Recyclable Stereoselective Catalysts

RuPHOX-Ru Catalyzed Asymmetric Cascade Hydrogenation of 3-Substituted Chromones for the Synthesis of Corresponding Chiral Chromanols

An efficient RuPHOX-Ru catalyzed asymmetric cascade hydrogenation of 3-substituted chromones has been achieved under mild reaction conditions, affording the corresponding chiral 3-substituted chromanols in high yields with excellent enantio- and diastereoselectivities (up to 99 % yield, >99 % ee and >20 : 1 dr). Control reactions and deuterium labelling experiments revealed that a dynamic kinetic resolution process occurs during the subsequent hydrogenation of the C=O double bond, which is responsible for the high performance of the asymmetric cascade hydrogenation. The resulting products allow for several transformations and it was shown that the protocol provides a practical and alternative strategy for the synthesis of chiral 3-substituted chromanols and their derivatives.

Bimetallic Ru/Ru-Catalyzed Asymmetric One-Pot Sequential Hydrogenations for the Stereodivergent Synthesis of Chiral Lactones

Asymmetric sequential hydrogenations of α-methylene γ- or δ-keto carboxylic acids are established in one-pot using a bimetallic Ru/Ru catalyst system, achieving the stereodivergent synthesis of all four stereoisomers of both chiral γ- and δ-lactones with two non-vicinal carbon stereocenters in high yields (up to 99%) and with excellent stereoselectivities (up to >99% ee and >20:1 dr). The compatibility of the two chiral Ru catalyst systems is investigated in detail, and it is found that the basicity of the reaction system plays a key role in the sequential hydrogenation processes. The protocol can be performed on a gram-scale with a low catalyst loading (up to 11000 S/C) and the resulting products allow for many transformations, particularly for the synthesis of several key intermediates useful for the preparation of chiral drugs and natural products.

Enantioselective Synthesis in Continuous Flow: Polymer-Supported Isothiourea-Catalyzed Enantioselective Michael Addition-Cyclization with α-Azol-2-ylacetophenones

A packed reactor bed incorporating a polymer-supported isothiourea HyperBTM catalyst derivative has been used to promote the enantioselective synthesis of a range of heterocyclic products derived from α-azol-2-ylacetophenones and -acetamides combined with alkyl, aryl, and heterocyclic α,β-unsaturated homoanhydrides in continuous flow via an α,β-unsaturated acyl-ammonium intermediate. The products are generated in good to excellent yields and generally in excellent enantiopurity (up to 97:3 er). Scale-up is demonstrated on a 15 mmol scale, giving the heterocyclic product in 68% overall yield with 98:2 er after recrystallization.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Stepwise and Controllable Synthesis of Mesoporous Heterotrimetallic Catalysts Based on Predesigned Al4 Ln4 Metallocycles

The motivation for making heterometallic compounds stemmed from their emergent synergistic properties and enhanced capabilities for applications. However, the atomically precisely controlled synthesis of heterometallic compounds remains a daunting challenge of the complications that arise when applying several metals and linkers. Herein, a stepwise and controlled method is reported for the accurate addition of second and third metals to homometallic aluminum macrocycles based on the synergistic coordination and hard-soft acid-base theory. These heterometallic compounds showed a good Lewis acid catalytic effect, and the addition of hetero-metals significantly improved the catalytic effect and rate, among that the conversion rate of compound AlOC-133 reached 99.9% within half an hour. This method combines both the independent controllability of stepwise assembly with the universality of one-step methods. Based on the large family of clusters, the establishment of this method paves the way for the controllable and customized molecular-level synthesis of heterometallic materials and creates materials customized for preferential application.

Primary amines as heterogeneous catalysts in an enantioselective [2,3]-Wittig rearrangement reaction

A series of heterogeneous catalysts anchored to different polystyrene-based supports has been prepared and applied in an asymmetric [2,3]-Wittig rearrangement reaction of cyclohexanone derivatives. Among them, primary amino acid-derived (aminomethylated)polystyrene-supported catalysts showed excellent reactivity leading to the formation of rearranged products in good enantioselectivities of both diastereomers. Reusability issues connected to the deactivation of the catalyst were proved to be dependent on the end-capping strategy chosen for the blocking of the unreacted active sites of the resin. This issue of end-capping has not previously been in focus. Using bulkier pivaloyl end-capping moiety, we were able to recycle the catalyst in six consecutive cycles with only marginal deceleration of the reaction. Moreover, the epimerization of the product that occurred while conducting a rearrangement reaction in the presence of a homogeneous catalyst was almost fully eliminated by switching the catalytic system to heterogeneous.

A copper(ii) complex containing pyridine-2-carbaldehyde and its direct binding onto ethylenediamine functionalized with Fe3O4@SiO2 nanoparticles for catalytic applications

In the present study, a copper(ii) complex containing a pyridine-2-carbaldehyde ligand and its direct binding onto ethylenediamine functionalized with Fe3O4@SiO2 nanoparticles [Cu(ii)-Schiff base-(CH2)3-SiO2@Fe3O4] as a heterogeneous magnetic nanocatalyst can be easily prepared using a multi-step method. Next, the structural and magnetic properties of the synthesized nanoparticles were identified using Fourier-transform infrared spectroscopy (FT-IR), inductively coupled plasma (ICP), vibrating-sample magnetometry (VSM), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), PXRD (Powder X-ray diffraction), Brunauer-Emmett-Teller (BET), and energy-dispersive X-ray spectrometry (EDX) techniques. TEM images reveal that the average particle size distribution was found to be in the range of 45-55 nm with spherical shape. The PXRD analysis indicated that the crystallite size was found to be 35.2 nm. The synthesized nanocatalyst exhibited a very good catalytic ability in the synthesis reaction of pyran derivatives and 2-benzylidenemalononitrile derivatives. Product 2-amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydrobenzo[b]pyran 4e was achieved in 97% yield with a TON of 129.3 and a TOF of 646.6 h-1 and product 2-(4-cyanobenzylidene)malononitrile 3j was achieved in 96% yield with a TON of 128 and a TOF of 984.6 h-1. In addition, the synthesized nanocatalyst was easily separated from the reaction mixture by a magnet and used 7 consecutive times without significant loss of catalytic activity. Also, leaching of copper metal from the synthesized nanocatalyst was very insignificant for this reaction.

One-step assembly of a MacMillan catalyst-based phenolic-type polymer

We report herein a "bottom-up" approach for the one-step assembly of a MacMillan catalyst-based phenolic-type polymer (Mac-CP). The resulting self-supported polymeric organocatalyst possesses homogeneously distributed and highly concentrated catalytic sites. Furthermore, Mac-CP is soluble in CH₃CN but insoluble in hexane. This unique property can be used to employ the polymer as an efficient catalyst in homogeneous organocatalysis and heterogeneous recycling. As a result, Mac-CP possesses comparable catalytic activity and enantioselectivity to its homogeneous counterpart in the asymmetric Diels-Alder reaction (95% yield, 93% enantiomeric excess (ee) for endo and 92% ee for exo).

Enantioselective construction of ortho-sulfur- or nitrogen-substituted axially chiral biaryls and asymmetric synthesis of isoplagiochin D

Axially chiral biaryl motifs possessing ortho-heteroatom-substituted functionalities exist widely in the structures of natural products and have served as foundation for constructing prominent chiral organocatalysts, ligands, functional materials, and even bioactive molecules. However, a general and enantioselective synthesis of such chiral structures with high synthetic value is rare. Taking advantage of the BaryPhos-facilitated asymmetric Suzuki-Miyaura cross-coupling, we have established a general, efficient and enantioselective construction of the ortho sulfur- or nitrogen-substituted axially chiral biaryls. The protocol shows excellent compatibility to various functional groups and structural features, delivering chiral biaryl structures with ortho-sulfonyl groups or with ortho-nitro groups at a broad range of molecular diversity and complexity. The immobilization of BaryPhos on polyethylene glycol (PEG) support has enabled homogeneous enantioselective cross-coupling in aqueous media and the palladium catalyst recycling for multiple times. The method has enabled a concise 10-step asymmetric synthesis of isoplagiochin D as well as the construction of chiroptical molecules with circularly polarized luminescence (CPL) properties.

Ortho-heteroatom-substituted axially chiral biaryls are valuable structures in synthetic and medicinal chemistry. Here, the authors established an efficient synthesis of these chiral structures via asymmetric cross-coupling.

Efficient biodiesel production from oleic and palmitic acid using a novel molybdenum metal-organic framework as efficient and reusable catalyst

In this study, metal–organic framework based on molybdenum and piperidine-4-carboxylic acid, was synthesized through a simple solvothermal method and employed as an effective catalyst for biodiesel production from oleic acid and palmitic acid via esterification reaction. The prepared catalyst was characterized by XRD, FTIR, TGA, DSC, BET, SEM, TEM, ICP-OES, X-ray mapping and EDX analysis. The resulting Mo–MOF catalyst exhibit a rod-like morphology, specific surface area of 56 m²/g, and thermal stability up to 300 °C. The solid catalyst exhibited high activities for esterification of oleic acid and palmitic acid. Moreover, the catalyst could be simply recovered and efficiently reutilized for several times without significant loss in its activity, also obtained results revealed that metal–organic framework could be used for the appropriate and rapid biodiesel production.

Chirality-driven self-assembly: application toward renewable/exchangeable resin-immobilized catalysts

Resin-immobilized catalysts were prepared through chirality-driven self-assembly. The method allows the resin-immobilized catalyst to be regenerated under mild conditions and in situ catalyst exchange to be carried out quantitatively. The uniqueness of the methodology was demonstrated by the preparation of a catalyst for TEMPO oxidation as well as a two-step sequential TEMPO oxidation/aldol condensation sequence enabled by facile catalyst exchange.

Template synthesis, DNA binding, antimicrobial activity, Hirshfeld surface analysis, and 1D helical supramolecular structure of a novel binuclear copper(ii) Schiff base complex

A new binuclear copper(ii) Schiff base complex [Cu₂ L₂⁻ (NO₃)₂]·2CH₃OH (1) [L = 2,6-bis((E)-(p-tolylimino)methyl)-4-methoxyphenol] was synthesized using a template method in which the tridentate N₂O Schiff base ligand was derived from [1 + 2] condensation of 2,6-diformyl-4-methoxyphenol and p-methyl aniline in the presence of copper(ii) ions as the template agent. The X-ray diffraction analyses revealed that this complex crystallizes in the monoclinic system with space group P2₁/n. The most remarkable structural feature of 1 is that it contains two types of 1D right-handed helical chains. The molecules are linked by intermolecular hydrogen bonds and π⋯π interactions, then a 3D supramolecular network was constructed. Moreover, the intermolecular interactions on the crystal packing of 1 have been further studied using Hirshfeld surface analysis and corresponding 2D fingerprint plots. Binding interaction of this complex with calf thymus DNA (CT-DNA) has been investigated using absorption and emission studies, viscosity experiments and circular dichroism studies. Complex 1 shows significant binding to the DNA. The results of fluorescence spectroscopy and UV absorption spectroscopy, CD spectroscopy and viscosity indicated that this complex interacted with CT-DNA in a groove binding mode where the binding constant was 1.3 ± 0.2 × 10⁴ L mol⁻¹. Our fluorimeteric study showed that the reaction between 1 and CT-DNA was exothermic (ΔH = 59.6 kJ mol⁻¹; ΔS = 268.79 J mol⁻¹ K⁻¹). Antibacterial activities of the complex were screened by the disc diffusion method against three Gram-positive bacteria (Staphylococcus aureus ATCC 25923, Enterococcus faecalis ATCC 23212 and S. epidermidis ATCC 34384), and three Gram-negative bacteria (Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853 and Klebsiella pneumonia ATCC 70063). The results indicated that this complex demonstrated acceptable antibacterial activities.

A new binuclear copper(ii) Schiff base complex was synthesized using a template method in which the tridentate N₂O Schiff base ligand was derived from [1 + 2] condensation of 2,6-diformyl-4-methoxyphenol and p-methyl aniline.

An environment-friendly methodology for the construction of diversified bicycloacenaphtho[1,2-d]imidazole-8-thione scaffolds using spinel NiFe2O4 nanoparticles as a sustainable catalyst

Herein, we successfully developed an easy access to bicycloacenaphtho[1,2-d]imidazole-8-thione by one-pot three-component MCRs of acenaphthoquinone, aryl or alkyl isothiocyanates and amines using environmentally benevolent and recyclable spinel NiFe₂O₄ nanocatalyst in aqueous ethanol. A broad number of products have been synthesized with both EDGs and EWGs present in the ring which increases the diversity of the protocol. The NiFe₂O₄ nanopowder has been synthesized and thoroughly characterized by powdered XRD, HRTEM, EDX, BET and ICP-AES analysis. The protocol to this bicyclic-heterocycle is noteworthy due to good to excellent yields, practical simplicity and high regioselectivity without any troublesome or hazardous by-products and its easy recovery and reusability of the catalyst. Spinel NiFe₂O₄ NPs-catalysed synthesis of various bicycloacenaphtho[1,2-d]imidazole-8-thione scaffolds under mild and sustainable conditions.

Synthesis of new chiral Mn(iii)–salen complexes as recoverable and reusable homogeneous catalysts for the asymmetric epoxidation of styrenes and chromenes

New chiral Mn(iii)–salen complexes 1a–e and 2a–e were synthesized from the reaction of C2-symmetric chiral salen ligands and Mn(CH3COO)2·4H2O under an inert atmosphere followed by aerobic oxidation.

Recent advances in the asymmetric reduction of imines by recycled catalyst systems

Recent advances relating to the asymmetric reduction of imines to afford optically active amines via recyclable catalyst systems are reviewed.

Synthesis, Biological Evaluation and In silico Studies of 1, 2, 3-triazolyl- Metronidazole Derivatives Against Leishmania Major

The simple and effective approach for the preparation, of 1,2,3-triazolyl-based metronidazole hybrid analogues as promising anti-leishmania agents using of [CuL-SiO-HA] as a catalyst were described. The catalyst was fully characterized...

Microporous Volumes from Nitrogen Adsorption at 77 K: When to Use a Different Standard Isotherm?

This work reviews the application of various standard isotherms to evaluate the micropore volume in a range of microporous materials. The selected materials have quite different surface chemistry, and are relevant due to their properties for adsorption and catalysis: zeolites, activated carbons, clay-based materials and MOFs. Some cases were analysed before and after being used as supports in the heterogenization of homogeneous catalysts. The discussion is centred, but not limited, to the three standard isotherms that are mostly employed in the literature (t-curve, non-porous carbon and non-porous hydroxylated silica) for the assessment of the micropore volume. For a given material the values of the micropore volumes from the different standard isotherms were compared, particularly against the values from the largely used t-curve. The cases where major discrepancies were found could normally be ascribed to samples that have a broad micropore size distribution.

Supramolecular Strategies for the Recycling of Homogeneous Catalysts

Supramolecular approaches are increasingly used in the development of homogeneous catalysts and they also provide interesting new tools for the recycling of metal-based catalysts. Various non-covalent interactions have been utilized for the immobilization homogeneous catalysts on soluble and insoluble support. By non-covalent anchoring the supported catalysts obtained can be recovered via (nano-) filtration or such catalytic materials can be used in continuous flow reactors. Specific benefits from the reversibility of catalyst immobilization by non-covalent interactions include the possibility to re-functionalize the support material and the use as "boomerang" type catalyst systems in which the catalyst is captured after a homogeneous reaction. In addition, new reactor design with implemented recycling strategies becomes possible, such as a reverse-flow adsorption reactor (RFA) that combines a homogeneous reactor with selective catalyst adsorption/desorpion. Next to these non-covalent immobilization strategies, supramolecular chemistry can also be used to generate the support, for example by generation of self-assembled gels with catalytic function. Although the stability is a challenging issue, some self-assembled gel materials have been successfully utilized as reusable heterogeneous catalysts. In addition, catalytically active coordination cages, which are frequently used to achieve specific activity or selectivity, can be bound to support by ionic interactions or can be prepared in structured solid materials. These new heterogenized cage materials also have been used successfully as recyclable catalysts.

A TEMPO-Functionalized Ordered Mesoporous Polymer as a Highly Active and Reusable Organocatalyst

The properties of high stability, periodic porosity, and tunable nature of ordered mesoporous polymers make these materials ideal catalytic nanoreactors. However, their application in organocatalysis has been rarely explored. We report herein for the first time the incorporation of a versatile organocatalyst, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), into the pores of an FDU-type mesoporous polymer via a pore surface engineering strategy. The resulting FDU-15-TEMPO possesses a highly ordered mesoporous organic framework and enhanced stability, and shows excellent catalytic activity in the selective oxidation of alcohols and aerobic oxidative synthesis of 2-substituted benzoxazoles, benzimidazoles and benzothiazoles. Moreover, the catalyst can be easily recovered and reused for up to 7 consecutive cycles.

Chiral Yolk-Shell MOF as an Efficient Nanoreactor for Asymmetric Catalysis in Organic-Aqueous Two-Phase System

It remains a great challenge to introduce large and efficient homogeneous asymmetric catalysts into MOFs and other microporous materials as well as retain their degrees of freedom. Herein, a new heterogeneous strategy of homogeneous chiral catalysts is proposed, that is, to construct a yolk-shell MOFs-confined, large-size, and highly efficient homogeneous chiral catalyst, which can be used as a nanoreactor for asymmetric catalytic reactions.

Exploiting Continuous Processing for Challenging Diazo Transfer and Telescoped Copper-Catalyzed Asymmetric Transformations

Generation and use of triflyl azide in flow enables efficient synthesis of a range of α-diazocarbonyl compounds, including α-diazoketones, α-diazoamides, and an α-diazosulfonyl ester, via both Regitz-type diazo transfer and deacylative/debenzoylative diazo-transfer processes with excellent yields and offers versatility in the solvent employed, in addition to addressing the hazards associated with handling of this highly reactive sulfonyl azide. Telescoping the generation of triflyl azide and diazo-transfer process with highly enantioselective copper-mediated intramolecular aromatic addition and C–H insertion processes demonstrates that the reaction stream containing the α-diazocarbonyl compound can be obtained in sufficient purity to pass directly over the immobilized copper bis(oxazoline) catalyst without detrimentally impacting the catalyst enantioselectivity.

Application of an Electrochemical Microflow Reactor for Cyanosilylation: Machine Learning-Assisted Exploration of Suitable Reaction Conditions for Semi-Large-Scale Synthesis

Cyanosilylation of carbonyl compounds provides protected cyanohydrins, which can be converted into many kinds of compounds such as amino alcohols, amides, esters, and carboxylic acids. In particular, the use of trimethylsilyl cyanide as the sole carbon source can avoid the need for more toxic inorganic cyanides. In this paper, we describe an electrochemically initiated cyanosilylation of carbonyl compounds and its application to a microflow reactor. Furthermore, to identify suitable reaction conditions, which reflect considerations beyond simply a high yield, we demonstrate machine learning-assisted optimization. Machine learning can be used to adjust the current and flow rate at the same time and identify the conditions needed to achieve the best productivity.

Advances in Chiral Metal-Organic and Covalent Organic Frameworks for Asymmetric Catalysis

Asymmetric catalysis is of crucial importance owing to the huge and rising demand for optically pure substances. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), as two emerging crystalline porous materials, have presented great promising applications for heterogeneous asymmetric catalysis. The unique properties, such as, highly regular porous structures, prominent structural tunability, and well-ordered catalytic sites, render chiral MOFs (CMOFs) and chiral COFs (CCOFs) highly active and enantioselective for a large number of asymmetric catalytic organic transformations. Furthermore, they provide a useful platform for facile mechanistic understanding and catalyst design. This review provides an overview of the advancements in CMOFs and CCOFs for asymmetric catalysis. The designs, syntheses and structures of these crystalline porous materials, and their asymmetric catalytic performance are described. And the perspectives on challenges and opportunities in development of CMOFs and CCOFs are discussed. It is anticipated that this review will shed light on the heterogeneous asymmetric catalysis with CMOFs and CCOFs and motivate further research in this promising field.

Self-Promoted Stereoselective Glycosylations - Past, Present, Future

Self-promoted glycosylations have generally not received much attention, despite having the advantages of being easy to perform and often highly stereoselective. This account covers the work done in this field and the mechanistic aspects of self-promoted glycosylations are discussed, with a main focus on the stereoselectivity of the reactions. Most self-promoted glycosylations utilize trichloroacetimidate donors, but examples of self-promoted reactions with other donors have been described and will be discussed. Self-promoted glycosylation strategies can provide a basis for new stereoselective glycosylation methodologies.

Amino Acid-Functionalized Metal-Organic Frameworks for Asymmetric Base-Metal Catalysis

We report a strategy to develop heterogeneous single-site enantioselective catalysts based on naturally occurring amino acids and earth-abundant metals for eco-friendly asymmetric catalysis. The grafting of amino acids within the pores of a metal-organic framework (MOF), followed by post-synthetic metalation with iron precursor, affords highly active and enantioselective (>99 % ee for 10 examples) catalysts for hydrosilylation and hydroboration of carbonyl compounds. Impressively, the MOF-Fe catalyst displayed high turnover numbers of up to 10 000 and was recycled and reused more than 15 times without diminishing the enantioselectivity. MOF-Fe displayed much higher activity and enantioselectivity than its homogeneous control catalyst, likely due to the formation of robust single-site catalyst in the MOF through site-isolation.

Horeau amplification in the sequential acylative kinetic resolution of (±)-1,2-diols and (±)-1,3-diols in flow

The sequential acylative kinetic resolution (KR) of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols using a packed bed microreactor loaded with the polystyrene-supported isothiourea, HyperBTM, is demonstrated in flow. The sequential KRs of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols exploits Horeau amplification, with each composed of two successive KR processes, with each substrate class significantly differing in the relative rate constants for each KR process. Optimisation of the continuous flow set-up for both C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diol substrate classes allowed isolation of reaction products in both high enantiopurity and yield. In addition to the successful KR of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols, the application of this process to the more conceptually-complex scenario involving the sequential KR of C1-symmetric (±)-1,3-anti-diols was demonstrated, which involves eight independent rate constants.

An azo-bridged ring system enabled by-standing immobilization of a chiral diene ligand

A family of 9-azabicyclo[3.3.1]nonadiene ligands were developed, and the nitrogen atom in the bridged ring enables a facile immobilization of diene ligands to silica.

First example of Ugi's amine as a platform for the construction of chiral coordination polymers: synthesis and properties

Here, the first example of phosphinic acid containing Ugi's amine (HL) is presented.

Chiral Rhodium Nanoparticle-Catalyzed Asymmetric Arylation Reactions

The development of heterogeneous catalyst systems for enantioselective reactions is an important subject in modern chemistry as they can be easily separated from products and potentially reused; this is particularly favorable in achieving a more sustainable society. Whereas numerous homogeneous chiral small molecule catalysts have been developed to date, there are only limited examples of heterogeneous ones that maintain high activity and have a long lifetime. On the other hand, metal nanoparticle catalysts have attracted much attention in organic chemistry due to their robustness and ease of deposition on solid supports. Given these advantages, metal nanoparticles modified with chiral ligands, defined as "chiral metal nanoparticles", would work efficiently in asymmetric catalysis. Although asymmetric hydrogenation catalyzed by chiral metal nanoparticles was pioneered in the late twentieth century, the application of chiral metal nanoparticle catalysis for asymmetric C-C bond-forming reactions that give a high level of enantioselectivity with wide substrate scope was very limited.This Account summarizes recent investigations that we have carried out in the field of chiral rhodium (Rh) nanoparticle catalysis for asymmetric arylation reactions. We initially utilized composites of polystyrene-based copolymers with cross-linking moieties and carbon black incarcerated Rh nanoparticle catalysts for the asymmetric 1,4-addition of arylboronic acids to enones. We found that chiral diene-modified heterogeneous Rh nanoparticles were effective in these reactions, with excellent enantioselectivities and without causing metal leaching, and that bimetallic Rh/Ag nanoparticle catalysts enhanced activity. The catalyst could be easily recovered and reused more than ten times, thus demonstrating the robustness of metal nanoparticle catalysts.We then developed a secondary amide-substituted chiral diene modifier designed as a bifunctional ligand that possesses a metal biding site and a NH group to activate a substrate through hydrogen bonding. This chiral diene was very effective for the Rh/Ag nanoparticle-catalyzed asymmetric arylation of various electron-deficient olefins, including enones, unsaturated esters, unsaturated amides and nitroolefins, and imines to afford the corresponding products in excellent yields and with outstanding enantioselectivities. The system was also applicable for the synthesis of intermediates of various useful compounds. Furthermore, the compatibility of chiral Rh nanoparticles with other catalysts was confirmed, enabling the development of tandem reaction systems and cooperative catalyst systems.The nature of the active species was investigated. Several characteristic features of the heterogeneous nanoparticle systems that were completely different from those of the corresponding homogeneous metal complex systems were found.

Pristine and modified chitosan as solid catalysts for catalysis and biodiesel production: A minireview

Chitosan is one of the readily available polymers with relatively high abundance, biodegradable and sustainable materials with divergent functional groups that are employed in broad range of applications. Chitosan is widely used in many fields like adsorption, drug carrier for therapeutic activity, environmental remediation, drug formulation and among others. One of the unique features of chitosan is that it can be transformed to other forms like beads, films, flakes, sponges and fibres depending upon the applications. This review is aimed at showing the potential applications of chitosan and its modified solids in organic transformations. The number of existing articles is organized based on the nature of materials and subsequently with the types of reactions. After a brief description on the structural features of chitosan, properties, characterization methods including various analytical/microscopic techniques and some of the best practices to be followed in catalysis are also discussed. The next section of this review describes the catalytic activity of native chitosan without any modifications while the subsequent sections provide the catalytic activity of chitosan derivatives, chitosan covalently modified with metal complexes/salts through linkers and chitosan as support for metal nanoparticles (NPs). These sections discuss number of organic reactions that include Knoevenagel condensation, oxidation, reduction, heterocycles synthesis, cross-coupling reactions and pollutant degradation among others. A separate section provides the catalytic applications of chitosan and its modified forms for the production of fatty acid methyl esters (FAME) through esterification/transesterification reactions. The final section summarizes our views on the future directions of this field in the coming years.