Johannes Teske

Bio: Johannes Teske is an academic researcher from University of Stuttgart. The author has contributed to research in topics: Cycloisomerization & Lewis acids and bases. The author has an hindex of 6, co-authored 10 publications receiving 140 citations.

Carbocations as Lewis Acid Catalysts: Reactivity and Scope

Abstract: The strength of efficient metal-free catalysis will be examined in this thesis. Efforts towards more sustainable processes will be demonstrated through implementation of strategies that meet several of the 12 principles of Green Chemistry.In the first part, a stereoselective total synthesis of multiple alkaloids from the Corynantheine and Ipecac families together with their non-natural analogues will be disclosed. A highly efficient, common synthetic strategy is applied leading to high overall yields starting from easily available starting material. Overall operational simplicity and sustainability have been the main focus. Time-consuming and waste-generating isolations and purifications of intermediates have been minimized, as well as the introduction of protection-group chemistry. Moreover, the first example of the total synthesis of Hydroxydihydrocorynantheol together with its non-natural epimer has been accomplished in multi-gram scale without protection groups and without a single isolation or purification step in high overall yield and diastereoselectivity.In the second part, carbocations will be presented as highly effective and versatile non-metal Lewis acid catalysts. Lewis acidity-tuning of carbocations will be introduced and applied in several reactions to suppress competing reactions. Finally, the broad scope of carbocation catalyzed transformations will be exposed.At large, evident progress has been made towards more sustainable chemistry.

Olefin Metathesis in Confined Geometries: A Biomimetic Approach toward Selective Macrocyclization.

Abstract: The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.55, corresponding to 35% MMC product obtained with the homogeneous catalyst, up to 1.49, corresponding to 60% MMC product. A correlation between the MMC/O ratio and the substrate-to-pore-size ratio was successfully established. Modification of the inner pore surface with dimethoxydimethylsilane allowed fine-tuning the effective pore size and reversing surface polarity, which resulted in a further increase of the MMC/O ratio up to 2.2, corresponding to >68% MMC product. Molecular-level simulations in model pore geometries help to rationalize the complex interplay between spatial confinement, specific (substrate and product) interaction with the pore surface, and diffusive transport. These effects can be synergistically adjusted for optimum selectivity by suitable surface modification.

Non-decarbonylative photochemical versus thermal activation of Bu4N[Fe(CO)3(NO)] – the Fe-catalyzed Cloke–Wilson rearrangement of vinyl and arylcyclopropanes

Abstract: The base metal complex Bu4N[Fe(CO)3(NO)] (TBA[Fe]) catalyzes the rearrangement of vinyl and arylcyclopropanes both under thermal or photochemical conditions to give the corresponding vinyl or aryldihydrofurans in good to excellent yields. Under photochemical conditions the reaction is performed at room temperature. Spectroscopic investigations show that the metal carbonyl catalyst is not decarbonylated. The best performance was observed at a wavelength of 415 nm. icMRCI+Q analysis of the excited singlet and triplet states of the [Fe(CO)3(NO)] anion was performed and used to calculate the vertical excitation energies which are in good agreement with the experimental data. CASSCF analysis indicates that the Fe center in all excited states of the ferrate becomes more electrophilic while adopting a distorted tetrahedral configuration. Both aspects have a positive synergistic effect on the formation of the initial π-complex with the incoming organic substrate.

A Redox-Neutral Fe-Catalyzed Cycloisomerization of Enyne Acetates

Abstract: The cationic complex [(Ph3P)2Fe(CO)(NO)]BF4 catalyzes the redox-neutral cycloisomerization of enyne acetates to diastereomerically pure allenyl pyrrolidines in good to excellent yields.

Organocatalysis in Inert C–H Bond Functionalization

Abstract: As two coexisting and fast-growing research fields in modern synthetic chemistry, the merging of organocatalysis and C–H bond functionalization is well foreseeable, and the joint force along this line has been demonstrated to be a powerful approach in making inert C–H bond functionalization more viable, predictable, and selective. In this review, we provide a comprehensive summary of organocatalysis in inert C–H bond functionalization over the past two decades. The review is arranged by types of inert C–H bonds including alkane C–H, arene C–H, and vinyl C–H as well as those activated benzylic C–H, allylic C–H, and C–H bonds alpha to the heteroatom such as nitrogen and oxygen. In each section, the discussion is classified by the explicit organocatalytic mode involved.

Allosteric initiation and regulation of catalysis with a molecular knot

Abstract: Molecular knots occur in DNA, proteins, and other macromolecules. However, the benefits that can potentially arise from tying molecules in knots are, for the most part, unclear. Here, we report on a synthetic molecular pentafoil knot that allosterically initiates or regulates catalyzed chemical reactions by controlling the in situ generation of a carbocation formed through the knot-promoted cleavage of a carbon-halogen bond. The knot architecture is crucial to this function because it restricts the conformations that the molecular chain can adopt and prevents the formation of catalytically inactive species upon metal ion binding. Unknotted analogs are not catalytically active. Our results suggest that knotting molecules may be a useful strategy for reducing the degrees of freedom of flexible chains, enabling them to adopt what are otherwise thermodynamically inaccessible functional conformations.

Automated Construction of Molecular Active Spaces from Atomic Valence Orbitals

Abstract: We introduce the atomic valence active space (AVAS), a simple and well-defined automated technique for constructing active orbital spaces for use in multiconfiguration and multireference (MR) electronic structure calculations. Concretely, the technique constructs active molecular orbitals capable of describing all relevant electronic configurations emerging from a targeted set of atomic valence orbitals (e.g., the metal d orbitals in a coordination complex). This is achieved via a linear transformation of the occupied and unoccupied orbital spaces from an easily obtainable single-reference wave function (such as from a Hartree–Fock or Kohn–Sham calculations) based on projectors to targeted atomic valence orbitals. We discuss the premises, theory, and implementation of the idea, and several of its variations are tested. To investigate the performance and accuracy, we calculate the excitation energies for various transition-metal complexes in typical application scenarios. Additionally, we follow the homoly...

Synthetic applications of vinyl cyclopropane opening.

Abstract: Vinyl cyclopropanes are amongst the most useful building blocks in organic synthesis. Their easy opening and capacity to generate dipoles have been exploited for the synthesis of cyclopentanes with good yields and sometimes excellent stereoselectivities. In this review we give an overview of their applications, focusing on the present century.

Iron‐Catalyzed Intramolecular C(sp2)−H Amination

Abstract: The nucleophilic iron complex Bu4N[Fe(CO)3(NO)] (TBA[Fe]) catalyzes the direct intramolecular C-H amination of α-azidobiaryls and (azidoaryl)alkenes into the corresponding carbazoles and indoles, respectively, under mild conditions and with low catalyst loadings. These features and the broad functional-group tolerance render this method a particularly attractive alternative to established noble-metal-based procedures.