Homogeneous Transition Metal Catalysts Research Articles

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation.

The dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130-140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF-mediated dehydrogenative synthesis. The TTT-DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT-DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well-suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates' functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation

AbstractThe dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130–140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF‐mediated dehydrogenative synthesis. The TTT‐DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT‐DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well‐suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates’ functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Asymmetric Hydrogenation of Ketones by Simple Alkane-Diyl-Based Ir(P,N,O) Catalysts: A Comparative Study.

The development of new chiral ligands with simple and modular structure represents a challenging direction in the design of efficient homogeneous transition metal catalysts. Herein, we report on the asymmetric hydrogenation of prochiral ketones catalyzed by the iridium complexes of simple alkane-diyl-based P,N,O-type chiral ligands with a highly modular structure. The role of (i) the P-N and N-O backbone in the potentially tridentate ligands, (ii) the number, position and relative configuration of their stereogenic elements and (iii) the effect of their NH and OH subunits on the activity and enantioselectivity of the catalytic reactions are studied. The systematic variation in the ligand structure and the comparative catalytic experiments shed light on different mechanistic aspects of the iridium-catalyzed reaction. The catalysts containing the simple alkane-diyl-based ligands with central chirality provided high enantioselectivities (up to 98% ee) under optimized reaction conditions and proved to be active and selective even at very high substrate concentrations (100 mmol substrate/mL solvent).

Metalated covalent organic frameworks as efficient catalysts for multicomponent tandem reactions.

Multicomponent tandem reactions have become indispensable synthetic methods due to their economic advantages and efficient usage in natural products and drug synthesis. The emergence of metalated covalent organic frameworks (MCOFs) has opened up new opportunities for the advancement of multicomponent tandem reactions. In contrast to commonly used homogeneous transition metal catalysts, MCOFs possess regular porosity, high crystallinity, and rich metal chelation sites that facilitate the uniform distribution and anchoring of metals within their cavities. Thus, they show extremely high activity and have recently been widely employed as catalysts for multicomponent tandem reactions. It is timely to conduct a review of MCOFs in multicomponent tandem reactions, in order to offer guidance and assistance for the synthesis of MCOF catalysts and their application in multicomponent tandem reactions. This review provides a comprehensive overview of the design and synthesis of MCOFs, their application and progress in multicomponent tandem reactions, and the primary challenges encountered during their current development with the aim of contributing to the promotion of the field.

Developing homogeneous first row early transition metal catalysts for the oxygen reduction reaction.

The oxygen reduction reaction (ORR) remains an important fixture in biological and synthetic systems for energy conversion and chemical functionalization. Late transition metals continue to dominate in the development of new catalyst systems, inspired by well-characterized metallocofactors and prior successes. By comparison, metals to the left of Fe on the periodic table are relatively understudied for the ORR. This Frontier article summarizes advancements related to the use of Mn, Cr, and V in homogeneous catalyst systems for the ORR and discusses the implications of these results for the development of catalyst systems from these metals and those earlier in the transition metal series.

Mild aqueous metal catalyzed oxidative conversion of low-density polyethylene to low molecular weight aliphatic carboxylic acids

Aqueous oxidative deconstruction of low-density polyethylene (LDPE) using homogeneous first-row transition metal catalysts under mild conditions (130–150 °C and 100 PSI oxygen pressure).

Visible-light-induced C-H Annulation by Metal-decorated Covalent Organic Frameworks with Fe-bipyridyl linkages

The integration of transition metal and photoredox catalyst achieves a harmonious cooperation between sophisticated chemical transformations and judicious utilization of light energy. The previous work was exclusively accomplished through employment of homogeneous transition-metal catalysts or photoredox catalysts. In this study, we present the synthesis of iron-decorated covalent organic frameworks (Fe-COF) and its application on C-H annulation of amides with alkynes by irradiation of visible light. The iron center prompts robust chelation with amides, leading to C-H activation, subsequently. Meanwhile, the photoactive covalent organic framework facilitates visible light absorption and accelerates the C-H activation step. Significantly, isoquinolin-1(2H)-one derivatives with high yield and selectivity were obtained in the presence of Fe-COF under visible light, and highlighting the covalent organic framework's stability and inherent heterogeneous property with repeatable chemical recyclability.

Homogeneous Metal-Catalyzed Hydrogenation of CO2 Derivatives: Towards Indirect Conversion of CO2 to Methanol

The increase in anthropogenic CO2 concentrations and associated environmental issues have demanded the development of technologies for CO2 utilization. Among various potential solutions to decrease CO2 emissions and achieve carbon neutrality, the recycling of post-combustion CO2 into value-added chemicals and fuels is considered one of the most economically attractive processes. In this regard, due to its large global demand and versatile applications in the chemical and energy sectors, methanol serves as the most appealing target for the chemical utilization of CO2. However, direct hydrogenation of CO2 to MeOH has proved challenging due to selectivity issues and high energy input, mainly dependent on CO2-emitting fossil energy sources. To address these challenges, an alternative indirect CO2-to-MeOH methodology has been proposed, which involves the hydrogenation of CO2 via the intermediate formation of well-known CO2 derivatives, such as formates, carbonates, formamides, carbamates, and urea derivatives. Homogeneous transition metal catalysts have been at the center of this research avenue, potentially allowing for more selective and low-temperature alternative routes from CO2 to MeOH. This review aims to highlight the advances and challenges in homogeneous transition metal-catalyzed hydrogenation of major CO2 derivatives to MeOH. Special attention is paid to the mechanisms of such transformations.

The Fast Formation of a Highly Active Homogeneous Catalytic System upon the Soft Leaching of Pd Species from a Heterogeneous Pd/C Precursor

Understanding the interface between soluble metal complexes and supported metal particles is important in order to reveal reaction mechanisms in a new generation of highly active homogeneous transition metal catalysts. In this study, we show that, in the case of palladium forming on a carbon (Pd/C) catalyst from a soluble Pd(0) complex Pd2dba3, the nature of deposited particles on a carbon surface turns out to be much richer than previously assumed, even if a very simple experimental procedure is utilized without the use of additional reagents and procedures. In the process of obtaining a heterogeneous Pd/C catalyst, highly active “hidden” metal centers are formed on the carbon surface, which are leached out by the solvent and demonstrate diverse reactivity in the solution phase. The results indicate that heterogeneous catalysts may naturally contain trace amounts of molecular monometallic centers of a different nature by easily transforming them to the homogeneous catalytic system. In line with a modern concept, a heterogenized homogeneous catalyst precursor was found to leach first, leaving metal nanoparticles mostly intact on the surface. In this study, we point out that the previously neglected soft leaching process contributes to high catalyst activity. The results we obtained demand for leaching to be reconsidered as a flexible tool for catalyst construction and for the rational design of highly active and selective homogeneous catalytic systems, starting from easily available heterogeneous catalyst precursors.

Boron-catalysed hydrogenolysis of unactivated C(aryl)–C(alkyl) bonds

The hydrogenolysis of C–C bonds is one of the most important processes in the petroleum industry. These transformations typically rely on heterogeneous catalysts and take place at high temperatures and high pressures with limited selectivity. Employing homogeneous transition metal catalysts, while allowing the hydrogenolysis of C–C bonds to proceed under much milder conditions, is only suitable for substrates containing strained C–C bonds or directing groups. Here we report that a borenium complex can catalyse the selective hydrogenolysis of unstrained C(aryl)–C(alkyl) bonds of alkylarenes in the absence of directing groups at ambient temperature, affording the corresponding alkanes and arenes. Mechanistic studies suggest a reaction pathway that involves a synergistic activation of dihydrogen by the borenium complex and alkylarenes, followed by retro-Friedel–Crafts reaction to cleave the C(aryl)–C(alkyl) bonds. The synthetic utility of this protocol is demonstrated by the conversion of post-consumer polystyrene into valuable benzene and phenylalkanes with mass recovery rates above 90%.

Pushing Boundaries—Selective Cooling Crystallization as Tool for Selectivity Compensation and Product Purification Using a Recyclable Pd/Xantphos Catalyst in the Methoxycarbonylation of Methyl 10‐Undecenoate

AbstractThe homogeneously catalyzed methoxycarbonylation of methyl 10‐undecenoate allows for the synthesis of dimethyl 1,12‐dodecandioate as an interesting bio‐based drop‐in alternative for 1,12‐dodecandioic acid as polymer building block. Although the benchmark catalyst system of Pd/1,2‐bis((di‐tert‐butylphosphino)methyl)benzene and methane sulfonic acid is very active and selective, long‐term stability over a potential catalyst recycling is limited. In this work, modifications of this catalyst system in terms of protonation of the ligand and its replenishment during recycling are first investigated, proving that the reaction system is tolerant against minor changes. Finally, the commercially available ligand Xantphos, featuring higher stability but comes with a rather low l:b selectivity of 70:30, is applied. However, through the application of cooling crystallization, 58 g product (52% isolated yield) with an overall purity of 94% is obtained from the crude reaction solution without further treatment and a ∑TON of 4000 after ten reaction runs, while catalyst loss into the product is low.Practical Applications: Selective syntheses on the basis of renewable resources are a powerful tool for the production of value products in terms of green chemistry. Thereby, homogeneous transition metal catalysts are beneficial regarding selectivity. However, their separation and recycling are challenging due to their limited stability. The combination of a stable, commercially available catalyst with a selective purification method allows for isolation and purification from a crude reaction mixture without the need for any auxiliary or further purification steps. In this work, cooling crystallization is applied for subsequent purification of the linear diester dimethyl 1,12‐dodecandioate. Thereby, a lower selectivity from the methoxycarbonylation reaction using the stable Xantphos ligand is compensated and combined with recycling of the homogeneous catalyst. Thus, the development of an integrated process covering a stable catalyst system in the reaction, and high selectivity in the purification is the key toward an efficient homogeneous catalyst recycling.

Exporting Homogeneous Transition Metal Catalysts to Biological Habitats.

The possibility of performing designed transition‐metal catalyzed reactions in biological and living contexts can open unprecedented opportunities to interrogate and interfere with biology. However, the task is far from obvious, in part because of the presumed incompatibly between organometallic chemistry and complex aqueous environments. Nonetheless, in the past decade there has been a steady progress in this research area, and several transition‐metal (TM)‐catalyzed bioorthogonal and biocompatible reactions have been developed. These reactions encompass a wide range of mechanistic profiles, which are very different from those used by natural metalloenzymes. Herein we present a summary of the latest progress in the field of TM‐catalyzed bioorthogonal reactions, with a special focus on those triggered by activation of multiple carbon‐carbon bonds.

Homogeneous first-row transition metal catalyst for sustainable hydrogen production and organic transformation from methanol, formic acid, and bio-alcohols

The recent advances in homogeneous first-row transition-metal catalyzed for hydrogen production and important organic transformations from methanol, formic acid (FA), and bio-alcohols are the current interest, which is an important paradigm in chemical synthesis for the development of sustainable catalysis. Early work mainly based on noble metal (Ru, Ir, and Pt) catalysts, but at present the focused on mainly earth-abundant 3d base-metal (Mn, Fe, Co, and Ni) complexes, which are showing growing interest, highly elusive and demanding in the modern science due to its low cost, less toxic and more abundance. In this review, we mainly aim to describe the topics such as a) hydrogen formation and organic transformations from methanol, formic acid, and bio-alcohols; b) C-methylation, N-methylation, and formamide formation using methanol; c) N-heterocycle, lactam, lactone and hydrogen formation from diol or bioalcohols; d) hydrogen formation and storage capability of the liquid organic hydrogen carrier (LOHC) and additionally a special attention is paid to the utility of these reactions including brief reaction mechanism, and scope of reaction.

In situ activation of therapeutics through bioorthogonal catalysis.

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Catalytic Functionalization of 1,3-Butadiene by Carbon Electrophiles

1,3-Butadiene is a useful C4 carbon source for the production of basic chemicals; but use as carbon feedstock often presents regioselectivity problems arising from its conjugated diene structure. This review summarizes the functionalization of 1,3-butadiene with carbon electrophiles by using homogeneous transition metal catalysts. A copper catalyst generated by treatment of Cu salt with alkyl Grignard reagent catalyzed internal carbon selective reductive alkylation of 1,3-butadiene with alkyl fluoride as a carbon electrophile and the alkyl Grignard reagent as a hydride source. In contrast, nickel promoted dimerization and alkylarylation of 1,3-butadiene with a similar combination of substrates. Using polyfluoroarenes as the carbon electrophiles, similar transformations proceeded to achieve the functionalization of 1,3-butadiene. The substrate scope as well as details of the reaction mechanism of these transformations are discussed.

The Cascade C─H functionalization with sequential hydroxylation and oxidation through heterogeneous BINAP‐copper on hydrotalcite

AbstractBACKGROUNDAromatic thioether derivatives exist widely in natural products and have significant applications in modern synthesis and the pharmaceutical industry; for example, 2‐sulfanylphenols and arylthioquinones are important intermediates in the processing of anticancer, antibiotic, antivirus and antimalarial drugs. Direct C─H hydroxylation was considered as one of the efficient methods for the synthesis of these compounds. Homogeneous transition metal catalysts usually were used through a cross‐coupling of halogen‐substituted phenols with thiophenols. However, from the perspectives of sustainable development and easy recovery, it is necessary to develop an efficient and recoverable catalytic system to realize the synthesis of aromatic thioether derivatives with good efficiency and recyclability.RESULTSThe heterogeneous catalyst [Cu(binap)I]2@HT in the present work showed good catalytic activity for the C─H hydroxylation of benzenethiol and phenylboronic acid. The catalytic system was tolerant to various substituents including alkyl, alkoxy, ester, halogen or trifluoromethyl on the phenyl ring of the thiophenols or arylboric acids. In addition to electron‐donating groups, the products with strong electron‐withdrawing groups, such as methyl formylate and trifluoromethyl also were achieved with good yields at 90 °C for 12 h (68% and 66%, respectively). Repeated experiment results showed that the catalyst could be reused at least five times without obvious decrease in catalytic activity.CONCLUSIONA BINAP‐Cu complex supported on hydrotalcite was prepared and used as a heterogeneous catalyst for the C─H activation of thiophenol under mild reaction conditions. Through a cascade C─H hydroxylation and further oxidation process, 2‐Sulfanylphenols and arylthioquinones could be obtained with moderate‐to‐good yields for a series of 2‐(phenylthio)phenol derivatives using the heterogeneous catalyst [Cu(binap)I]2@HT. In addition, this heterogeneous catalyst showed good recylbility. © 2020 Society of Chemical Industry

Conceptual study of co-product separation from catalyst-rich recycle streams in thermomorphic multiphase systems by OSN

Homogeneous transition metal catalysts allow for high selectivity at mild reaction conditions. However, very high recoveries of the precious catalyst are decisive for the development of economic processes, especially in case of reactions with an atom economy below 100% since co-products and their potential accumulation have to be considered. To address this issue, an innovative process concept is investigated, which exploits thermomorphic multiphase systems combined with organic solvent nanofiltration for homogeneous catalyst recovery and co-product removal. In this concept, the reaction takes place at elevated temperature in a homogeneous liquid phase, while the reactor effluent is cooled down, inducing a phase split. This allows for the separation of a product-rich non-polar and a catalyst-rich polar phase that is recycled into the reactor. The current article investigates this concept considering a reductive amination reaction as case study based on an experimental membrane screening and a model-based evaluation. DuraMem 150 (T1) proved to be the best membrane for all tested solvents as catalyst ligand rejections higher than 99% and a removal of the co-product water were achieved. The model-based evaluation indicated a final OSN-process of three stages for the solvent methanol and the requirement of a subsequent solvent recovery that is realized by distillation.

Recent Achievements in the Hydrogenation of Nitriles Catalyzed by Transitional Metals

Amines are important and valuable intermediates in the pharmaceutical, plastic and agrochemical industry. Hence, there is an increasing interest in developing improved process for the synthesis of amines. The heterogeneous catalytic hydrogenation of nitriles is one of the most frequently applied methods for the synthesis of diverse amines, but the homogeneous catalysis has also received a growing attention from the catalysis community. This mini-review provides an overview of the recent achievements in the selective reduction of nitriles using both homogeneous and heterogeneous transition metal catalysts.

Thermomorphic Multiphase Systems: Switchable Solvent Mixtures for the Recovery of Homogeneous Catalysts in Batch and Flow Processes.

Over the past 20 years, thermomorphic multiphase systems (TMS) have been used as a versatile and elegant strategy for the recovery and recycling of homogeneous transition-metal catalysts, in both batch-scale experiments and continuously operated processes. TMS ensure a homogeneous reaction in a monophasic reaction mixture at reaction temperature and the recovery of the homogeneous transition-metal catalyst through liquid-liquid separation at a lower separation temperature. This is achieved by using at least two solvents, which have a highly temperature-sensitive miscibility gap. The suitability of commercially available solvents makes this approach highly interesting from an industrial point of view. For the first time, herein, all studies in the area of TMS are reviewed, with the aim of providing a concise and integral representation of this approach for homogeneous catalyst recovery. In addition to the discussion of examples from the literature, the thermodynamic fundamentals of the temperature-dependent miscibility of solvents are also presented. This review also gives key indicators to compare different TMS approaches, for instance. In this way, new solvent combinations and in-depth research, as well as improvements to existing approaches, can be addressed and promoted.

Substantially enhancing the catalytic performance of bis(imino)pyridylcobaltous chloride pre‐catalysts adorned with benzhydryl and nitro groups for ethylene polymerization

Variations in the ligand structure of homogeneous late transition metal catalysts through judicious choice and location of substituent is the foremost strategy in improving their catalytic performance for ethylene polymerization. In this contribution, symmetrical and unsymmetrical bis(imino)pyridylcobaltous chloride complexes adorned with nitro and benzhydryl groups {2‐[1‐(2,6‐dibenzhydryl‐4‐nitrophenylimino)ethyl]‐6‐[1‐(alkylphenylimino)ethyl]pyridylcobaltous chloride (alkyl: R1 = Me and R2 = H, Co1; R1 = Et and R2 = H, Co2; R1 = iPr and R2 = H, Co3; R1 and R2 = Me, Co4; R1 = Et and R2 = Me, Co5; R1 = benzhydryl and R2 = NO2, Co6)} have been prepared and applied as catalysts for ethylene polymerization. The molecular structure of Co1 and Co2 revealed the unequal steric protection of the cobalt center induced by bis(imino)pyridine chelate. In the presence of methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) activators at different ethylene feeding rates (1 and 10 atm), catalysts Co1–Co5 displayed high activities at 10 atm ethylene and produced strictly linear polyethylene (PE) with high molecular weight, Co2/MMAO being the most highly active catalytic system showing the highest activity of 9.41 × 106 g of PE (mol of Co)−1 h−1 which is three times higher than that of prototypal cobalt catalyst (Co0) under identical conditions. Moreover, high melt temperature and unimodal molecular weight distribution are the characteristics of the resulting polyethylene.