Constructing a triangular metallacycle with salen-Al and its application to a catalytic cyanosilylation reaction

A triangular metallosalen-based metallacycle was constructed in quantitative yield by the self-assembly of a 180° bis(pyridyl)salen-Al complex and a 60° diplatinum(II) acceptor in a 1 : 1 stoichiometric ratio. This metallacycle was then successfully used to cyanosilylate a wide range of benzaldehydes with trimethylsilyl cyanide.

3D-Boronic Ester Architectures: Synthesis, Host-Guest Chemistry, Dynamic Behavior, and Supramolecular Catalysis

Boronic esters are known to be formed simply by mixing boronic acids and alcohols under neutral conditions, and the equilibrium is in favor of the boronic esters when 1,2- or 1,3-diols are employed as alcohols. By utilizing the dynamic nature of the boronic ester formation, our group successfully constructed unique boron-containing 3D structures, such as ring-shaped macrocycles, cages, and tubes, based on the boronic ester formation of various aromatic di-, tri-, or hexaboronic acids with an originally designed tetrol 1 containing two sets of fixed 1,2-diol units oriented on the same face of an indacene framework. Various functions of the obtained boronates were further pursued to disclose the characteristic features of this system. This personal account describes our self-assembled boronate system using tetrol 1 including synthesis, host-guest chemistry, kinetic connection, characteristic dynamic behaviors, and supramolecular catalysis.

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.

Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations.

Computational Discovery of Transition-metal Complexes: From High-throughput Screening to Machine Learning

Transition-metal complexes are attractive targets for the design of catalysts and functional materials. The behavior of the metal-organic bond, while very tunable for achieving target properties, is challenging to predict and necessitates searching a wide and complex space to identify needles in haystacks for target applications. This review will focus on the techniques that make high-throughput search of transition-metal chemical space feasible for the discovery of complexes with desirable properties. The review will cover the development, promise, and limitations of "traditional" computational chemistry (i.e., force field, semiempirical, and density functional theory methods) as it pertains to data generation for inorganic molecular discovery. The review will also discuss the opportunities and limitations in leveraging experimental data sources. We will focus on how advances in statistical modeling, artificial intelligence, multiobjective optimization, and automation accelerate discovery of lead compounds and design rules. The overall objective of this review is to showcase how bringing together advances from diverse areas of computational chemistry and computer science have enabled the rapid uncovering of structure-property relationships in transition-metal chemistry. We aim to highlight how unique considerations in motifs of metal-organic bonding (e.g., variable spin and oxidation state, and bonding strength/nature) set them and their discovery apart from more commonly considered organic molecules. We will also highlight how uncertainty and relative data scarcity in transition-metal chemistry motivate specific developments in machine learning representations, model training, and in computational chemistry. Finally, we will conclude with an outlook of areas of opportunity for the accelerated discovery of transition-metal complexes.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Azide···Oxygen Interaction: A Crystal Engineering Tool for Conformational Locking

We have designed, synthesized and crystallized 36 compounds, each containing an azide group and an oxygen atom separated by three bonds. Crystal structure analysis revealed that each of these molecules adopts a conformation in which the azide and oxygen groups orient syn to each other with a short O ··· N b  contact. Geometry-optimized structures [using  M06-2X/6-311G(d,p) level of theory ] also showed the syn conformation in all 36 of these cases, suggesting that this not merely a crystal packing effect. Quantum topological analysis using Bader's Atoms in Molecules (AIM) theory revealed bond paths and bond critical points (BCP) in these structures suggesting its nature and energetics to be similar to weak hydrogen bonding. The NCI-RDG plot clearly revealed the attractive interaction consisting of electrostatic or dispersive components in all the 36 systems. NBO analysis suggested a weak orbital-relaxation (charge-transfer) contribution of energy for a few  (sp2) O-donor systems. Natural population analysis (NPA) and molecular electrostatic potential mapping (MESP) of these crystal structures further revealed the existence of favorable azide-oxygen interaction.   A CSD search indicated the frequent and consistent occurrence of this interaction and its role dictating the syn conformation of azide and oxygen in molecules where these groups are separated by 2-4 bonds.

Enzyme-like Supramolecular Iridium Catalysis Enabling C-H Bond Borylation of Pyridines with meta-Selectivity

The use of secondary interactions between substrates and catalysts is a promising strategy to discover selective transition metal catalysts for atom-economy C-H bond functionalization. The most powerful catalysts are found via trial-and-error screening due to the low association constants between the substrate and the catalyst in which small stereo-electronic modifications within them can lead to very different reactivities. To circumvent these limitations and to increase the level of reactivity prediction in these important reactions, we report herein a supramolecular catalyst harnessing Zn⋅⋅⋅N interactions that binds to pyridine-like substrates as tight as it can be found in some enzymes. The distance and spatial geometry between the active site and the substrate binding site is ideal to target unprecedented meta-selective iridium-catalyzed C-H bond borylations with enzymatic Michaelis-Menten kinetics, besides unique substrate selectivity and dormant reactivity patterns.

Encapsulation of Aromatic Guests in the Bisporphyrin Cavity of a Double-Stranded Spiroborate Helicate: Thermodynamic and Kinetic Studies and the Encapsulation Mechanism

A double-stranded spiroborate helicate bearing a bisporphyrin unit in the middle forms an inclusion complex with electron-deficient aromatic guests that are sandwiched between the porphyrins. In the present study, we systematically investigated the effects of size, electron density, and substituents of a series of aromatic guests on inclusion complex formations within the bisporphyrin. The thermodynamic and kinetic behaviors during the guest-encapsulation process were also investigated in detail. The guest-encapsulation abilities in the helicate increased with the increasing core sizes of the electron-deficient aromatic guests and decreased with the increasing bulkiness and number of substituents of the guests. Among the naphthalenediimide derivatives, those with bulky N-substituents at both ends hardly formed an inclusion complex. Instead, they formed a [2]rotaxane-like inclusion complex through the water-mediated dynamic B-O bond cleavage/reformation of the spiroborate groups of the helicate, which enhanced the conformational flexibility of the helicate to enlarge the bisporphyrin cavity and form an inclusion complex. Based on the X-ray crystal structure of a unique pacman-like 1:1 inclusion complex between the helicate and an ammonium cation as well as the molecular dynamics simulation results, a plausible mechanism for the inclusion of a planar aromatic guest within the helicate is also proposed.

Noncovalent Interactions at Lanthanide Complexes

Lanthanide complexes have attracted a widespread attention due to their structural diversity, as well as multifunctional and tunable properties. The development of lanthanide based functional materials has often relied on the design of the secondary coordination sphere of the corresponding lanthanide complexes. For instance, usually simple lanthanide salts (solvento complexes) do not catalyze effectively organic reactions or provide low yield of the expected product, whereas the presence of a suitable organic ligand with a noncovalent bond donor or acceptor centre (secondary coordination sphere) modifies the symmetry around the metal centre in lanthanide complexes which then successfully can act as catalysts in both homogenous and heterogenous catalysis. In this minireview, we discuss several relevant examples, based on X-ray crystal structure analyses, in which the hydrogen, halogen, chalcogen, pnictogen, tetrel and rare-earth bonds, as well as cation-π, anion-π, lone pair-π, π-π and pancake interactions, are used as a synthon in the decoration of the secondary coordination sphere of lanthanide complexes.

Possible effects of fluxionality of a cavitand on its catalytic activity through confinement

The discovery of fullerenes was a huge milestone in the scientific community, and with it came the urge to discover and analyze various small and large atomic and molecular clusters having a cavity. These cavitands of varied shapes and sizes have wide applications in the encapsulation of rare gas atoms to induce bond formation between them, storage of hydrogen and hydrocarbons to be used as alternative sources of fuel, catalyzation of otherwise slow reactions without using a catalyst, activation of small gas molecules, etc. Various cavitands like fullerenes, [ExBox]4+, cucurbit[n]urils, borospherenes, octa acid, etc. have been used for this purpose. Some clusters including cavitands exhibit fluxional behaviour. Systems in a confined environment often manifest interesting variations in their properties and behaviour, compared to their unconfined counterparts, facilitating the aforementioned applications. In this perspective article, we explore the possibility of making use of this extra degree of freedom, viz., the fluxionality, in changing the catalytic activity of the cavitand.

Biomimetic electrochemical sensors: New horizons and challenges in biosensing applications

The urge to meet the ever-growing needs of sensing technology has spurred research to look for new alternatives to traditional analytical methods. In this scenario, the glucometer is the flagship of commercial electrochemical sensing platforms, combining selectivity, reliability and portability. However, other types of enzyme-based biosensors seldom achieve the market, in spite of the large and increasing number of publications. The reasons behind their commercial limitations concern enzyme denaturation, and the high costs associated with procedures for their extraction and purification. In this sense, biomimetic materials that seek to imitate the desired properties of natural enzymes and biological systems have come out as an appealing path for robust and sensitive electrochemical biosensors. We herein portray the historical background of these biomimicking materials, covering from their beginnings until the most impactful applications in the field of electrochemical sensing platforms. Throughout the discussion, we present and critically appraise the major benefits and the most significant drawbacks offered by the bioinspired systems categorized as Nanozymes, Synzymes, Molecularly Imprinted Polymers (MIPs), Nanochannels, and Metal Complexes. Innovative strategies of fabrication and challenging applications are further reviewed and evaluated. In the end, we ponder over the prospects of this emerging field, assessing the most critical issues that shall be faced in the coming decade.

Photochemical Deracemization of Primary Allene Amides by Triplet Energy Transfer: A Combined Synthetic and Theoretical Study

The photochemical deracemization of 2,4-disubstituted 2,3-butadienamides (allene amides) was investigated both experimentally and theoretically. The reaction was catalyzed by a thioxanthone which is covalently linked to a chiral 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton providing a U-shaped arrangement of the sensitizing unit relative to a potential hydrogen-bonding site. Upon irradiation at λ = 420 nm in the presence of the sensitizer (2.5 mol %), the amides reached at -10 °C a photostationary state in which one enantiomer prevailed. The enantioenriched allene amides (70-93% ee) were isolated in 74% to quantitative yield (19 examples). Based on luminescence data and DFT calculations, energy transfer from the thioxanthone to the allene amides is thermodynamically feasible, and the achiral triplet allene intermediate was structurally characterized. Hydrogen bonding of the amide enantiomers to the sensitizer was monitored by NMR titration. The experimental association constants (K_a) were similar (59.8 vs 25.7 L·mol^-1). DFT calculations, however, revealed a significant difference in the binding properties of the two enantiomers. The major product enantiomer exhibits a noncovalent dispersion interaction of its arylmethyl group to the external benzene ring of the thioxanthone, thus moving away the allene from the carbonyl chromophore. The minor enantiomer displays a CH-π interaction of the hydrogen atom at the terminal allene carbon atom to the same benzene ring, thus forcing the allene into close proximity to the chromophore. The binding behavior explains the observed enantioselectivity which, as corroborated by additional calculations, is due to a rapid triplet energy transfer within the substrate-catalyst complex of the minor enantiomer.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

Noncovalent Protection for Direct Synthesis of α-Amino-ω-hydroxyl Poly(ethylene oxide)

The synthesis of poly(ethylene oxide) (PEO) with amino end group, a key functionality for PEGylation, is a long-standing challenge. Multistep routes based on postmodification or covalent protection have been adopted to circumvent ethoxylation of the amino group by ethylene oxide (EO). Here, we report a noncovalent protection strategy for one-step synthesis of PEO amine. An amino (di)alcohol is mixed with a small amount of mild phosphazene base and excess triethylborane (Et₃B) before addition of EO. The complexation of the amino group with Et₃B guarantees that polymerization of EO occurs selectively from the hydroxyl group through the bicomponent metal-free catalysis. Simply by precipitation in diethyl ether, the protective Et₃B as well as the catalyst can be removed to afford α-amino-ω-hydroxyl PEO with controlled molar mass, low dispersity, and complete end functionality. The effect of initiator structure and retention of Et₃B on the storage (oxidative) stability of PEO amine is also revealed.

Sulfuroxygen interaction-controlled (Z)-selective anti-Markovnikov vinyl sulfides

The sulfur oxygen (SO) interaction was used herein to obtain (Z)-selective anti-Markovnikov vinyl sulfides from the addition of thiyl radicals to terminal alkynes. DFT calculations predicted that SO interaction originated from the delocalization of the lone-pair of the carbonyl oxygen to the adjacent σ* orbital of the S atom of C-S.

Silver-Catalyzed Enantioselective Sulfimidation Mediated by Hydrogen Bonding Interactions

An enantioselective sulfimidation of 3-thiosubstituted 2-quinolones and 2-pyridones was achieved with a stoichiometric nitrene source (PhI=NNs) and a silver-based catalyst system. Key to the success of the reaction is the use of a chiral phenanthroline ligand with a hydrogen bonding site. The enantioselectivity does not depend on the size of the two substituents at the sulfur atom but only on the binding properties of the heterocyclic lactams. A total of 21 chiral sulfimides were obtained in high yields (44-99 %) and with significant enantiomeric excess (70-99 % ee). The sulfimidation proceeds with high site-selectivity and can also be employed for the kinetic resolution of chiral sulfoxides. Mechanistic evidence suggests the intermediacy of a heteroleptic silver complex, in which the silver atom is bound to one molecule of the chiral ligand and one molecule of an achiral 1,10-phenanthroline. Support for the suggested reaction course was obtained by ESI mass spectrometry, DFT calculations, and a Hammett analysis.

Supramolecular Chirality Suppresses Molecular Chirality: Selective Chiral Recognition in Hierarchically Coassembled Pyridine-Benzimidazole Conjugates with Precise ee% Detection

Incorporation of aromatic chiral species with axial, helical, or propeller chirality in surapmolecular chiral motifs would potentially facilitate the chiroptical applications such as enantiomeric excess detection, chiral sensing, and displays, which however suffer from inevitable competition between supramolecular chirality and molecular chirality and remain major challenges. Here, we show the programmable coassembly of pyridine-cored benzimidazole derivatives with intrinsic propeller chirality, which shall form binary and ternary aggregates with chiral acids as well as metal ions though H-bonds and metal-ligand coordination interactions in an orthogonal manner, to enhance and flexibly control the chiroptical properties. Solid-state X-ray structures of pyridine-benzimidazole derivatives suggested they adopted the propeller molecular chirality. Competition between molecular and supramolecular chirality and dynamic binding toward enantiomers of pyridine-benzimidazole derivatives was observed in the coassembly systems based on the chiroptical responses and molecular dynamic simulation. Compared to the intrinsic racemic assembly, coassembly systems produced chiroptical responses including the Cotton effect and circularly polarized luminescence (CPL) with relatively high dissymmetry factor (g_abs up to 4.9 × 10^-2, g_lum up to 9.6 × 10^-3). Furthermore, chiroptical responses were further controlled by introducing metal ions, achieving inverted handedness and tunable dissymmetry factors. This work provides feasible strategies to efficiently regulate and enhance the chiroptical properties of intrinsic aromatics via multiple interactions, which also expressed great potential in quantitative ee% sensing for chiral acids.

Phosphorus Ligands in Hydroformylation and Hydrogenation: A Personal Account

Metal-catalyzed hydroformylation and hydrogenation heavily rely on ligands, among which phosphorous ligands play a pivotal role. This personal account presents a selection of three distinct classes of phosphorous ligands, namely, monodentate meta-substituted phosphinites, bis-phosphites, and P-chiral supramolecular phosphines, developed in our group. The synthesis of these ligands, isolation, characterization, and their performance in transition metal-catalyzed hydroformylation, isomerizing hydroformylation, and asymmetric hydrogenation of olefins is summarized. The state of the art development in iron-catalyzed hydroformylation of alkenes and our contributions to the field is discussed. Use of phosphines enabled iron-catalyzed hydroformylation of alkenes under mild conditions. Thus, this account demonstrates the central role of phosphorus ligands in industrially relevant transformations such as hydrogenation and hydroformylation. The seemingly matured field of ligand discovery still holds significant potential and will steer the field of homogeneous catalysis.

Water-Mediated Reversible Control of Three-State Double-Stranded Titanium(IV) Helicates

A stimuli-responsible reversible structural transformation is of key importance in biological systems. We now report a unique water-mediated reversible transformation among three discrete double-stranded dinuclear titanium(IV) achiral meso- and chiral rac-helicates linked by a mono(μ-oxo) or a bis(μ-hydroxo) bridge between the titanium ions through hydration/dehydration or its combination with a water-mediated dynamic cleavage/re-formation of the titanium-phenoxide (Ti-OPh) bonds. The bis(μ-hydroxo) bridged titanium(IV) meso-helicate prepared from two tetraphenol strands with titanium(IV) oxide was readily dehydrated in CD₃CN containing a small amount of water upon heating, accompanied by Ti-OPh bond cleavage/re-formation catalyzed by water, resulting in the formation of the mono(μ-oxo)-bridged rac-helicate, which reverted back to the original bis(μ-hydroxo)-bridged meso-helicate upon hydration in aqueous CD₃CN. These reversible transformations between the meso- and rac-helicates were also promoted in the presence of a catalytic amount of an acid, which remarkably accelerated the reactions at lower temperature. Interestingly, in anhydrous CD₃CN, the bis(μ-hydroxo)-bridged meso-helicate was further slowly converted to a different helicate, while its meso-helicate framework was maintained, namely the mono(μ-oxo)-bridged meso-helicate, through dehydration upon heating and its meso to meso transformation was significantly accelerated in the presence of cryptand[2.2.1], which contributes to removing Na⁺ ions coordinated to the helicate. Upon cooling, the backward meso to meso transformation took place via hydration. Hence, three different, discrete double-stranded chiral rac- and achiral meso-titanium(IV) helicates linked by a mono(μ-oxo) or a bis(μ-hydroxo) bridge were successfully generated in a controllable manner by a change in the water content of the reaction media.

Theoretical Insight into the Reversal of Chemoselectivity in Diels-Alder Reactions of α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted and Lewis Acids

Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, the use of tris(pentafluorophenyl)borane B(C6F5)3 bulky Lewis acid as catalyst gave mainly the cycloadduct of α,β-unsaturated aldehyde as a major product. Our aim in the present work is to put in evidence the role of the catalyst in the reversal of the chemoselectivity of the catalyzed Diels–Alder reactions of (2E)-but-2-enal and 2-Hexen-4-one with cyclopentadiene. The calculations were performed at the ωB97XD/6-311G(d,p) level of theory and the solvent effects of dichloromethane were taken into account using the PCM solvation model. The obtained results are in good agreement with experimental outcomes.

Enantioselective Synthesis of Diaryl Sulfoxides Enabled by Molecular Recognition

The enantioselective sulfoxidation of diaryl-type sulfides was accomplished using a chiral manganese porphyrin complex equipped with a remote molecular recognition site. Despite the marginal size difference between the two substituents at the prostereogenic sulfur center, hydrogen bonding enabled the formation of chiral sulfoxides with exquisite enantioselectivities (16 examples, up to 99% ee). Aside from the precise orientation of a distinct substrate, the quinolone lactam offers an excellent entry point for further derivatization.

A general strategy to prepare atomically dispersed biomimetic catalysts based on host-guest chemistry

Herein, we report a general strategy based on host-guest interactions to fabricate atomically dispersed biomimetic catalysts, which were evaluated by diboration of phenylacetylene. The structure and function of these mimics are quite similar to those of enzymes, namely, the atomically dispersed metal serves as an active site, the external macromolecular structure plays a role as an enzyme catalytic pocket to stabilize the reaction intermediates and the interactions between the intermediates and functional groups near to the active site can reduce the reaction activation energy.

A Switchable Catalyst Duo for Acyl Transfer Proximity Catalysis and Regulation of Substrate Selectivity

Enzymes are encoded with a gamut of information to catalyze a highly selective transformation by selecting the proper reactants from an intricate mixture of constituents. Mimicking biological machinery, two switchable catalysts with differently sized cavities and allosteric control are conceived that allow complementary size-selective acyl transfer in an on/off manner by modulating the effective local concentration of the substrates. Selective activation of one of two catalysts in a mixture of reactants of similar reactivity enabled upregulation of the desired product.

Half-Sandwich Ruthenium Complexes of Amide-Phosphine Based Ligands: H-Bonding Cavity Assisted Binding and Reduction of Nitro-substrates

We present synthesis and characterization of two half-sandwich Ru(II) complexes supported with amide-phosphine based ligands. These complexes presented a pyridine-2,6-dicarboxamide based pincer cavity, decorated with hydrogen bonds, that participated in the binding of nitro-substrates closer to the Ru(II) centers, which is further supported with binding and docking studies. These ruthenium complexes functioned as the noteworthy catalysts for the borohydride mediated reduction of assorted nitro-substrates. Mechanistic studies not only confirmed the intermediacy of [Ru-H] in the reduction but also asserted the involvement of several organic intermediates during the course of the catalysis. A similar Ru(II) complex that lacked pyridine-2,6-dicarboxamide based pincer cavity substantiated its unique role both in the substrate binding and the subsequent catalysis.

The Size-Accelerated Kinetic Resolution of Secondary Alcohols

The factors responsible for the kinetic resolution of alcohols by chiral pyridine derivatives have been elucidated by measurements of relative rates for a set of substrates with systematically growing aromatic side chains using accurate competitive linear regression analysis. Increasing the side chain size from phenyl to pyrenyl results in a rate acceleration of more than 40 for the major enantiomer. Based on this observation a new catalyst with increased steric bulk has been designed that gives enantioselectivity values of up to s=250. Extensive conformational analysis of the relevant transition states indicates that alcohol attack to the more crowded side of the acyl-catalyst intermediate is favoured due to stabilizing CH-π-stacking interactions. Experimental and theoretical results imply that enantioselectivity enhancements result from accelerating the transformation of the major enantiomer through attractive non-covalent interactions (NCIs) rather than retarding the transformation of the minor isomer through repulsive steric forces.

Beyond hydrogen bonding: recent trends of outer sphere interactions in transition metal catalysis

The implementation of interactions beyond hydrogen bonding in the 2^nd coordination sphere of transition metal catalysts is rare. However, it has already shown great promise in last 5 years, providing new tools to control the activity and selectivity as here reviewed.

Carbon (sp3) tetrel bonding mediated BODIPY supramolecular assembly via unprecedented synergy of Csp3⋯N and Csp3⋯F pair interactions

Here, we present the first example of sp³ hybridized carbon centered (C_sp3) tetrel bonding mediated 3D BODIPY assembly via the exceptional synergy of C_sp3⋯N and C_sp3⋯F pair interactions.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Systematic Variation of Ligand and Cation Parameters Enables Site-Selective C-C and C-N Cross-Coupling of Multiply Chlorinated Arenes through Substrate-Ligand Electrostatic Interactions

Use of attractive noncovalent interactions between ligand and substrate is an emerging strategy for controlling positional selectivity. A key question relates to whether fine control on molecules with multiple, closely spaced reactive positions is achievable using typically less directional electrostatic interactions. Herein, we apply a 10-piece "toolkit" comprising of two closely related sulfonated phosphine ligands and five bases, each possessing varying cation size, to the challenge of site-selective cross-coupling of multiply chlorinated arenes. The fine tuning provided by these ligand/base combinations is effective for Suzuki-Miyaura coupling and Buchwald-Hartwig coupling on a range of isomeric dichlorinated and trichlorinated arenes, substrates that would produce intractable mixtures when typical ligands are used. This study develops a practical solution for site-selective cross-coupling to generate complex, highly substituted arenes.