‘Green’ Catalysts for ecologically responsible Oxidation of Organic Substrates and Renewable Resources
Sustainable development, responsible care, and process integrated protection of the environment, which will result in enhanced customer satisfaction, are the inevitable guidelines which will carry modern chemistry into the new millennium. A high percentage of industrially applied, catalyst mediated processes are based on oxidative transformations of substrates. Especially in the production of pharmaceuticals, fine- and special-chemicals, the amount of by-products is still too high with regard to the corresponding volume of value product, in spite of significant process improvements during the last decades. Thus, a continuous effort is necessary to develop more selective oxidation catalysts, which are able to activate cheap and ecologically most favorable oxidants: oxygen and hydrogen peroxide. With the intensive investigation of the chemistry of organorhenium-oxides in high oxidation states, a new class of catalysts with outstanding properties as well as with an unexpected broad field of possible applications was discovered.
Methyltrioxorhenium (MTO) and its congeners turned out to be highly active catalysts, due to their expressed Lewis acidity at the metal center. In the pole position, oxidation catalysis should be mentioned first. Alkylrhenium-oxides as catalyst precursors form with hydrogen peroxide, almost at any concentration of the latter, oxidative highly active mono- and bis-peroxo complexes which can transform oxygen atoms onto a very broad variety of substrates, to be chosen from various classes of organic compounds e.g.: olefins to epoxides, diols, aldehydes or acids, alcohols to aldehydes or acids, arenes to hydroxy arenes or quinones, amines to N-oxides, sulfides to sulfoxides or sulfones.
Now, our main focus lies on the investigation and development of sustainable ‘green’ oxidation technologies, optional working in two- or multi-phase systems, saving energy and resources: after the development of a new, rhenium based, route to vitamin K3 and vitamin E derivatives, we are currently working on the clean oxidative conversion of various renewable resources like carbohydrates (e.g. starch oxidation) and the synthesis of fragrances, which are derived from natural precursors. Therefore, we developed a novel four-component system with MTO as catalyst for the selective oxidation of terminal alcohols, either to the corresponding aldehyde or to the carboxylic acid depending on reaction parameters. Applied on the terminal alcohol group (C6) in carbohydrates (especially starch) this new system opens perspectives for the broadly desired industrial application in this area.
1. Herrmann; W.A.; J. Organomet. Chem. 1995, 500, 149.
2. Herrmann, W.A.; Kühn, F.E.; Acc. Chem. Res. 1997, 30, 169.
3. Herrmann, W.A.; Kratzer, R.M.; Ding, H.; Thiel, W.R.; Glas, H.; J. Organomet. Chem. 1998, 555, 293.
4. Herrmann, W.A.; Zoller, J.P.; Fischer, R.W.; J. Organomet. Chem. 1999, 581, 404.