Overview: Research in the group
The group is generally interested in the development of synthetic methods with a high atom efficiency, in order to reduce the overall waste generated (green and sustainable chemistry). In this vein, the use of dihydrogen (H2) in catalytic transformations as possible replacement for stoichiometrically used reducing agents has been a main focus of research.
In the projects dealing with method development, the group strives to develop catalysts which allow for high control of chemo-, regio- and/or stereoselectivity in the production of useful synthetic building blocks in organic chemistry. In this vein, we take two main approaches: On the one hand, we are interested in new reactivity employing readily available 3d metals (base metals). On the other hand, we aim at the use of dihydrogen (H2) not primarily in hydrogenation reactions, but rather in catalytic hydride transfer reactions and H2-mediated reductive coupling reactions, among others.
The group is also interested in the development of bifunctional catalysts, which allow for fundamentally new transformations. Especially the combination of transition metal catalysis and organocatalysis has been a center of attention.
Next to the projects more oriented at method development, we have also started a program on target-oriented synthesis, with a current focus on caged hydrocarbon framworks. Stay tuned.Research Projects in More Detail:
Research Projects in More Detail:
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Catalytic Homogeneous Hydrogenation
The group has pushed the field of homogeneous copper-catalyzed alkyne semihydrogenation by putting forward copper(I)/N-heterocyclic carbene (NHC) complexes that effect highly chemo- and stereoselective alkyne semihydrogenations. The NHC ligands steer the reactivity to bring about a high Z-selectivity and negligible overreduction to the alkane, thus displaying a high chemoselectivity. Key to the approach is the presence of a Copper-Oxygen bond that allows for the heterolytic H-H-bond activation. Several catalysts have been developed (Chem. Eur. J. 2015, 21, 15934-15938; Org. Biomol. Chem. 2016, 14, 10660-10666; Synthesis 2017, 49, 2470-2482; Synlett 2019, 30, 783-786 and the substrate scope has been widened to diynes and enynes (Tetrahedron 2017, 73, 5023-5028). The related conjugate reduction of enoates with H2 has also been developed (Chem. Commun. 2019, 55, 2293-2296).
For an overview, see: Homogeneous Hydrogenation with Copper Catalysts (Book Chapter) in Homogeneous Hydrogenation with Non-Precious Catalysts, Wiley, Weinheim, 2019.
More recently, our group has developed a simple and practical protocol to effect the synthetically much more challenging E-selective alkyne semihydrogenation. In this method, a wide variety of internal alkynes can be converted with high stereoselectivity to E-alkenes with a remarkably high tolerance of functional groups (Chem. Eur. J. 2020, 26, 1597-1603).
Catalytic Transfer Hydrogenation
While our main focus lies on the use of cheap and readily available dihydrogen (H2), we have shown that our copper(I)/NHC catalysts can also be employed in a transfer hydrogenation setting (alkyne semihydrogenation as well as conjugate reduction). In these protocols either ammonia borane (Chem Commun. 2017, 53, 732-735) or simple alcohols (Chem. Commun. 2019, 55, 13410-13413) can be used as H2 equivalents. The overall transformation mirror the high selectivity of the hydrogenative variants, however, they do not require high pressure equipment, making the protocols much simpler.
Reductive Coupling Reactions
Emanating from the abovementioned catalytic hydrogenation protocols, we strive to develop reductive coupling reactions by trapping the reactive intermediates of the (transfer) alkyne semihydrogenation with electrophiles or coupling reactions. In this manner, we have developed a highly stereo- and regioselective formal hydrohalogenation of alkynes using the transfer hydrogenation protocol with ammonia borane (Org. Lett. 2018, 20, 4926-4929).
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Catalytic Hydride Transfer from H2
Building upon a heterolytic H2 activation with copper catalysts, we have been able to develop a catalytic hydride transfer from H2. In this manner, the stoichiometric use of common complex reducing agents (such as borohydrides or aluminum hydrides) can be circumvented by a catalytic protocol and H2, rendering the overall transformation much more atom efficient. We have showcased this reactivity in an allylic reduction, namely the regioselective hydride transfer to allylic acceptors (Chem. Eur. J. 2019, 25, 985-988), a reaction that we had develop with hydrosilanes in a stereoselective fashion earlier (Org. Lett. 2016, 18, 2455-2458 ;Chem. Commun. 2017, 53, 11686-11689). One of the highlights of the catalysts is their remarkable chemoselectivity: Whereas H2 is activated, alkenes are not hydrogenated.
© AK Teichert
Combining Transition Metal Catalysis and Organocatalysis
In the realm of the excellence cluster Unifying Concepts in Catalysis, we are exploring bifunctional catalyst frameworks in which we combine catalytic units constructed with transition metals and with organocatalysts. In this manner, we hope not only to tune the reactivity of the transition metal, but to access fundamentally new reactivities in general. In this project, ligand design and synthesis of new multifunctional molecules is the main focus.
© AK Teichert
In our group, we look through the eyes of synthetic organic chemists, which is why we also move forward some synthesis-based projects. At the moment, we are in the midst of the synthesis of several molecules with attractive molecular frameworks, but the whole thing is still going on. Stay tuned... :-