Putting Anion-π Interactions at Work for Catalysis

Synthesis of NIR-II Fluorophores by a C(sp2)-C(sp2/sp) Coupling Reaction Driven by Charge-Transfer Interaction

Here, we report a C(sp2)-C(sp2/sp) coupling reaction driven by charge transfer interaction. Due to the strong intermolecular noncovalent interaction, an essential electron donor-acceptor (EDA) complex is formed, confirmed solidly by absorption spectra, 1H NMR and single crystal X-ray diffraction. The EDA complex lowers the activation barrier (Ea=9.17 kcal/mol) and facilitates the formation of the C-C bond, which is the rate-limiting step revealed by H/D and 12C/13C KIE studies. Kinetic investigations reveal that the reaction is a second-order reaction. Furthermore, high-level theoretical calculations support the proposed mechanism. The mono- and di-substituted products were obtained by varying the reaction conditions. A series of structurally diverse and novel second near-infrared (NIR-II) fluorophores based on benzo[1,2-c : 4,5-c']bis([1,2,5]thiadiazole) (BBTD) were synthesized by utilizing this coupling reaction.

Chalcogen Bonding Catalysis Enables Ring-Opening of Cyclopropene and Ring Expansion of Aryl Ketones

Catalytic transformation of carbene species constitutes a fundamental part in organic synthesis, and the research in this direction has been dominated by transition metals while organic catalysts are difficult to mimic such transition-metal-like reactivity. It would significantly advance carbene chemistry if organic catalysts enable achieving classical metal-carbene approaches otherwise unrealizable reactions. Herein, we report that chalcogen bonding catalysis can solve reactivity problem to achieve an elusive Buchner ring expansion of aryl ketones appending a cyclopropene moiety as carbene precursor. In this work, the ring-opening of cyclopropene and the ring-expansion of aryl systems were added in the field of noncovalent catalysis. This work demonstrates that chalcogen bonding can mediate carbene transformations beyond the known ionic approach. In contrast, transition metals such as rhodium, palladium, and copper complexes, could not solve the reactivity problem to achieve ring expansion of aryl ketones but instead these metal-carbene approaches prefer the formation of furan side product. In addition, a thermal approach even being conducted at 200 °C could not achieve this ring expansion reaction. Mechanistic investigation suggests Se⋅⋅⋅π interaction with cyclopropene facilitates the ring-opening of cyclopropene and chalcogen bonding with carbene intermediate changes the reaction pathway, thus overriding the furan reaction pathway.

Cooperative Anion-π Catalysis with Chiral Molecular Cages toward Enantioselective Desymmetrization of Anhydrides

Exploiting novel noncovalent interactions for catalysis design represents a fascinating direction. For the flexible and relatively weak anion-π interactions, manipulation of two or more π-acidic surfaces for cooperative activation is highly desirable. Here, we demonstrate the strategy of cooperative anion-π catalysis based on chiral molecular cages with V-shaped electron-deficient cavities for synergic binding and activation of dicarbonyl electrophiles toward highly enantioselective desymmetrization transformation. The chiral cages were readily synthesized by incorporation of additional chiral base sites in one step. The cages efficiently catalyzed methanolytic desymmetrization of a series of meso cyclic anhydrides in nearly quantitative yields and up to 94% ee. In contrast, the non-cage analogues and simple control catalysts showed sluggish conversion and much lower enantioselectivity. Crystal structure, substrate binding studies, and theoretical modeling consistently suggested the essential role of the cage electron-deficient cavities in harnessing cooperative anion-π interactions for efficient activation and excellent selectivity control.

Electric-Field Catalysis on Carbon Nanotubes in Electromicrofluidic Reactors: Monoterpene Cyclizations

The control over the movement of electrons during chemical reactions with oriented external electric fields (OEEFs) has been predicted to offer a general approach to catalysis. Recently, we suggested that many problems to realize electric-field catalysis in practice under scalable bulk conditions could possibly be solved on multiwalled carbon nanotubes in electromicrofluidic reactors. Here, we selected monoterpene cyclizations to assess the scope of our system in organic synthesis. We report that electric-field catalysis can function by stabilizing both anionic and cationic transition states, depending on the orientation of the applied field. Moreover, electric-field catalysis can promote reactions which are barely accessible by general Brønsted and Lewis acids and field-free anion-π and cation-π interactions, and drive chemoselectivity toward intrinsically disfavored products without the need for pyrene interfacers attached to the substrate to prolong binding to the carbon nanotubes. Finally, interfacing with chiral organocatalysts is explored and evidence against contributions from redox chemistry is provided.

Small Molecule π-π Stacking Promotes Efficient Photoelectrocatalytic Splitting of Aqueous Hydrogen Production from Polyaniline

Photoelectrocatalysis efficiency depends on light absorption and the effective use of photogenerated carriers but is often limited by inefficient charge transfer and catalytic surface reactivity. In this study, π-π stacking of polar small molecules on aromatic ring-rich polyaniline (PANI) was carried out to improve its photoelectrocatalytic splitting of water for hydrogen production. Detailed photoelectrochemical experiments and density-functional theory (DFT) calculations show that small molecules of p-aminobenzoic acid (PABA) and PANI have the best π-π stacking (compared to p-toluenesulfonic acid (PTA)), which promotes the separation of carriers on the PANI surface. In addition, the polar effect of the small molecules also improves the reactivity of the PANI surface and also reduces the potential barrier for H2 evolution. The current density of PANI-PABA reached -0.12 mA/cm2 (1.23 V vs. RHE) 2.53 times higher than that of pure PANI in linear voltammetric scanning tests under light. This strategy of introducing polar small molecules into organocatalysts via π-π stacking will provide new ideas for the preparation of efficient organic photoelectrocatalysis.

Lone Pair-π Interactions in Organic Reactions

Noncovalent interactions between a lone pair of electrons and π systems can be categorized into two types based on the nature of π systems. Lone pair-π(C═O) interactions with π systems of unsaturated, polarized bonds are primarily attributed to orbital interactions, whereas lone pair-π(Ar) interactions with π systems of aromatic functional groups result from electrostatic attractions (for electron-deficient aryls) or dispersion attractions and Pauli repulsions (for electron-rich/neutral aryls). Unlike well-established noncovalent interactions, lone pair-π interactions have been comparatively underappreciated or less used to influence reaction outcomes. This review emphasizes experimental and computational studies aimed at integrating lone pair-π interactions into the design of catalytic systems and utilizing these interactions to regulate the reactivity and selectivity of chemical transformations. The role of lone pair-π interactions is highlighted in the stabilization or destabilization of transition states and ground-state binding. Examples influenced by lone pair-π interactions with both unsaturated, polarized bonds and aromatic rings as π systems are included. At variance with previous reviews, the present review is not structured according to the physical origin of particular classes of lone pair-π interactions but is divided into chapters according to ways in which lone pair-π interactions affect kinetics and/or selectivity of reactions.

Synthesis of highly activated polybenzene-grafted carbon nanoparticles for supercapacitors assisted by solution plasma

The growing demand for electronic storage devices with faster charging rates, higher energy capacities, and longer cycle lives has led to significant advancements in supercapacitor technology. These devices typically utilize high-surface-area carbon-based materials as electrodes, which provide excellent power densities and cycling stability. However, challenges such as inadequate electrolyte interaction, hydrophobicity that impedes ion transport, and high manufacturing costs restrict their effectiveness. This study aims to enhance carbon-based materials by grafting polymer chains onto their surfaces for supercapacitor applications. A simple solution plasma process (SPP), followed by heating, prepared the polymer-grafted carbon materials. Carbon nanoparticles were synthesized from benzene through plasma discharge in liquid under ambient conditions, forming free radical sites on the carbon surface. Subsequently, benzene molecules were grafted onto the surface via radical polymerization during heating. We investigated the structural and morphological properties of the synthesized materials using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), and Raman spectroscopy. Additionally, N2 absorption-desorption isotherms were measured, pore structure was analyzed with the Dubinin-Astakhov (DA) average pore size model, and specific surface area was determined using the Brunauer-Emmett-Teller (BET) equation for all synthesized samples. The results indicated that the grafting process was influenced by heating time and drying temperature. Furthermore, the electrical properties of the samples were evaluated using cyclic voltammetry (CV), which demonstrated enhancements in both areal capacitance and cycling stability for the polybenzene-grafted carbon compared to the non-grafted variant. This research illustrates that polymer grafting can effectively improve the performance and stability of carbon-based materials for supercapacitor applications. Future work will aim to optimize these materials for broader applications.

Anion Recognition-Directed Supramolecular Catalysis with Functional Macrocycles and Molecular Cages

ConspectusThe development of supramolecular chemistry has provided a variety of host molecules and noncovalent tools for boosting catalytic processes, stimulating the emergence and advance of supramolecular catalysis, among which macrocyclic and cage-like compounds have attracted great attention due to their possession of an enzyme-mimetic cavity and recognition ability. While the privileged scaffolds such as crown ethers, cyclodextrins, cucurbiturils, calixarenes, and metal-coordinated cages have been widely used, their skeletons usually do not contain a directional binding site; binding and activation mainly rely on cation-associated interactions or hydrophobic effects. In this context, the recent advance of anion supramolecular chemistry has drawn our attention to developing an anion recognition-directed approach by using tailor-made functionalized macrocycles and cages. Anions are important widely existing species in both biological and chemical systems and play an important role in regulating the structure and function of enzymes. We envisioned that by taking advantage of anions, including their rich variety, diverse geometry, and multiple interaction sites, the sophisticated cooperation of multiple noncovalent interactions can be manipulated in a confined cavity for directing efficient and selective catalysis.Following this concept, we initiated our study by introducing typical thiourea H-bonding groups to design and synthesize a series of bis-thiourea macrocycles, especially chiral macrocycles, by incorporating chiral linkers. Taking advantage of the obtained strong, cooperative anion binding, a macrocycle-enabled counteranion trapping strategy was developed, which afforded greatly enhanced catalytic efficiency and excellent stereocontrol in acid-catalyzing reactions. Furthermore, inspired by sulfate-induced macrocyclic dimerization assembly, we built a substrate-induced assembly system, enabling an induced-fit cooperative activation network for efficient and enantioselective catalysis. In addition, anion recognition-driven chirality gearing with a more sophisticated trithiourea cage was revealed, which could provide a basis for implementing anion-triggered allosteric catalysis within the induced helical space. Not limited to hydrogen bonding, the emerging anion-π interactions were largely exploited. A series of triazine-based prism cages containing three V-shaped electron-deficient π-cavities were constructed, and their anion-π binding properties were studied. Based on this system, cooperative anion-π activation was established for driving highly efficient and selective catalysis, which paved a way to push anion-π interactions toward more practical and useful catalyst design.These results demonstrated that the anion-recognition direction can serve as a powerful, versatile approach for boosting highly efficient and selective supramolecular catalysis. It is feasible not only for employing exogenous anions (e.g., counteranion) as a handle but also for recognition and regulation of anionic active intermediates/transition states, from use in conventional H-bonding to emerging anion-π recognition.

Pnictogen-Bonding Enzymes

The objective of this study was to create artificial enzymes that capitalize on pnictogen bonding, a σ-hole interaction that is essentially absent in biocatalysis. For this purpose, stibine catalysts were equipped with a biotin derivative and combined with streptavidin mutants to identify an efficient transfer hydrogenation catalyst for the reduction of a fluorogenic quinoline substrate. Increased catalytic activity from wild-type streptavidin to the best mutants coincides with the depth of the σ hole on the Sb(V) center, and the emergence of saturation kinetic behavior. Michaelis-Menten analysis reveals transition-state recognition in the low micromolar range, more than three orders of magnitude stronger than the millimolar substrate recognition. Carboxylates preferred by the best mutants contribute to transition-state recognition by hydrogen-bonded ion pairing and anion-π interactions with the emerging pyridinium product. The emergence of challenging stereoselectivity in aqueous systems further emphasizes compatibility of pnictogen bonding with higher order systems catalysis.

Weak Interaction Activates Esters: Reconciling Catalytic Activity and Turnover Contradiction by Tailored Chalcogen Bonding

The activation of esters by strong Lewis acids via the formation of covalent adducts is a classic strategy to give reactivity; however, this approach frequently incurs limited turnover due to the low efficiency in the dissociation of catalyst from a stable catalyst-product complex. While the use of some weak interaction catalysts that can easily dissociate from any bonding complexes in the reaction system would solve this catalyst turnover problem, the poor catalytic activity in the ester activation that can be provided by these noncovalent forces in turn sets up a formidable challenge. Herein, we describe the activation and catalytic transformation of esters by weak interactions, which provides a promising platform to reconcile the catalytic activity and turnover problems. Several tailored chalcogen-bonding catalysts were developed for the activation of esters, enabling achieving several inherently low reactive Diels-Alder reactions as well as the ring-opening polymerization of lactones through weak chalcogen bonding interactions. This supramolecular catalysis approach is particularly highlighted by its capability to promote some uncommon Diels-Alder reactions involving using dienes bearing electron-withdrawing groups coupled by α,β-unsaturated ester as dienophiles and substrate incorporating competitive Lewis basic sites, in which typical strong Lewis acids showed low catalytic efficiency, while representative hydrogen and halogen bonding catalysts were inactive.

Highly Enantioselective Dihydroxylation of 1,1-Disubstituted Aliphatic Alkenes Enabled by Orchestrated Noncovalent π-Interactions

The elusive nature of noncovalent π-interactions leads to their infrequent use as a design element in challenging chemical reactions. Stereocontrolling models based on coordinated noncovalent π-interactions were used for the asymmetric dihydroxylation of 1,1-disubstituted aliphatic alkenes. By introduction of a substituted phthalazine ring into the alkene substrates, the enantioselectivity reached 99% under the catalysis of bis-cinchona alkaloid ligands. Density functional theory calculations indicated a well-orchestrated, π-π interaction-directed "sandwich-like" transition state.

Arsenate Anion-π Interactions on Amine-Modified Polydivinylbenzene in Aqueous Systems: Experimental and Theoretical Investigation

Anion-π interactions aiding in the adsorption of anions in the solution phase, though challenging to quantify, have attracted a lot of attention in supramolecular chemistry. We present the design of a polymer adsorbent that quantifies the adsorption of arsenate ions experimentally by optimizing anion-π interactions in a purely aqueous system and use density functional theory to compare these results with theoretical data. Arsenate anions are removed from water by amine-functionalized polydivinylbenzene using the comonomer 1-vinyl-1,2,4-triazole, which was cross-linked with divinylbenzene via radical polymerization in a hydrothermal procedure. The amine-functionalized polydivinylbenzene successfully removed arsenate anions from water with a capacity of 46 mg g-1, a 70% increase compared to the nonfunctionalized polydivinylbenzene (27 mg g-1) capacity under the same conditions. Adsorption is best described by the Sips isotherm model with a correlation coefficient R2 factor of 0.99, indicating that adsorption sites are homogeneous, and adsorption occurred by forming a monolayer. Kinetic studies indicated that adsorption is second order in the amine-functionalized polydivinylbenzene. Computational studies using density functional theory showed that the 1-vinyl-1,2,4-triazole comonomer improved the thermodynamic stability of the anionic-π interactions of polydivinylbenzene with arsenate anions. Electrostatic interactions dominate the mechanism of adsorption in polydivinylbenzene compared to the anion-induced interactions that dominate adsorption in amine-functionalized polydivinylbenzene.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Anion-(π)n -π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)n -π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)n -π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)n -π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.

The Origin of Anion-π Autocatalysis

The autocatalysis of epoxide-opening ether cyclizations on the aromatic surface of anion-π catalysts stands out as a leading example of emergent properties expected from the integration of unorthodox interactions into catalysis. A working hypothesis was proposed early on, but the mechanism of anion-π autocatalysis has never been elucidated. Here, we show that anion-π autocatalysis is almost independent of peripheral crowding in substrate and product. Inaccessible asymmetric anion-π autocatalysis and sometimes erratic reproducibility further support that the origin of anion-π autocatalysis is more complex than originally assumed. The apparent long-distance communication without physical contact calls for the inclusion of water between substrate and product on the catalytic aromatic surface. Efficient anion-π autocatalysis around equimolar amounts but poor activity in dry solvents and with excess water indicate that this inclusion of water requires high precision. Computational models suggest that two water molecules transmit dual substrate activation by the product and serve as proton shuttles along antiparallel but decoupled hydrogen-bonded chains to delocalize and stabilize evolving charge density in the transition state by "anion-π double bonds". This new transition-state model of anion-π autocatalysis provides a plausible mechanism that explains experimental results and brings anion-π catalysis to an unprecedented level of sophistication.

Chalcogen Bonding Catalysis with Phosphonium Chalcogenide (PCH)

ConspectusThe exploration of new catalysis concepts and strategies to drive chemical reactions is of vital importance for the sustainable development of organic synthesis. Recently, chalcogen bonding catalysis has emerged as a new concept for organic synthesis and has been demonstrated to be an important synthetic tool capable of addressing elusive reactivity and selectivity issues. This Account describes our progress in the research field of chalcogen bonding catalysis, including (1) the discovery of phosphonium chalcogenide (PCH) as highly efficient chalcogen bonding catalyst; (2) the development of "chalcogen-chalcogen bonding catalysis" and "chalcogen···π bonding catalysis" modes; (3) the demonstration that chalcogen bonding catalysis with PCH can activate hydrocarbons to achieve cyclization and coupling reactions of alkenes; (4) the discovery of unusual results that chalcogen bonding catalysis with PCH can solve elusive reactivity and selectivity issues that are inaccessible by classic catalysis approaches; and (5) the elucidation of chalcogen bonding mechanisms.With PCH catalysts, we systematically studied their chalcogen bonding properties, the relationship between structure and catalysis, and their application in facilitating a diverse array of reactions. Enabled by chalcogen-chalcogen bonding catalysis, an efficient assembly reaction of three molecules of β-ketoaldehyde and one indole derivative in a single operation was realized, delivering heterocycles with a newly constructed seven-membered ring. In addition, a Se···O bonding catalysis approach achieved an efficient synthesis of calix[4]pyrroles. We developed a "dual chalcogen bonding catalysis" strategy to solve reactivity and selectivity issues in the Rauhut-Currier-type reactions and related cascade cyclizations, thus shifting conventionally covalent Lewis base catalysis to a cooperative Se···O bonding catalysis approach. This strategy enables the cyanosilylation of ketones to take place in the presence of a ppm-level amount of PCH catalyst loading. Furthermore, we established chalcogen···π bonding catalysis for catalytic transformation of alkenes. In the research field of supramolecular catalysis, the activation of hydrocarbons such as alkenes by weak interactions is a highly interesting unresolved topic. We showed that the Se···π bonding catalysis approach could efficiently activate alkenes to achieve both coupling and cyclization reactions. Chalcogen···π bonding catalysis with PCH catalysts is particularly highlighted by the capability of facilitating strong Lewis-acid inaccessible transformations, such as the controlled cross coupling of triple alkenes. Overall, this Account presents a panoramic view of our research on chalcogen bonding catalysis with PCH catalysts. The works described in this Account provide a significant platform to solve synthetic problems.

Unfurling Anion-π Interactions Involving Graphynes

Ever since the inception of anion-π interactions, their nature and functional relevance have intrigued researchers. We address the twin challenge of elucidation of the role of extended conjugation and design of all-carbon neutral anion receptors by computations on the anion-π complexes of the halide ions with graphynes. Leveraging on the extended π-conjugation effects, we unfurl the functional relevance of graphynes as anion receptors using descriptors such as electrostatic potential, quadrupole moments, molecular polarizabilities and binding energies. Further, employing natural energy decomposition analysis, we assert that anion-π interactions are not merely dominated by electrostatic interactions. The polarization components do indeed play a crucial role in governing the binding of the anions to the graphynes.

Donor-Acceptor Dyads and Triads Employing Core-Substituted Naphthalene Diimides: A Synthetic and Spectro (Electrochemical) Study

Donor-acceptor dyads and triads comprising core-substituted naphthalene diimide (NDI) chromophores and either phenothiazine or phenoxazine donors are described. Synthesis combined with electrochemical and spectroelectrochemical investigations facilitates characterisation of the various redox states of these molecules, confirming the ability to combine arrays of electron donating and accepting moieties into single species that retain the redox properties of these individual moieties.

Inherently Chiral Cages via Hierarchical Desymmetrization

A new type of cage inherent chirality was accessed by hierarchical desymmetrization of a D_3h-symmetric prismlike cage motif. The dissymmetric C_3v cage precursor C1 bearing two different phloroglucinol caps was first synthesized. The subsequent progressive substitutions on the three triazine arms by different nucleophiles furnished the desired C₁-symmetric inherently chiral cages C3 and C4 with rich structural diversity. Resolution of the racemic cages was achieved by chiral chromatography, and the enantiopure cages were readily obtained on the gram scale. Convenient post-synthetic transformations of the chiral cages with retention of enantiomeric purity were also realized. The absolute configuration was determined by X-ray crystallography, and a chirality descriptor was provided to define the cage chirality. With the inherently chiral array of the electron-deficient triazine surfaces constituting three individual chiral V-shaped π cavities, regio- and enantioselective anion-π binding was probed for the first time with minimum interference of other interactions. As exemplified with chiral phosphate anions (CPAs), it was found that cage (-)-C3a preferably binds (S)-CPA^- in the most electron-deficient cavity through synergistic anion-π interactions with considerable chiral selectivity.

A Se···O bonding catalysis approach to the synthesis of calix[4]pyrroles

Described herein is a chalcogen bonding catalysis approach to the synthesis of calix[4]pyrrole derivatives. The Se···O bonding interactions between selenide catalysts and ketones gave rise to the catalytic activity in the condensation reactions between pyrrole and ketones, leading to the generation of calix[4]pyrrole derivatives in moderate to high yields. This chalcogen bonding catalysis approach was efficient since only 5 mol % catalyst loading was used to promote the consecutive condensation processes while the reactions could be carried out at room temperature, thus highlighting the potential of this type of nonclassical interactions in catalyzing relative complex transformations.