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Selective Hydrogenation of Arenes - A Dream Reaction

Periodic Reporting for period 4 - HyDream (Selective Hydrogenation of Arenes - A Dream Reaction)

Okres sprawozdawczy: 2023-04-01 do 2023-09-30

Conceptually, the direct hydrogenation of unsaturated compounds is an ideal reaction, one able to fulfil the principles of Green Chemistry. It exhibits perfect atom economy, as all employed atoms end up in the product and uses inexpensive hydrogen gas, which can be obtained from renewable sources. Hence, the process is both ecologically sustainable and cost-efficient. Its catalytic nature obviates the need for stoichiometric hazardous reagents and reduces waste. The high energy efficiency enabled by the often mild conditions further adds to its sustainability. Consequently, the hydrogenation of unsaturated substrates, such as ketones or olefins, has been widely explored and has been awarded with several Nobel Prizes.
The relatively unexplored hydrogenation of aromatic systems offers additional benefits, such as the conversion of readily available flat frameworks into elusive complex saturated (hetero)cycles. These are attractive motifs throughout the chemical sciences, especially for pharmaceutical and agrochemical research. Given effective control over chemo-, diastereo- and enantioselectivity, an astonishing amount of complexity can be built up in a single chemical step making it a dream reaction for use in an ideal synthesis. As the ultimate goal, this should provide efficient and unique access to valuable building blocks for the synthesis of pharmaceuticals and other important compounds.
Many different challenges have been tackled and interesting results have been obtained. These include homogeneous and heterogeneous catalytic methods. Whereas the latter ones are literally more powerful and can be used for the hydrogenation of less reactive substrates, such as benzene derivatives, the former ones are more easily developed into enantioselective methods, suitable for the hydrogenation of heteroarenes (which are typically less aromatic, less stabilized). In both fields we have been able to develop new catalysts and – very challenging - explore the mode of action of some of these catalysts. In general, we have a focus on practical methods (for example, low-pressure hydrogenation or being able to tolerate sulfur-based functional groups) and on innovative methods. A current highlight is the challenging use of chlorine-containing substrates (while keeping the C-Cl bond intact) and the design of new enantioselective catalysts, homogeneous and heterogeneous.
The production of fluorinated piperidines by hydrogenation of fluoropyridines has been a highlight of our work, which is beyond the art. The same is true of other goals we have published on already, such as low pressure hydrogenation. This could lead to a popularization of aromatic hydrogenation, because no special high pressure autoclaves would be needed anymore. In addition, more robust, chemoselective arene hydrogenations (tolerating sulfur-based groups, tolerating chlorine-substituents and related) as well as enantioselective variants for many classes of substrates, would also expand the scope of this important, rather green transformation.
Pyridine Hydrogenation