Catalysis by Heterogenized Metal Polymers: Advances and Prospects

The current status, trends, and a specific role for macroligands in catalysis by heterogenized metallopolymeric complexes are considered. Relations between homogeneous catalysis, enzyme catalysis, and catalysis by heterogenized metal complexes are traced. The effects of various factors on the catalysis of the main reactions used in organic synthesis—hydrogenation, polymerization (in particular, under the action of immobilized metallocene and postmetallocene catalysts), and redox processes (such as the catalysis of oxygenation, hydroperoxide oxidation, epoxidation, and hydroformylation)—are analyzed. In this review, attention is focused on the nondestructive identification of intermediates and catalytically active species in heterogenized systems. Experimental evidence is presented in support of the fact that the high activity, stability, and selectivity of immobilized catalysts are associated with a dramatic inhibition of concerted reactions in the coordination sphere of a transition metal, which result in catalyst deactivation, as well as with substrate enrichment. Prospects for the development of these highly organized hybrid systems and possibilities to consider the main requirements imposed on metal complex catalysis even at the stage of designing them are predicted.     

Water-Soluble Ligands,Metal Complexes,and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis

Rapid developments in the field of catalysis are leading to an increased demand for tailor-made catalysts. Water-soluble complex catalysts, which are being intensively investigated at the present time, combine the advantages of homogeneous and heterogeneous catalysis: simple and complete separation of the product from the catalyst, high activity, and high selectivity. From the large number of available water-soluble ligands, the appropriate catalysts can be developed for many reactions. The industrial applications in the fields of hydrogenation and hydroformylation have already indicated the wide scope of this type of catalyst. In addition, the annual production of 300 000 tons of butyraldehyde through application of water-soluble rhodium complexes at Hoechst AG in Oberhausen, Germany, has demonstrated the industrial importance of the concept of complex-catalyzed reactions in aqueous two-phase systems. The efficient operation of catalytic processes increasingly requires the loss-free recycling of the noble metal catalyst, and this can be simply and economically realized in two-phase systems. Special applications in biochemical problems open up developments in the field of water-soluble transition metal complexes that far transcend the familiar kinds of homogeneous catalysis. In the near future, the investigation and application of metal complex catalysts that are compatible with the physiological, cheap, and environmentally friendly solvent, water, is likely to become a high priority in catalysis research.     

同核铁系双金属络合物及其在均相催化体系中的应用

随着合成化学的不断发展,开发高活性催化剂来活化一些惰性化学键或者惰性分子受到越来越多的关注。双核金属络合物作为一类特殊的催化剂展现出了不同于单核金属催化剂的催化活性。在双核过渡金属催化体系中,因两个金属中心存在协同作用而表现出了独特的催化活性。铁、钴、镍为第四周期第VIII族元素,也称为铁系元素。该类金属廉价易得且参与的催化反应种类繁多,近年来引起了人们的广泛关注。本综述重点介绍了近年来同核双金属铁系络合物的合成及其表征。同时,对相关同核铁、钴以及镍催化剂在均相催化体系中的应用也进行了详细的介绍和总结。     

Biphasic catalysis in petrochemical processes

Main features of the concept of homogeneous metal complex biphasic catalysis are considered. Different techniques for immobilization of metal complexes in water, ionic liquids, supercritical carbon dioxide, and fluorous solvents are discussed. The use of discussed approaches in petrochemical processes is analyzed.     

Bimetallic catalysis using transition and Group 11 metals: an emerging tool for C-C coupling and other reactions

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group?11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.     

Nickel: An Element with Wide Application in Industrial Homogeneous Catalysis

The efficiency and future development of the chemical industry are closely linked to catalysis. It has been estimated, for example, that 60 to 70% of all industrial chemicals have involved the use of a catalyst at some point during their manufacture. In the past two decades the share of the market credited to homogeneous transition metal catalysis increasead to 10–15%. Besides cobalt, which is used mainly in hydroformylation reactions, nickel is the most frequently used metal. Many carbon–carbon bond formation reactions can be carried out with high selectivity if catalyzed by organonickel complexes. Such reactions include, inter alia, carbonylation reactions, cyclic and linear oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions. It was Reppe and Wilke who pioneered and shaped the field of homogeneous nickel catalysis. Great impetus was also given to the development of organonickel chemistry by Wilke and his students. Research in this area has contributed immensely towards an understanding of the reactions involved in catalysis.—This review is primarily concerned with nickel-catalyzed reactions which are of interest both preparatively and industrially; some mechanistic aspects are also dealt with.     

Transition metal-mediated P-C/X exchange at bound phosphine ligands (X=aryl, alkyl, NR2, OR and F): scope and mechanisms

The range of transition metal-mediated P-C/X exchange reactions that result in the replacement of a phosphine substituent with another group, X, are categorised according to the nature of the replacing group (X=aryl or alkyl, N- and O-based species and fluoride). Proposed mechanisms for P-C/X exchange are described and the factors promoting these unusual-and often undesirable-reactions are discussed. This tutorial review should be of relevance for those engaged in homogeneous catalysis, C-F activation and the synthesis of complexes combining soft metal centres and hard donor ligands.     

Bimetallic Catalysis using Transition and Group 11 Metals: An Emerging Tool for C?C Coupling and Other Reactions

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C? C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.     

Synthesis of heterocyclic compounds in the presence of transition metal complexes (review)

The most recent data on reactions involving the formation of heterocycles that take place in the presence of palladium, nickel, cobalt, and iron complexes are examined. The mechanisms of the reactions are discussed in the light of modern concepts of catalysis by metal complexes. The particular significance of metal complex catalysts for the synthesis of heterocycles with unique structures is demonstrated. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 147–163, February, 1980.     

镍N-杂环卡宾配合物在均相催化偶联反应中的应用

 近十几年来, N-杂环卡宾的配位化学和金属有机化学发展迅速, 已成为均相催化反应中研究最为广泛的配体之一. 在许多过渡金属催化的有机合成反应中特别是偶联反应中, N-杂环卡宾与传统有机膦配体相比具有较高的反应性. 镍价格低廉, 在很多反应中有望替代贵金属钯催化剂. 本文总结了镍 N-杂环卡宾化合物在催化交叉偶联反应和还原偶联反应中的最新应用进展.     

Reactivity and Recyclability of Ligand-Protected Metal Cluster Catalysts for CO2 Transformation

It remains a significant challenge to construct an integrated catalyst that combines advantages of homogeneous and heterogeneous catalysis with clarified mechanism and high performance. Here we show atomically precise CuAg cluster catalysts for CO₂ capture and utilization, where two functional units are combined into the clusters: metal and ligand. Due to atomic resolution on total and local structures of such catalysts to be achieved, which disentangles heterogeneous imprecise systems and permits tracing the reaction processes via experiments coupled with theory, site-specific catalysis induced by metal-ligand synergy can be accurately elucidated. The CuAg cluster catalysts exhibit excellent reactivity and recyclability to forge the C−N bonding from CO₂ formylation with secondary amines that can make the cluster catalysts more unique compared with typically homogeneous complexes.     

Homogene Metallkatalyse – effektiv,aber auch effizient?

The high selectivity makes transition metal catalyzed reactions valuable for organic synthesis. Although very effective, not all of them are also efficient. This circumstance is illustrated by the small number of homogeneous metal-catalyzed reactions that is used in industrial production processes compared to heterogeneous catalysis. Using important examples of the field, criteria and measures are discussed that can be used to evaluate the efficiency of homogeneous metal-catalyzed processes. While the concept of atom economy shows the efficiency of a transformation only if very small amounts of catalyst are needed, turn over numbers give the efficiency of the catalyst itself.     

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.     

Anti-Markovnikov hydration of α-olefins and the other pathways to n-alcohols

Three possible mechanisms of the direct hydration of α-olefins under conditions of catalysis with metal complexes in solution are considered, and information that confirms the possible realization of the reaction steps of various types is presented. Factors determining the regioselectivity of reaction steps are considered in the framework of various mechanisms, and the available published data about catalytic systems capable of ensuring the direct hydration of α-olefins are analyzed. New approaches to organization of the synthesis of n-alcohols from olefins and dienes are discussed, which are based on the systems of consecutive reactions, conjugated, and combined (associated) processes.     

Contributions of quantum chemistry to the development of ring opening polymerizations and chemical recycling

Quantum chemistry has increasingly become an integral component for the development of homogeneous catalytic processes to form polymers and to break them down. This review examines how quantum chemistry has been utilized to gain insights on mechanisms and selectivities in polymerization and depolymerization reactions by homogeneous catalysts, from some of the earliest uses of theory to the most recent efforts. Key aspects of catalysis by transition-metal catalysts, organocatalytic bases and organic acids will be highlighted.     

Metal complex-assisted activation of small molecules. From NO to superoxide and peroxides

Transition metal centres are one of the primary targets for nitric oxide (NO), superoxide (O2(-)) and hydrogen peroxide (H2O2), which are small molecules present in a biological milieu, and of industrial and environmental interest. Coordination to a metal centre modulates their redox behaviour in such a way that they become activated for an inner-sphere oxidation or reduction, depending on the electronic and redox properties of a particular transition metal ion. Since the related redox reactions play multiple roles in physiological and pathophysiological processes, as well as in chemical catalysis in terms of synthetic applications and exhaust gas purification, the elucidation of the mechanisms of the elementary reaction steps behind these complex processes is of fundamental importance. This review concentrates on our work in this area, where by applying low temperature and high pressure kinetic and thermodynamic techniques we shed more light on the mechanisms of the particular reaction steps involved in the activation of NO, O2(-) and various peroxides. The studies include work on solvent exchange reactions that control the binding of small molecules to the metal centre and subsequent electron-transfer processes. We paid special attention to different iron and manganese complexes with heme and non-heme ligand systems.     

Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non‐Innocent Scaffolds for Homogeneous Catalysis

Ligands in coordination chemistry and homogeneous catalysis are traditionally “static” spectators that do not actively participate in the catalytic cycle. However, such classic systems do not provide additional “handles” that could facilitate or trigger alternative productive reaction pathways. Recent advances in the use of novel nitrogen‐centered pincer systems have unveiled interesting opportunities for cooperative catalysis. The chemistry of pyridine‐derived, neutral ligands is discussed, with a specific focus on their non‐innocent behavior and potential as facilitators for metal‐mediated organic transformations. This overview should provide inspiration and an incentive to incorporate non‐innocent ligands and their metal complexes within old and new homogeneously catalyzed reactions.     

Leveraging Metal and Ligand Reactive Sites for One Pot Reactions: Ligand-Centered Borenium Ions for Tandem Catalysis with Palladium

Tandem catalysts that perform two different organic transformations in a single pot are highly desirable because they enable rapid and efficient assembly of simple organic building blocks into more complex molecules. Many examples of tandem catalysis rely on metal-catalyzed reactions involving one or more metal complexes. Remarkably, despite surging interest in the development of chemically reactive (i. e., non-innocent) ligands, there are few examples of metal complexes that leverage ligand-centered reactivity to perform catalytic reactions in tandem with separate catalytic reactions at the metal. Here we report how multifunctional Pd complexes with triaminoborane-derived diphosphorus ligands, called TBDPhos, appear to facilitate borenium-catalyzed cycloaddition reactions at the ligand, and Pd-catalyzed Stille and Suzuki cross-coupling reactions at the metal. Both transformations can be accessed in one pot to afford rare examples of tandem catalysis using separate metal and ligand catalysis sites in a single complex.     

Combinatorial synthesis of bimetallic complexes with three halogeno bridges

Methods for the synthesis of bimetallic complexes in which two different metal fragments are connected by three chloro or bromo bridges are reported. The reactions are general, fast, and give rise to structurally defined products in quantitative yields. Therefore, they are ideally suited for generating a library of homo- and heterobimetallic complexes in a combinatorial fashion. This is of special interest for applications in homogeneous catalysis. Selected members of this library were synthesized and comprehensively characterized; single-crystal X-ray analyses were performed for 15 new bimetallic compounds.     

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

The sustainable use of the resources on our planet is essential. Noble metals are very rare and are diversely used in key technologies, such as catalysis. Manganese is the third most abundant transition metal of the Earth's crust and based on the recently discovered impressive reactivity in hydrogenation and dehydrogenation reactions, is a potentially useful noble‐metal “replacement”. The hope of novel selectivity profiles, not possible with noble metals, is also an aim of such a “replacement”. The reactivity of manganese complexes in (de)hydrogenation reactions was demonstrated for the first time in 2016. Herein, we summarize the work that has been published since then and especially discuss the importance of homogeneous manganese catalysts in comparison to cobalt and iron catalysts.