Die Kation-π-Wechselwirkung in der Katalyse mit niedermolekularen Verbindungen

Boronic Acid-Rich Lanthanide Metal-Organic Frameworks Enable Deep Proteomics with Ultratrace Biological Samples

Label-free proteomics is widely used to identify disease mechanism and potential therapeutic targets. However, deep proteomics with ultratrace clinical specimen remains a major technical challenge due to extensive contact loss during complex sample pretreatment. Here, a hybrid of four boronic acid-rich lanthanide metal-organic frameworks (MOFs) with high protein affinity is introduced to capture proteins in ultratrace samples jointly by nitrogen-boronate complexation, cation-π and ionic interactions. A MOFs Aided Sample Preparation (MASP) workflow that shrinks sample volume and integrates lysis, protein capture, protein digestion and peptide collection steps into a single PCR tube to minimize sample loss caused by non-specific absorption, is proposed further. MASP is validated to quantify ≈1800 proteins in 10 HEK-293T cells. MASP is applied to profile cerebrospinal fluid (CSF) proteome from cerebral stroke and brain damaged patients, and identified ≈3700 proteins in 1 µL CSF. MASP is further demonstrated to detect ≈9600 proteins in as few as 50 µg mouse brain tissues. MASP thus enables deep, scalable, and reproducible proteome on precious clinical samples with low abundant proteins.

Asymmetric Organocatalysis Under Mechanochemical Conditions

Asymmetric organocatalysis is a robust methodology providing access to numerous valuable compounds while having green chemistry principles in mind. The realization of organocatalytic transformation under solvent-free mechanochemical conditions brings additional benefits in terms of yields, selectivities, and, last but not least overall improved sustainability. This overview describes developments in the use of mechanochemistry as a vehicle for asymmetric organocatalytic transformations. The material is organized according to main catalytic activation modes, starting with covalent activation and proceeding to non-covalent activation modes. The advantages of mechanochemical organocatalytic reactions are particularly highlighted, but in some cases also, limitations are mentioned. Possibilities for target compound synthesis are also discussed.

π-Face Promoted Catalysis in Water: From Electron-deficient Molecular Cages to Single Aromatic Slides

Exploiting noncovalent π-interactions particularly emerging anion-π interactions to drive efficient catalysis is fascinating. Even with exciting progresses, can anion-π activation operate in water remains elusive. Here we report the design, synthesis and catalytic studies of a class of water-soluble electron-deficient molecular cages and relevant aromatic slide compounds. The prism-like cages contain three divided, long, cationic aromatic walls which constitute three highly electron-deficient V-shape cavities. They were efficiently synthesized in two steps from a parent triformyl cage in gram-scale. Crystal structure showed the π-walls bind to the counter bromide through strong anion-π interactions. Just 5 mol% of cages were effective in catalyzing decarboxylative Aldol reactions of aldehydes and malonic acid half thioesters in water but not in organic solvents, showing a pronounced hydrophobic amplification effect. Meantime, a series of single π-slides resembling the π-wall of the cage performed equally well, while those lacking an extended π-surface were ineffective, highlighting the essential role of electron-deficient π-face on promoting the conversion.

Putting Anion-π Interactions at Work for Catalysis

Since its discovery two decades ago, anion-π interaction has been increasingly recognized as an important driving force. Extensive theoretical and experimental efforts on the ground-state anion-π binding and recognition have laid the bases for exploring its relevance in catalysis. Accordingly, the concept of "anion-π catalysis" that employing an electron-deficient π surface (π-acidic surface) for anionic reaction intermediate and transition state stabilization has emerged. This article shortly reviews the emergence and development of this concept, aiming to provide an emphasis on the general concept and key progress in this exciting area. To highlight the essential contribution of anion-π interactions, the contents are organized according to their role engaged in catalytic process, for example from both ground-state and transition-state stabilization to solely transition-state stabilization, mainly by a single π-face, and to cooperative π-face activation. A concluding remark and outlook on future development of this field is also given.

Exploiting Anion-π Interactions for Efficient and Selective Catalysis with Chiral Molecular Cages

Exploiting anion-π interactions in catalyst design is a fascinating direction to develop new and fundamental catalysis. For the appealing yet flexible π-face activation, can two or more π-acidic surfaces be manipulated for cooperative activation to achieve efficient transformation and particularly selectivity control is highly desirable. Here, we demonstrate a supramolecular π-catalysis strategy by establishing cooperative π-face activation in a confined electron-deficient cage cavity. The catalysts have a triazine based prism-like cage core and pendant chiral base sites. Only 2 mol % of cage catalyst efficiently catalyzed the decarboxylate Mannich reactions of sulfamate-headed cyclic aldimines and a series of malonic acid half thioesters in nearly quantitative yields and up to 97 % ee, enabling an unprecedent organocatalytic approach. The supramolecular π-cavity is essential in harnessing cooperative anion-π interactions for the efficient activation and excellent selectivity control.

π-π Catalysis in Carbon Flatland-Flipping [8]Annulene on Graphene

Noncovalent interactions are an integral part of the modern catalysis toolbox. Although stronger noncovalent interactions such as hydrogen bonding are commonly the main driving force of catalysis, π-π interactions typically provide smaller additional stabilizations, for example, to afford selectivity enhancements. Here, it is shown computationally that pristine graphene flakes may efficiently catalyze the skeletal inversions of various benzannulated cyclooctatetraene derivatives, providing an example of a catalytic process driven solely by π-π stacking interactions. Hereby, the catalytic effect results from disproportionate shape complementarity between catalyst and transition structure compared with catalyst and reactant. An energy decomposition analysis reveals electrostatic and, especially with increasing system size, to a larger extent, dispersion interactions as the origin of stabilization.

Polyether Natural Product Inspired Cascade Cyclizations: Autocatalysis on π-Acidic Aromatic Surfaces

Anion-π catalysis functions by stabilizing anionic transition states on aromatic π surfaces, thus providing a new approach to molecular transformation. The delocalized nature of anion-π interactions suggests that they serve best in stabilizing long-distance charge displacements. Aiming therefore for an anionic cascade reaction that is as charismatic as the steroid cyclization is for conventional cation-π biocatalysis, reported here is the anion-π-catalyzed epoxide-opening ether cyclizations of oligomers. Only on π-acidic aromatic surfaces having a positive quadrupole moment, such as hexafluorobenzene to naphthalenediimides, do these polyether cascade cyclizations proceed with exceptionally high autocatalysis (rate enhancements k_auto /k_cat >10⁴  m^-1 ). This distinctive characteristic adds complexity to reaction mechanisms (Goldilocks-type substrate concentration dependence, entropy-centered substrate destabilization) and opens intriguing perspectives for future developments.

Peptide Stapling with Anion-π Catalysts

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core-bridged NDIs matches that of one turn of an α helix. NDI-stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino-acid sidechains above the π-acidic NDI surface affords operational anion-π catalysts. According to an enolate chemistry benchmark reaction, anion-π catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated α-helix turns, and acyclic peptide controls. Elongation of isolated turns into short α helices significantly increases activity. This increase is consistent with remote control of anion-π catalysis from the α-helix macrodipole.

The Role of Organoferrates in Iron-Catalyzed Cross-Couplings

Recent groundbreaking studies on organoferrates have demonstrated that coordinatively unsaturated three-coordinate-σ-alkylferrates are active catalysts in Fe-catalyzed cross-couplings with Grignard reagents and that pronounced solvent and counterion effects dictate metalate speciation and catalyst activity. Thanks to modern spectroscopic methods, sensitive catalyst intermediates could be analyzed.

Enantioselective Catalysis of an Anionic Oxy-Cope Rearrangement Enabled by Synergistic Ion Binding

Charge-accelerated rearrangements present interesting challenges to enantioselective catalysis, due in large part to the competing requirements for maximizing reactivity (ion-pair separation) and stereochemical communication. Herein, we describe application of a synergistic ion-binding strategy to catalyze the anionic oxy-Cope rearrangement of a symmetric bis-styrenyl allyl alcohol in up to 75:25 e.r. Structure-reactivity-selectivity relationship studies, including linear free-energy-relationship analyses, with bifunctional urea catalysts indicate that H-bonding and cation-binding interactions act cooperatively to promote the chemo- and enantioselective [3,3]-rearrangement. Implications for catalyst designs applicable to other transformations involving oxyanionic intermediates are discussed.

A Facile Enantioselective Alkynylation of Chromones

The first catalytic enantioselective alkynylation of chromones is reported. In this process, chromones are silylated to form silyloxybenzopyrylium ions that lead to silyl enol ethers after Cu-catalyzed alkyne addition using StackPhos as a ligand. The outcome of the reaction is impacted by distal ligand substituents with differing electronic character and it was found that successful reactions could be achieved with different ligand congeners by using different solvents. This sequence enables access to different products by protonation or further functionalization, thus increasing complexity in a divergent manner. The transformation is high yielding over a broad scope to provide a variety of useful chromanones in high enantioselectivity.

Understanding the Reactivity and Selectivity of Fluxional Chiral DMAP-Catalyzed Kinetic Resolutions of Axially Chiral Biaryls

Fluxional chiral DMAP-catalyzed kinetic resolutions of axially chiral biaryls were examined using density functional theory. Computational analyses lead to a revised understanding of this reaction in which the interplay of numerous non-covalent interactions control the conformation and flexibility of the active catalyst, the preferred mechanism, and the stereoselectivity. Notably, while the DMAP catalyst itself is confirmed to be highly fluxional, electrostatically driven π⋅⋅⋅π⁺ interactions render the active, acylated form of the catalyst highly rigid, explaining its pronounced stereoselectivity.

Remote Control of Anion-π Catalysis on Fullerene-Centered Catalytic Triads

The design, synthesis and evaluation of catalytic triads composed of a central C60 fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene-fullerene-amine triads display the highest selectivity in anion-π catalysis observed so far, whereas NDI-fullerene-amine triads are not much better than fullerene-amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene-fullerene-amine triads. Activation and deactivation of the fullerene-centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self-induced, the anionic intermediates and transition states create their own anion-π catalyst.

Catalysis with Pnictogen, Chalcogen, and Halogen Bonds

Halogen‐ and chalcogen‐based σ‐hole interactions have recently received increased interest in non‐covalent organocatalysis. However, the closely related pnictogen bonds have been neglected. In this study, we introduce conceptually simple, neutral, and monodentate pnictogen‐bonding catalysts. Solution and in silico binding studies, together with high catalytic activity in chloride abstraction reactions, yield compelling evidence for operational pnictogen bonds. The depth of the σ holes is easily varied with different substituents. Comparison with homologous halogen‐ and chalcogen‐bonding catalysts shows an increase in activity from main group VII to V and from row 3 to 5 in the periodic table. Pnictogen bonds from antimony thus emerged as by far the best among the elements covered, a finding that provides most intriguing perspectives for future applications in catalysis and beyond.

Effect of Cation-π Interaction on Macroionic Self-Assembly

A series of rod-shaped polyoxometalates (POMs) [Bu₄ N]₇ [Mo₆ O₁₈ NC(CH₂ O)₃ MnMo₆ O₁₈ (OCH₂ )₃ CNMo₆ O₁₈ ] and [Bu₄ N]₇ [ArNMo₆ O₁₇ NC(CH₂ O)₃ MnMo₆ O₁₈ (OCH₂ )₃ CNMo₆ O₁₇ NAr] (Ar=2,6-dimethylphenyl, naphthyl and 1-methylnaphthyl) were chosen to study the effects of cation-π interaction on macroionic self-assembly. Diffusion ordered spectroscopy (DOSY) and isothermal titration calorimetry (ITC) techniques show that the binding affinity between the POMs and Zn²⁺ ions is enhanced significantly after grafting aromatic groups onto the clusters, leading to the effective replacement of tetrabutylammonium counterions (TBAs) upon the addition of ZnCl₂ . The incorporation of aromatic groups results in the significant contribution of cation-π interaction to the self-assembly, as confirmed by the opposite trend of assembly size vs. ionic strength when compared with those without aromatic groups. The small difference between two aromatic groups toward the Zn²⁺ ions is amplified after combining with the clusters, which consequently triggers the self-recognition behavior between two highly similar macroanions.