In Silico Design and Synthesis of Novel Metal-Free Systems for Bond Activation and Catalysis

• Applicant: Dr. Urs Gellrich
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405989762

In this interim report, we report on our work towards the computational design and the synthesis of novel, metal-free systems for bond activation and catalysis. Specifically, we discuss the synthesis of pyridonate boranes, which can be described as frustrated Lewis Pairs (FLPs) and are able to split dihydrogen reversibly. A conceptually new aspect of these FLPs is that hydrogen activation turns the pyridonate borane into a datively bound pyridone borane complex. The dissociation of these pyridone borane complexes enables the hydroboration of olefins, alkynes, and allenes. We used the pyridonate boranes as catalysts for hydrogenations and alkyne-dimerizations as well as for the formation of nucleophilic allylboranes from molecular hydrogen and allenes. Furthermore, we report on our work towards the synthesis of self-associating and carbogenic FLPs. We further discuss the organocatalytic dehydrogenation of ammonia borane with a thiopyridone catalyst and the application of strong boron based Lewis acids, which react with alkynes and allenes via direct carboborations, in organic synthesis. In a renewal proposal for a sixth year, we propose three research projects. The goal of the first research project is the development of an atom-economic, metal-free allylation of aldehydes, ketones, and imines catalyzed by a borane-based frustrated Lewis pair. Here, we build on our earlier work on the formation of nucleophilic allylboranes from molecular hydrogen and allenes mediated by our pyridonate borane and extend this work regarding the synthesis and use of water-tolerant boranes. The second research project is based on our recently reported finding that a sterically encumbered imidazolone turns the simplest borane, i.e. BH3, into an active catalyst for the semihydrogenation of alkynes. The optimization of this catalyst system by an interplay of computational and experimental studies is the objective of the second research project. Having established a protocol for the organocatalytic dehydrogenation of ammonia borane, developing a method for the regeneration of spent ammonia borane, i.e. borazine and polyborazylene, is the third research objective for the sixth year. We propose a photocatalytic strategy for the hydrogenation of borazine. As the hydrogen source, we plan to use thioles, which will be oxidized to the respective disulfides during the photocatalytic process.

• DFG Programme: Independent Junior Research Groups