Harnessing the Unquenched Reactivity of Novel Frustrated Lewis Pairs for Transition Metal-Free Hydrogenation Catalysis

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The transition metal free heterolytic cleavage of molecular hydrogen via frustrated Lewis pairs (FLPs) pioneered by Stephan is an innovative concept in sustainable chemistry. It has the potential of replacing expensive, less abundant and toxic precious metals in their classical domain, the catalytic hydrogenation of unsaturated organic species. FLPs are sterically encumbered Lewis acid-base pairs of unquenched reactivity due to their inability to form classical Lewis acid-base complexes. Frequently used FLP catalysts are combinations of bulky phosphines and B(C6F5)3 or related fluorinated boranes. These hydrogenation catalysts are expensive and exhibit poor activities due to the strength of the product-boron bond, which inhibits product liberation. The goal of this collaborative research project is to improve transition-metal free catalytic hydrogenation of olefins, imines and ketones as sustainable alternative to established catalytic systems. In combination with the design of tailored FLP catalysts, recycling strategies will be developed. The investigated catalysts will initially be based on a set of established structural motifs as lead structures for more detailed investigations. Novel combinations of Lewis acids and bases, capable of reversibly cleaving dihydrogen, will be identified by a combined computational/experimental approach. Attractive weak Lewis acids are boronic and boric esters, as these compounds can readily be synthesized from inexpensive starting materials or are commercially available at low cost. Phosphazenes and guanidines will be used as base components. The progress in catalyst development will be amplified with the combination of modular FLP synthesis, detailed catalytic testing and computational investigations on novel catalysts and the underlying catalytic mechanisms. Spectroscopic studies using special high-pressure NMR equipment will provide refined information on the catalytic activity of the FLPs. The ultimate goal of this proposal, however, is the development of highly active, durable and recyclable FLP catalysts able to selectively hydrogenate olefins, imines and ketones under mild conditions.