Iron-catalyzed asymmetric hydrogenations and transfer hydrogenations of ketones

Platinum metal-based hydrogenations range amongst the most efficient processes for the large scale production of chemicals such as pharmaceuticals, herbicides or fragrances. Though, the constantly rising price of ruthenium, rhodium, iridium and platinum based catalysts as well as severe environmental concerns have initiated research aiming for a replacement of these precious metals by the much cheaper and, above all, less toxic iron. Very recently, first breakthroughs and promising results have been achieved in the iron catalyzed asymmetric transfer hydrogenation of polar double bonds and it is expected that in the near future, for transfer hydrogenation reactions, ruthenium can be replaced by iron. So far, efficient iron-based asymmetric hydrogenation catalysts have not yet been reported. Therefore, the main intention of this project is the further development of iron-based catalysts for the asymmetric hydrogenation and in part also for transfer hydrogenations of polar double bonds. For this purpose the coordination sphere of the existing iron complexes will be modified by planar chiral and tetradentate PNNP ferrocene derivatives and by tridentate PNP pincer ligands bearing chiral phospholane units. With these novel iron complexes mainly hydrogenations but also transfer hydrogenations will be carried out. Subsequently, within the framework of an international cooperation with the group of Prof. R. H. Morris (University of Toronto) extensive mechanistic studies will be carried out. These studies are directed towards a better understanding of the catalysis processes, but will also provide a rational for further optimization of such catalyst systems. Furthermore, it is intended to search for relations between the catalyst structures and the experimentally established product absolute configurations. It is expected that within the next few years many existing industrial productions will be replaced by not only more economically but also more environmentally benign processes and this project shall contribute to this fast evolving topic.

In a large number of industrial processes for the production of chemicals including pharmaceuticals, herbicides or fragrances catalysts based on noble transition metals are used. Though, the constantly rising price of ruthenium, rhodium, iridium and platinum based catalysts as well as severe environmental concerns have initiated research aiming for a replacement by the much cheaper iron. Iron represents the most abundant transition metal, non-toxic, inexpensive, and ubiquitously available. Driven by these ideas this project aimed for the development of iron based coordination complexes suitable as catalyst precursors for asymmetric hydrogenations. During the course of this project, a number of enantiopure pincer ligands and corresponding iron complexes were synthesized. Especially iron complexes of enantiopure and ferrocene based PNP pincer ligands turned out to be very promising catalyst precursors for asymmetric hydrogenations. In a number of test reactions it was shown that with these well-defined iron complexes ketones can be hydrogenated at room temperature, with full conversion and with high enantioselectivity to chiral, non-racemic alcohols. Based on these results, this concept was extended to manganese. At present, interest in manganese as an alternative to noble metals such as ruthenium, rhodium, iridium or platinum is enormous, not only since like iron manganese is a cheap and a very abundant element (third abundant in the earth crust) but also since resent research has shown its high potential in catalysis. Within this context, manganese complexes of enantiopure PNP ligands have been synthesized and tested as catalyst precursors. It was shown for the first time that with PNP ligands modified manganese complexes can be used efficiently as catalysts in asymmetric transfer hydrogenations of ketones. Alcohols with a very high enantioselectivity (>90%) were obtained. These results clearly show again the high potential of manganese in catalysis, especially in asymmetric hydrogenation. It is expected that within the next few years many existing industrial productions will be replaced by not only more economically but also more environmentally benign processes and it was the intention of this project to contributed to this fast evolving topic.