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Current Research (Update 2020)

Silicon Cation Chemistry

The chemistry of silicon cations is at the core of our research program. Our strong interest in fundamental aspects of their preparation, isolation, and characterization is paired with our desire to apply these super strong Lewis acids in catalysis. Our laboratory designed a new type of silicon cation where the electron deficiency at the silicon atom is compensated by the metallocenyl group, not by the metal center alone. An extreme dip angle of the silicon atom toward the metal atom is explained by two three-center/two-electron bonds with participation of both the upper and lower cyclopendienyl rings of the metallocene sandwich structure. These and other intramolecularly stabilized silylium ions are exceptionally potent in Lewis acid catalysis, e.g., in difficult Diels­–Alder reactions of cyclohexa-1,3-dienes at ambient or even lower temperatures. Recent developments include a protolysis approach to silylium ions, culminating in the synthesis of the ‘fat’ proton H3Si+. That strategy also opens new avenues in catalysis.


Transition-Metal-Free Transfer Processes

We discovered that adequately substituted cyclohexa-1,4-dienes are excellent surrogates for hydrosilanes (with the electrofugal silicon group in one of the bisallylic positions) and dihydrogen, respectively. Treatment of these precursors with strong main-group Lewis acids leads to hydride abstraction from the methylene group, thereby generating a Wheland intermediate that either releases a silylium ion or proton. The net reaction is a two-step formation of H/Si+ or H/H+. The latter is rather unexpected as the corresponding Wheland complex is a high-energy intermediate. These processes have been coupled with reductions catalyzed by the same Lewis acid. Ionic transfer hydrosilylation had been totally unprecedented, and transition metal-free transfer hydrogenation employing cyclohexa-1,4-dienes as reducing agents is exceptionally rare. Recently, this concept has been extended to a broad range of intriguing transition-metal-free transfer reactions such as transfer hydrocyanation and transfer hydromethallylation. Transfer hydrohalogenation reactions have been made possible by proton catalysis employing specifically designed cyclohexa-1,4-diene platforms.


Kinetic Resolution by Alcohol Silylation

The non‐enzymatic kinetic resolution of racemic mixtures of alcohols by silylation had been unknown before the turn of the century. This stands in stark contrast to established acylation techniques. Our laboratory introduced kinetic resolution by dehydrogenative Si–O coupling, initially employing silicon-stereogenic hydrosilanes. The current state of the art is catalyst-controlled variants that enable the kinetic resolution of a broad range of highly functionalized secondary and, remarkably, tertiary alcohols.


Cooperative Bond Activation of Interelement Bonds

We recognized at an early stage that transition-metal alkoxides/hydroxides are perfectly suited to react with various interelement bonds through σ-bond-metathesis-type processes. With our focus on synthetic silicon chemistry, we were particularly interested in the transmetalation of the silicon–boron linkage that yields a transition-metal silanide along with a boron byproduct. The catalytically generated silicon nucleophile then participates in nearly all typical asymmetric carbon–silicon bond-forming reactions, and we have been able to solve several long-standing challenges in enantioselective catalysis involving silicon, e.g., conjugate addition, 1,2-addition, and allylic substitution.


Recently, we discovered that subtle modification of the reaction setup can turn these ionic processes into radical reactions. This has been demonstrated for C(sp3)–Si cross-coupling reactions of a broad range of unactivated alkyl electrophiles and redox-active aliphatic carboxylic acid derivatives. Activated alkyl electrophiles still engage in ionic SN2-type displacements.


The same concept is the basis for our “bioinspired” approach to the catalytic generation of silicon as well as boron, aluminum, and tin electrophiles. Similar to the dihydrogen activation mechanism of hydrogenases, we accomplished the heterolytic splitting of element–hydrogen bonds at transition-metal thiolates into sulfur-stabilized silicon, boron, aluminum, and tin cations and a transition-metal hydride. As before, the activation mode is best described as a σ-bond metathesis. By this, we have been able to develop several unique transformations, e.g., electrophilic aromatic substitution with release of dihydrogen to achieve C–H bond silylation and borylation as well as partial reduction of pyridines and related benzannulated congeners.


Main-Group Lewis Acids for Si–H and H–H Bond Activation

With our expertise in silicon chemistry and Lewis acid catalysis, we became involved in the recent topic of frustrated Lewis pair (FLP) chemistry. Our contribution lies in the preparation of new chiral derivatives of the electron-deficient boron Lewis acid B(C6F5)3 and their application in enantioselective reduction of C=X bonds. However, these chiral Lewis acid catalysts are also useful for other challenging reactions such as Nazarov cyclizations. Defunctionalization reactions are another focus. Our contributions to FLP chemistry include difficult hydrogenations (of oximines without cleavage of the fragile N–O bond to yield hydroxylamines) as well as the characterization of frustrated radical pairs (FRPs) based on silicon.


Investigation of Reaction Mechanisms

Our group’s track record of mechanistic work is documented by a series of high-level publications in diverse areas of catalysis. We usually rely on experimental and spectroscopic techniques and, if needed, seek collaboration with theoretical groups (Stefan Grimme, Martin Kaupp, Mu-Hyun Baik, and Peter Hrobárik). Recently, we have been able to implement our knowledge in silicon cation chemistry to synthesize key intermediates and test their kinetic competence in catalytic cycles. These investigations have been crucial in the mechanistic understanding of fundamental processes such as catalysis with B(C6F5)3 (and related FLP chemistry) and various carbonyl hydrosilylation reactions.




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