Manganese complex catalysts for hydrogenation and dehydrogenation reactions

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The sustainable handling of the resources on our planet is a central challenge of humankind. Noble metals are rare, expensive and are used diversely in key technologies. Catalysis is such a key technology. Manganese is the third most abundant transition metal in the earth’s crust and a promising “replacement” for noble metal catalysts in hydrogenation and dehydrogenation reactions. Such a “replacement” becomes especially interesting if new reactivity or selectivity are accessible. In three recent (2016 and 17) preliminary work manuscripts published in Angewandte Chemie, we introduced novel highly active manganese catalysts for important hydrogenation and dehydrogenation reactions. We focus on three aims in the proposal. 1. A better understanding of how PNP-ligand stabilized manganese complexes act in hydrogenation or dehydrogenation reactions and thereof derived novel catalytic applications. 2. The development of novel manganese catalysts based on (chiral) PNN-pincer ligands. 3. The development of novel Mn-catalyzed reactions in which amino alcohols are converted into important classes of compounds. 1. Mechanistic studies of hydrogenation and dehydrogenation catalysis: In the preliminary work, we could show that manganese catalysts are an interesting alternative to noble metal catalysts in the hydrogenation of C=O bonds (Angew. Chem. 2016, paper 1). Right now we don’t know how the catalytic cycle proceeds and how the activation of substrates takes place. We thus propose mechanistic studies, which also permit a more rational catalyst design. The obtained insight should be used for the hydrogenation of imines in the presence of hydrogenation sensitive functional groups. Beside hydrogenation we are interested in mechanistic aspects of the dehydrogenation, especially the role of the base.2. Novel (chiral) catalysts: Progress in manganese based hydrogenation and dehydrogenation catalysis is strongly linked with the development of novel catalysts. Thus, we plan the development of a novel (also chiral) catalyst class. Therefore, a simple and broadly variable ligand synthesis is applied. The novel catalysts are used comparative in a variety of reactions. With regard to enantioselective catalysis, for instance, the hydrogenation of imines is of interest. 3. Novel alcohol re-functionalization reactions: In our preliminary work, we could demonstrate a Mn-Ir analogy in dehydrogenative condensation reaction for the synthesis of aromatic N-heterocyclic compounds (2 x Angew. Chem. 2017, paper 2 and 3). Thus, the development of novel syntheses of heterocycles using manganese catalysts should be possible in the future. Beside the base-tunable Imine-amine synthesis, the development of a novel pyrimidine and imidazole synthesis is proposed. Preliminary work support the feasibility of them.