Mechanisms in homogeneous catalysis
Dr. Hans-Joachim Drexler
Current research: asymmetric hydrogenation of α-, β-dehydroamino acid derivatives, itaconic acid derivatives and ketones.
In contrast to a huge number of selection inducing ligands, mainly known chiral ligands for homogeneous catalysis, there are only few investigations that deal with kinetic aspects of reactions. The asymmetric hydrogenation of prochiral olefins with cationic Rh(I)-complexes is one of the best understood selection processes. It is appropriate to regard the resulting so-called major/minor principle – the minor substrate complex, less stable in solution, dominates the selectivity due to its high reactivity –as a basic concept of homogeneous catalysis.
Recently, we succeeded in supplying the evidence that the dominating intermediate – the so called major substrate complex – can also lead to the main product of the selection process. This corresponds in principle to the lock-and-key concept well known from enzyme kinetics.
In our surveys on asymmetric hydrogenation as a model reaction, we are investigating the extension of the model, the transformation of pre-catalysts into active species, the influence of intramolecular and pre-equilibria, the concentration ratio of the intermediates and the origin of different levels of reactivity. In addition, the influence of solvent, temperature and pressure on selectivity and reactivity is of interest.
On the one hand, a better understanding of the mutual dependences offers a deep insight into the phenomenon of catalysis; on the other hand it provides a possibility for optimization.