catalyst. Detailed DFT studies showed that the reductive elimination is the selectivity-determining step. Multiple pathways were found for the reductive elimination, with the energetic order depending on the catalyst. Statistical modeling was performed to achieve an interpretable multivariate linear regression model. The model enabled the analysis of how ligand electronic and steric properties affect reductive elimination by stabilizing a cationic substructure.
Philipp Dominik Engel (Thu,) studied this question.