A review of enantioselective dual transition metal/photoredox catalysis

Transition metal catalysis is one of the most important tools to construct carbon-carbon and carbon-heteroatom bonds in modern organic synthesis. Visible-light photoredox catalysis has recently drawn considerable attention of the scientific community owing to its unique activation modes and significance for the green synthesis. The merger of photoredox catalysis with transition metal catalysts, termed metallaphotoredox catalysis, has become a popular strategy for expanding the synthetic utility of visiblelight photocatalysis. This strategy has led to the discovery of novel asymmetric transformations, which are unfeasible or not easily accessible by a single catalytic system. This contemporary area of organic chemistry holds promise for the development of economical and environmentally friendly methods for the asymmetric synthesis of chiral compounds. In this review, the advances in the enantioselective metallaphotoredox catalysis(EMPC) are summarized.     

Transition Metal Catalysis Using Functionalized Dendrimers

Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems. These novel polymers have inspired many chemists to develop new materials and several applications have been explored, catalysis being one of them. The recent impressive strides in synthetic procedures increased the accessibility of functionalized dendrimers, resulting in a rapid development of dendrimer chemistry. The position of the catalytic site(s) as well as the spatial separation of the catalysts appears to be of crucial importance. Dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes, their efficient natural counterparts, whereas the surface-functionalized systems have been proposed to fill the gap between homogeneous and heterogeneous catalysis. This might yield superior catalysts with novel properties, that is, special reactivity or stability. Both the core and periphery strategies lead to catalysts that are sufficiently larger than most substrates and products, thus separation by modern membrane separation techniques can be applied. These novel homogeneous catalysts can be used in continuous membrane reactors, which will have major advantages particularly for reactions that benefit from low substrate concentrations or suffer from side reactions of the product. Here we review the recent progress and breakthroughs made with these promising novel transition metal functionalized dendrimers that are used as catalysts, and we will discuss the architectural concepts that have been applied.     

Bis-(thiosemicarbazonato) Zn(II) complexes as building blocks for construction of supramolecular catalysts

In this paper we report the application of bis-(thiosemicarbazonato) Zn(II) complexes as building blocks in the construction of supramolecular transition metal assemblies. We investigated their coordination behaviour towards pyridylphosphine molecules and found these systems comparable to those based on Zn(porphyrin) and Zn(salphen) complexes. Additionally, catalytic experiments and an in situ high-pressure FTIR study of the supramolecular rhodium hydroformylation catalysts, assembled using the bis-(thiosemicarbazonato) Zn(II) complexes, demonstrate their applicability in supramolecular catalysis and their potential for application in other areas of supramolecular chemistry.     

Activation of Carboxylic Acids in Asymmetric Organocatalysis

Organocatalysis, catalysis using small organic molecules, has recently evolved into a general approach for asymmetric synthesis, complementing both metal catalysis and biocatalysis. 1 Its success relies to a large extent upon the introduction of novel and generic activation modes. 2 Remarkably though, while carboxylic acids have been used as catalyst directing groups in supramolecular transition‐metal catalysis, 3 a general and well‐defined activation mode for this useful and abundant substance class is still lacking. Herein we propose the heterodimeric association of carboxylic acids with chiral phosphoric acid catalysts as a new activation principle for organocatalysis. This self‐assembly increases both the acidity of the phosphoric acid catalyst and the reactivity of the carboxylic acid. To illustrate this principle, we apply our concept in a general and highly enantioselective catalytic aziridine‐opening reaction with carboxylic acids as nucleophiles.     

New supramolecular transition metal catalysts

The covalent connection of a catalytically active transition metal center with a water-soluble receptor (host molecule) generates a new type of supramolecular catalyst in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins, unusual degrees of substrate selectivity based on molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (water/organic) hydrogenation of nitro-aromatics also is a smooth process catalyzed by these supramolecular complexes. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system. Dendrimers having diphosphane moieties on the surface provide ligands for transition metals, the corresponding metal complexes (e.g., Pd) functioning as efficient catalysts which can be recycled due to their nanoscopic properties.     

3D Supramolecular Network Based on Hydrogen Bonds Between 1D [ Ni(μ2 -C2O4 ) (Him) 2 ] Zigzag Chains

Introduction In recent years, considerable attention has been paid to supramolecular networks based on metal organic building blocks because of their potential applications in diverse fields, such as, catalysis, optics, sensors,magnetism, and molecular recognition[1-3]. On the basis of the principles of crystal engineering and special synthesis strategies, several novel supramolecular frameworks have been assembled from various organic,inorganic and metal-organic moieties, which largely enriches the structure chemistry of solid state materials[4-9].     

Photoactive Nickel Complexes in Cross-Coupling Catalysis

Transition metal catalyzed cross-coupling reactions are important in chemical synthesis for the formation of C−C and C-heteroatom bonds. Suitable catalysts are frequently based on palladium or nickel, and lately the cheaper and more abundant first-row transition metal element has been much in focus. The combination of nickel catalysis with photoredox chemistry has opened new synthetic possibilities, and in some cases electronically excited states of nickel complexes play a key role. This is a remarkable finding, because photo-excited metal complexes are underexplored in the context of organic bond-forming reactions, and because the photophysics and the photochemistry of first-row transition metal complexes are underdeveloped in comparison with their precious metal-based congeners. Consequently, there is much potential for innovation at the interface of synthetic-organic and physical-inorganic chemistry. This Minireview highlights recent key findings in light-driven nickel catalysis and identifies essential concepts for the exploitation of photoactive nickel complexes in organic synthesis.     

Recent Advances in Supramolecular Gels and Catalysis

Over the past two decades, supramolecular gels have attracted significant attention from scientists in diverse research fields and have been extensively developed. This review mainly focuses on the significant achievements in supramolecular gels and catalysis. First, by incorporating diverse catalytic sites and active organic functional groups into gelator molecules, supramolecular gels have been considered as a novel matrix for catalysis. In addition, these rationally designed supramolecular gels also provide a variety of templates to access metal nanocomposites, which may function as catalysts and exhibit high activity in diverse catalytic transformations. Finally, as a new kind of biomaterial, supramolecular gels formed in situ by self‐assembly triggered by catalytic transformations are also covered herein.     

Catalytic Antibodies

The vast binding repertoire of the immune system has been exploited for the generation of tailor-made biological catalysts. A number of strategies have been developed to generate catalytic antibodies that carry out a wide range of reactions with exquisite specificities. The generation and characterization of these novel catalysts is not only providing new insights into the nature of biological recognition and catalysis, but may also lead to novel catalysts for applications in chemistry, biology, and medicine.     

Transition Metal Catalysis in Living Cells: Progress,Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.     

Macromolecules containing bipyridine and terpyridine metal complexes: towards metallosupramolecular polymers

The ability of a broad range of N-heterocycles to act as very effective and stable complexation agents for several transition metal ions, such as cobalt(II), copper(II), nickel(II), and ruthenium(II), has long been known in analytical chemistry. This behavior was later utilized in supramolecular chemistry for the construction of highly sophisticated architectures, such as helicates, racks, and grids. The discovery of macromolecules by Staudinger in 1922 opened up avenues towards sophisticated materials with properties hitherto completely unknown. In the last few decades, the combination of macromolecular and supramolecular chemistry has been attempted by developing metal-complexing and metal-containing polymers for a wide variety of applications that range from filtration to catalysis. The stability of the polymer-metal complex is a fundamental requirement for such applications. In this respect, the use of bi- and terpyridines as chelating ligands is highly promising, since these molecules are known to form highly stable complexes with interesting physical properties with transition-metal ions. A large number of different structures have been designed for many different applications, but polymers based on the application of coordinative forces have been prepared in a few cases only. Furthermore, the synthetic procedures applied frequently resulted in low yields. During the last few years, strong efforts have been made in the direction of self-assembling and supramolecular polymers as novel materials with "intelligent" and tunable properties. In this review, an overview of this active area at the interface of supramolecular and macromolecular chemistry is given.     

Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts.By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).     

Supramolecular Strategies for Controlling Reactivity within Confined Nanospaces

Nanospaces are ubiquitous in the realm of biological systems and are of significant interest among supramolecular chemists. Understanding chemical behavior within nanospaces offers new perspectives on biological phenomena in nature and opens the way to highly unusual and selective forms of catalysis. Supramolecular chemistry exploits weak, yet effective, intermolecular interactions such as hydrogen bonding, metal‐ligand coordination, and the hydrophobic effect to assemble nano‐sized molecular architectures, providing reactions with remarkable rate acceleration, substrate specificity, and product selectivity. In this minireview, the focus is on the strategies that supramolecular chemists use to emulate the efficiency of biological processes, and elucidating how chemical reactivity is efficiently controlled within well‐defined nanospaces. Approaches such as orientation and proximity of substrate, transition‐state stabilization, and active‐site incorporation will be discussed.     

Ruthenium Catalysis in Biological Habitats

Recent years have witnessed a considerable progress in research aimed at merging transition metal catalysis with chemical and cell biology. Therefore, a crescent number of metal-catalyzed transformations have been shown compatible with biological media and even with living settings. Of the different transition metals used to build these biocompatible catalysts, ruthenium has demonstrated to be particularly powerful, in part because the resulting complexes exhibit a very good balance between reactivity and biological stability. Indeed, ruthenium complexes have demonstrated utility to promote a great variety of reactions in biologically relevant contexts, from deprotection and redox processes to cycloadditions or photocatalytic transformations. Many of these reactions may enable the development of new type of biological tools and pharmacological strategies.     

多金属氧酸盐及其溶液自聚集行为研究进展

多金属氧酸盐(POM)是一类由过渡金属与氧原子桥连而成的阴离子簇合物,由于其特殊的分子结构及优异的物理化学性质,使其在催化、医药、材料科学、表面化学、超分子化学等领域有广泛的应用价值。 POM在稀的水溶液中能够发生自聚集,形成类似两亲分子溶液中的“有序聚集体结构”,赋予其新的结构和性质,以期开发出新型纳米器件及在催化、药物等领域得到应用。 本文介绍了POM的主要结构、性质和近年来的应用,阐述了其在溶液中自聚集行为的研究状况和新进展。     

Catalysis Concepts in Medicinal Inorganic Chemistry

Catalysis has strongly emerged in the field of medicinal inorganic chemistry as a suitable tool to deliver new drug candidates and to overcome drawbacks associated to metallodrugs. In this Concept article, we discuss representative examples of how catalysis has been applied in combination with metal complexes to deliver new therapy approaches. In particular, we explain key achievements in the design of catalytic metallodrugs that damage biomolecular targets and in the development of metal catalysis schemes for the activation of exogenous organic prodrugs. Moreover, we discuss our recent discoveries on the flavin-mediated bioorthogonal catalytic activation of metal-based prodrugs; a new catalysis strategy in which metal complexes are unconventionally employed as substrates rather than catalysts.     

Textural manipulation of mesoporous materials for hosting of metallic nanocatalysts

The preparation and stabilization of nanoparticles are becoming very crucial issues in the field of so-called "nanocatalysis". Recent developments in supramolecular self-assembled porous materials have opened a new way to get nanoparticles hosted in the channels of such materials. In this paper, a new approach towards monodisperse and thermally stable metal nanoparticles by confining them in ordered mesoporous materials is presented, and three aspects are illustrated. Firstly, the recent progress in the functional control of mesoporous materials will be briefly introduced, and the rational tuning of the textures, pore size, and pore length is demonstrated by controlling supramolecular self-assembly behavior. A novel synthesis of short-pore mesoporous materials is emphasized for their easy mass transfer in both biomolecule absorption and the facile assembly of metal nanocomposites within their pore channels. In the second part, the different routes for encapsulating monodisperse nanoparticles inside channels of porous materials are discussed, which mainly includes the ion-exchange/conventional incipient wetness impregnation, in situ encapsulation routes, organometallic methodologies, and surface functionalization schemes. A facile in situ autoreduction route is highlighted to get monodisperse metal nanoparticles with tunable sizes inside the channels of mesoporous silica. Finally, confinement of mesoporous materials is demonstrated to improve the thermal stability of monodisperse metal nanoparticles catalysts and a special emphasis will be focused on the stabilization of the metal nanoparticles with a low Tammann temperature. Several catalytic reactions concerning the catalysis of nanoparticles will be presented. These uniform nanochannels, which confine monodisperse and stable metal nanoparticles catalysts, are of great importance in the exploration of size-dependent catalytic chemistry and further understanding the nature of catalytic reactions.     

N‐Heterocyclic Carbenes: A New Concept in Organometallic Catalysis

N‐Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N‐heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C−H activation, C−C, C−H, C−O, and C−N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N‐heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.     

Bis(oxazolinyl)phenyl transition metal complexes: synthesis, asymmetric catalysis, and coordination chemistry

Molecular catalysts for organic synthesis should be constructed to be tailored to target reactions and their desirable conditions. In our search for them, we have studied new types of transition metal molecular catalysts dressed with a tridentate N,C,N modular ligand, which consists of a C2-symmetric side-by-side phenyl group with chiral bis(oxazolinyl) substituents. The ligand, 2,6-bis(oxazolinyl)phenyl abbreviated as Phebox, can connect covalently to transition metals by the central carbon atom. Here, we review our recent work on the chemistry of Phebox and its metal complexes, including preparation, structural analysis, asymmetric Lewis acid catalysis, asymmetric hydrosilylation, asymmetric conjugate reduction, asymmetric reductive aldol reaction, and organometallic reactions.     

Supramolecular Coordination Cages for Asymmetric Catalysis

Inspired by the high efficiency and specificity of enzymes in living systems, the development of artificial catalysts intrinsic to the key features of enzyme has emerged as an active field. Recent advances in supramolecular chemistry have shown that supramolecular coordination cages, built from non-covalent coordination bonds, offer a diverse platform for enzyme mimics. Their inherent confined cavity, analogous to the binding pocket of an enzyme, and the facile tunability of building blocks are essential for substrate recognition, transition-state stabilization, and product release. In particular, the combination of chirality with supramolecular coordination cages will undoubtedly create an asymmetric microenvironment for promoting enantioselective transformation, thus providing not only a way to make synthetically useful asymmetric catalysts, but also a model to gain a better understanding for the fundamental principles of enzymatic catalysis in a chiral environment. The focus here is on recent progress of supramolecular coordination cages for asymmetric catalysis, and based on how supramolecular coordination cages function as reaction vessels, three approaches have been demonstrated. The aim of this review is to offer researchers general guidance and insight into the rational design of sophisticated cage containers for asymmetric catalysis.