Linear scaling relationships and volcano plots in homogeneous catalysis – revisiting the Suzuki reaction

Linear free energy scaling relationships and volcano plots are common tools used to identify potential heterogeneous catalysts for myriad applications. Despite the striking simplicity and predictive power of volcano plots, they remain unknown in homogeneous catalysis. Here, we construct volcano plots to analyze a prototypical reaction from homogeneous catalysis, the Suzuki cross-coupling of olefins. Volcano plots succeed both in discriminating amongst different catalysts and reproducing experimentally known trends, which serves as validation of the model for this proof-of-principle example. These findings indicate that the combination of linear scaling relationships and volcano plots could serve as a valuable methodology for identifying homogeneous catalysts possessing a desired activity through a priori computational screening.     

Understanding the Surprising Oxidation Chemistry of Au−OH Complexes

Au is known to be fairly redox inactive (in catalysis) and bind oxygen adducts only quite weakly. It is thus rather surprising that stable Au−OH complexes can be synthesized and used as oxidants for both one- and two-electron oxidations. A charged Au^III−OH complex has been shown to cleave C−H and O−H bonds homolytically, resulting in a one-electron reduction of the metal center. Contrasting this, a neutral Au^III−OH complex performs oxygen atom transfer to phosphines, resulting in a two-electron reduction of the hydroxide proton to form a Au^III−H rather than causing a change in oxidation state of the metal. We explore the details of these two examples and draw comparisons to the more conventional reactivity exhibited by Au^I−OH. Although the current scope of known Au−OH oxidation chemistry is still in its infancy, the current literature exemplifies the unique properties of Au chemistry and shows promise for future findings in the field.     

Revised Role of Selectfluor in Homogeneous Au‐Catalyzed Oxidative CO Bond Formations

The pairing of transition metal catalysis with the reagent Selectfluor (F‐TEDA–BF₄) has attracted considerable attention due to its utility in myriad C?C and C?heteroatom bond‐forming reactions. However, little mechanistic information is available for Selectfluor‐mediated transition metal‐catalyzed reactions and controversy surrounds the precise role of Selectfluor in these processes. We present herein a systematic investigation of homogeneous Au‐catalyzed oxidative C?O bond‐forming reactions using density functional theory calculations. Currently, Selectfluor is thought to serve as an external oxidant in Au^I/Au^III catalysis. However, our investigations suggest that these reactions follow a newly proposed mechanism in which Selectfluor functions as an electrophilic fluorinating reagent involved in a fluorination/defluorination cycle. We have also explored Selectfluor‐mediated gold‐catalyzed homocoupling reactions, which, when cyclopropyl propargylbenzoate is used as a substrate, lead to an unexpected byproduct.     

Direct arylation and heterogeneous catalysis; ever the twain shall meet

The formation of aryl–aryl bonds and heteroaryl analogues is one of the most important C–C bond forming processes in organic chemistry. Recently, a methodology termed Direct Arylation (DA) has emerged as an attractive alternative to traditional cross-coupling reactions (Suzuki–Miyaura, Stille, Negishi, etc.). A parallel focus of the pharmaceutical and other chemical industries has been on the use heterogeneous catalysis as a favourable substitute for its homogeneous counterpart in cross-coupling reactions. Only very recently has heterogeneous catalysis been proposed and applied, to DA reactions. In this perspective, we consider the terms ‘heterogeneous’ and ‘homogeneous’ and the problems associated with their delineation in transition-metal catalysed reactions. We highlight the reports at the interface of DA and heterogeneous catalysis and we comment briefly on the methods used which attempt to classify reaction types as homo- or heterogeneous. In future work we recommend an emphasis be placed on kinetic methods which provide an excellent platform for analysis. In addition two analytical techniques are described which if developed to run in situ with DA reactions would illuminate our understanding of the catalysis. Overall, we provide an entry point, and bring together the mature, yet poorly-understood, subject of heterogeneous catalysis with the rapidly expanding area of DA, with a view towards the acceleration of catalyst design and the understanding of catalyst behaviour.     

Controllable Conversion of CO2 on Non-Metallic Gold Clusters

Gold nanoparticles in metallic or plasmonic state have been widely used to catalyze homogeneous and heterogeneous reactions. However, the catalytic behavior of gold catalysts in non-metallic or excitonic state remain elusive. Atomically precise Au_n clusters (n=number of gold atoms) bridge the gap between non-metallic and metallic catalysts and offer new opportunities for unveiling the hidden properties of gold catalysts in the metallic, transition regime, and non-metallic states. Here, we report the controllable conversion of CO₂ over three non-metallic Au_n clusters, including Au₉, Au₁₁, and Au₃₆, towards different target products: methane produced on Au₉, ethanol on Au₁₁, and formic acid on Au₃₆. Structural information encoded in the non-metallic clusters permits a precise correlation of atomic structure with catalytic properties and hence, provides molecular-level insight into distinct reaction channels of CO₂ hydrogenation over the three non-metallic Au catalysts.     

Gold-catalyzed cyanosilylation reaction: homogeneous and heterogeneous pathways

Gold had been considered to be an extremely inert metal, but recently it was found that nanometer-sized gold particles on metal-oxide supports acted as catalysts for simple organic reactions, such as oxidation and hydrogenation, even at or below room temperature. Herein, we report that gold nanoparticles (AuNPs) of zero oxidation state (Au0) are catalytically active for a C--C bond-forming reaction, the cyanosilylation of aldehydes. The AuNP-catalyzed cyanosilylation proceeded smoothly at room temperature with 0.2 wt % loading of AuNPs. The reactions of aromatic aldehydes were almost quantitative, except for benzaldehyde derivatives containing the electron-withdrawing NO2 group, and alpha,beta-unsaturated aromatic aldehydes were the most reactive substrates. The reactions also went smoothly for aliphatic aldehydes. Mechanistic studies indicated that the reactions proceeded both homogeneously and heterogeneously: homogeneous catalysis by leached gold species and heterogeneous catalysis by the adsorption of the reactants (aldehydes and trimethylsilyl cyanide) onto AuNPs. The ratio of homogeneous and heterogeneous catalysis was estimated to be approximately 4:1.     

Chiral Frustrated Lewis Pair@Metal-Organic Framework as a New Platform for Heterogeneous Asymmetric Hydrogenation

Asymmetric hydrogenation, a seminal strategy for the synthesis of chiral molecules, remains largely unmet in terms of activation by non-metal sites of heterogeneous catalysts. Herein, as demonstrated by combined computational and experimental studies, we present a general strategy for integrating rationally designed molecular chiral frustrated Lewis pair (CFLP) with porous metal–organic framework (MOF) to construct the catalyst CFLP@MOF that can efficiently promote the asymmetric hydrogenation in a heterogeneous manner, which for the first time extends the concept of chiral frustrated Lewis pair from homogeneous system to heterogeneous catalysis. Significantly, the developed CFLP@MOF, inherits the merits of both homogeneous and heterogeneous catalysts, with high activity/enantio-selectivity and excellent recyclability/regenerability. Our work not only advances CFLP@MOF as a new platform for heterogeneous asymmetric hydrogenation, but also opens a new avenue for the design and preparation of advanced catalysts for asymmetric catalysis.     

Polyoxometalates-based heterogeneous catalysts in acid catalysis

Acid catalysis, one of the most important industrial processes, suffers from the toxicity, corrosion and recyclability problems of conventional homogeneous acid catalysts. Thus, the development of green heterogeneous acid catalysts becomes the focus of fundamental research and industrial catalysis. As a class of discrete anionic metal-oxygen clusters with tunable structure at the molecular and atomic scales, polyoxometalates(POMs) benefit from their super strong Br?nsted acidity, high proton mobility,and thermal stability. POMs-based heterogeneous catalysts have been used as the potential green alternatives to conventional homogeneous acid catalysts. In this review, we summarize recent progress on the design strategies of the POMs-based heterogeneous catalysts and their catalytic properties in acid-catalyzed reactions, where they are combined with functionalized cations, modified through covalent interactions, supported onto the non-precious metal support, and introduced into the framework of porous polymers. The design, functional strategies and catalytic performance of these POMs-based heterogeneous catalysts in specific acid-catalyzed reactions are emphasized.     

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions

Excimers and exciplexes are defined as assemblies of atoms or molecules A / A ′ where interatomic/intermolecular bonding appears only in excited states such as [ A ₂]* (for excimers) and [ AA ′]* (for exciplexes). Their formation has become widely known because of their role in gas‐phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration Au^I(5d¹⁰)) through aurophilic contacts. In the corresponding excimers, a new type of Au???Au bonding arises, with bond energies and lengths approaching those of ground‐state Au?Au bonds between metal atoms in the Au⁰(5d¹⁰6s¹) and Au^II(5d⁹) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.     

Carbon–Phosphorus Coupling from C^N Cyclometalated AuIII Complexes

With the aim of exploiting new organometallic species for cross-coupling reactions, we report here on the Au^III-mediated C_aryl−P bond formation occurring upon reaction of C^N cyclometalated Au^III complexes with phosphines. The [Au(C^N)Cl₂] complex 1 featuring the bidentate 2-benzoylpyridine (C^CON) scaffold was found to react with PTA (1,3,5-triaza-7-phosphaadamantane) under mild conditions, including in water, to afford the corresponding phosphonium 5 through C−P reductive elimination. A mechanism is proposed for the title reaction based on in situ ³¹P{¹H} NMR and HR-ESI-MS analyses combined with DFT calculations. The C−P coupling has been generalized to other C^N cyclometalated Au^III complexes and other tertiary phosphines. Overall, this work provides new insights into the reactivity of cyclometalated Au^III compounds and establishes initial structure–activity relationships to develop Au^III-mediated C−P cross-coupling reactions.     

Controllable Conversion of CO2 on Non‐Metallic Gold Clusters

Gold nanoparticles in metallic or plasmonic state have been widely used to catalyze homogeneous and heterogeneous reactions. However, the catalytic behavior of gold catalysts in non‐metallic or excitonic state remain elusive. Atomically precise Au_n clusters (n=number of gold atoms) bridge the gap between non‐metallic and metallic catalysts and offer new opportunities for unveiling the hidden properties of gold catalysts in the metallic, transition regime, and non‐metallic states. Here, we report the controllable conversion of CO₂ over three non‐metallic Au_n clusters, including Au₉, Au₁₁, and Au₃₆, towards different target products: methane produced on Au₉, ethanol on Au₁₁, and formic acid on Au₃₆. Structural information encoded in the non‐metallic clusters permits a precise correlation of atomic structure with catalytic properties and hence, provides molecular‐level insight into distinct reaction channels of CO₂ hydrogenation over the three non‐metallic Au catalysts.     

Selective Homogeneous and Heterogeneous Gold Catalysis with Alkynes and Alkenes: Similar Behavior,Different Origin

The development of new sustainable chemical processes requires the implementation of ultra‐selective reaction processes. The enormous selectivity found for gold‐based catalysts when applied in several reactions has opened new frontiers. For instance, the selective activation of alkynes is a common feature for both homogeneous and heterogeneous gold catalysts. Herein, we employ experimental and theoretical methods to assess the similarities and differences in the performance of homogeneous and heterogeneous gold catalysts. Alkynophilicity, the selective activation of alkynes, is found to have a thermodynamic origin in the heterogeneous case and a kinetic one for homogeneous catalysis. Complex enyne rearrangements require the more active homogeneous (single gold) catalyst because it has more electrophilic character than its heterogeneous (nanoparticle) counterpart.     

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.     

Solid-supported cross-coupling catalysts derived from homogeneous nickel and palladium coordination complexes

Solid-supported catalysts derived from homogeneous nickel(II) and palladium(II) non-symmetrical salen-type coordination complexes have been prepared and shown to be effective in the heterogeneous catalysis of carbon-carbon cross-coupling reactions. The nickel catalyst has been used in room-temperature Tamao-Kumada-Corriu reactions and the palladium catalyst in the Heck reaction at elevated temperatures. The complexes were prepared by improved methods and characterised by spectroscopic techniques. Comparisons between the solid-supported catalysts and their homogeneous analogues are reported. The single-crystal structure determination of the nickel and palladium complexes [M(salenac-OH)][M = Ni, Pd; salenac-OH = 9-(2',4'-dihydroxyphenyl)-5,8-diaza-4-methylnona-2,4,8-trienato](2-)] is reported.     

Single-atom catalysis for organic reactions

Metal-based catalysis, including homogeneous and heterogeneous catalysis, plays a significant role in the modern chemical industry. Heterogeneous catalysis is widely used due to the high efficiency, easy catalyst separation and recycling. However, the metal-utilization efficiency for conventional heterogeneous catalysts needs further improvement compared to homogeneous catalyst. To tackle this, the pursing of heterogenizing homogeneous catalysts has always been attractive but challenging. As a recently emerging class of catalytic material, single-atom catalysts (SACs) are expected to bridge homogeneous and heterogeneous catalytic process in organic reactions and have arguably become the most active new frontier in catalysis field. In this review, a brief introduction and development history of single-atom catalysis and SACs involved organic reactions are documented. In addition, recent advances in SACs and their practical applications in organic reactions such as oxidation, reduction, addition, coupling reaction, and other organic reactions are thoroughly reviewed. To understand structure-property relationships of single-atom catalysis in organic reactions, active sites or coordination structure, metal atom-utilization efficiency (e.g., turnover frequency, TOF calculated based on active metal) and catalytic performance (e.g., conversion and selectivity) of SACs are comprehensively summarized. Furthermore, the application limitations, development trends, future challenges and perspective of SAC for organic reaction are discussed.     

Synthetic application of gold complexes on magnetic supports

Gold nanocatalysts (GNCs) have recently become a hot topic and are widely used in chemistry. Between heterogeneous and homogeneous catalyses, two main types of catalyses, heterogeneous catalysts have gained importance because of their ease of separation from the mixture with low contamination of the products. Currently, nano-heterogeneous catalysis is the main subject in a wide range of research studies because nanostructures have unique abilities that are found only at the nano-level of the molecular world. This review selectively discusses the magnetic phenomena of heterogeneous GNCs, in which their inherent magnetic property provides many advantages over the nonmagnetic counterparts and also highlights the synthesis, development, and recyclability of various types of magnetic GNCs in well-known organic reactions. These insights are useful for researchers working on the future of magnetic-type gold nanocatalysis to enhance organic transformations, in both rate and yield.     

Efficient anodic and direct phenol-arene C,C cross-coupling: the benign role of water or methanol

C,C cross-coupling reactions for the synthesis of nonsymmetrical biaryls represent one of the most significant transformations in contemporary organic chemistry. A variety of useful synthetic methods have been developed in recent decades, since nonsymmetrical biaryls play an evident role in natural product synthesis, as ligand systems in homogeneous catalysis and materials science. Transformation of simple arenes by direct C,H activation belongs to the cutting-edge strategies for creating biaryls; in particular the 2-fold C,H activation is of significant interest. However, in most examples very costly noble metal catalysts, ligand systems, and significant amount of waste-producing oxidants are required. Electrochemical procedures are considered as inherently "green" methods, because only electrons are required and therefore, no reagent waste is produced. Here, we report a metal-free electrochemical method for cross-coupling between phenols and arenes using boron-doped diamond (BDD) anodes in fluorinated media. Our sustainable approach requires no leaving functionalities. Employing water or methanol as mediator represents the key improvement for achieving nonsymmetrical biaryls with superb selectivity and synthetic attractive yields.     

Reductive Elimination or C−C Bond Activation with Model Ni,Pd, Pt Complexes? A High-Accuracy Comparative Computational Analysis of Reactivity

The development of Pd- and Ni-catalyzed reactions for C−C bond formation is one of the primary driving forces in modern organic synthesis and the fine chemical industry. However, understanding the role of conformational mobility in reaction mechanisms is a long-standing challenge. We highlight the effect of a multirotamer (multiconformer) system on the effective Gibbs free energy of activation in the key C−C coupling process and promote the use of a simplified version of multiconformer transition state theory that is straightforward to apply. Multivariate regression helped to quantitatively map the effect of coupled organic substituents (their structural and electronic parameters), as well as to determine the relative activity of metals. We provide computational evidence for solvent control of the equilibrium in RE/C−C-bond activation for some model complexes. We also demonstrate that Ni complexes, being unique in the catalysis of sp³-sp³ couplings, can be more challenging for machine learning and computational chemistry. The modeling was performed at an exceptionally high level, DLPNO-CCSD(T)/CBS//RIJCOSX-PBE0-D4/def2-TZVP. The Conclusions section contains an infographic summarizing the key findings related to the fields of cross-coupling catalysis, machine learning in catalysis, and computational chemistry.     

均相催化多相化新进展——氟两相体系

氟两相体系实现了均相催化多相化, 可以方便地分离溶解在PFC 中的催化剂。本文综述了氟两相体系及氟合成领域, 包括氟溶配体、催化剂和反应物等方面的最新进展。对氟两相体系在烯烃的氢甲酰化反应、硼氢化反应以及其它有机反应中的广泛应用作了讨论。     

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI/AuIII Catalysis

In the last decade, major advances have been made in homogeneous gold catalysis. However, Au^I/Au^III catalytic cycle remains much less explored due to the reluctance of Au^I to undergo oxidative addition and the stability of the Au^III intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for Au^I/Au^III redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.