Towards Enantioselective Carboxylation and Hydrogenation Reactions (Quantum Chemical Modelling of Homogeneous Reactions)

Abstract

With the aid of DFT methods, it is possible to get insights into the mechanistic details of homogeneous reactions, the substrate preferences and activities of catalysts. Computational methods can also help to identify the selectivity-determining factors that govern asymmetric reactions. In this thesis, DFT methods are applied in order to study the enantioselective addition of small molecules, such as CO2 and H2, to alkenes in order to form saturated carboxylic acids and alkanes.

Both rhodium-mediated hydrocarboxylation and cobalt-mediated hydrogenation reactions were investigated using the popular DFT functionals PBE and B3LYP, including dispersion corrections. First, the nonselective Rh-cyclooctadiene(COD)-catalyzed hydrocarboxylation of alkenes with CO2 was studied by employing the PBE-D2 functional. Several styrene derivatives and α,β-unsaturated compounds were analyzed. Our computational investigation of Rh-COD-benzyl complexes revealed an unusual TS for the C-CO2 bond formation step, where CO2 does not interact with the metal center and the substrate is coordinated to the metal in a η6-fashion via a phenyl ring. The study was expanded by analyzing the potential of five chiral ligands, (S)-SEGPHOS, (R,R)-BDPP, (R,R)-tBu-BOX, (S)-iPr-PHOX, and (R)-StackPhos, to form Rh-based catalysts for asymmetric hydrocarboxylations. Interestingly, the preferred carboxylation TSs with chiral Rh-complexes display a similar substrate binding mode as with the achiral COD ligand and also show a preference for outer sphere CO2 insertion. However, the results indicate that different CO2 insertion paths, frontside or backside, are possible, dependent on the nature of the ligand. For ligands containing an N-heterocyclic ring, it is shown that CO2 is able to form stacking interactions with the ring, which for several ligands results in a preference for frontside CO2 insertion.

Second, a detailed mechanistic investigation of Co-catalyzed asymmetric hydrogenation of enamides was performed at the B3LYP-D3 level of theory. The study of enamides with very different molecular structures shows that two mechanistic pathways appear accessible, both a non-redox Co(II) mechanism proceeding through metallacycle intermediates and a more classical redox Co(0)/Co(II) mechanism. The obtained barriers indicate that these mechanisms may be competing. It is also shown that explicit solvent affects the computed barriers significantly and that its inclusion appears to be crucial for the proper estimation of the enantiomeric excesses of Co-catalyzed hydrogenation of enamides.