Dehydrogenation of Alcohols to Carboxylic Acid Catalyzed by in Situ-Generated Facial Ruthenium-CPP Complex

Phosphine-assisted synthesis of a nanostructured iridium catalyst for acceptorless dehydrogenation of alcohols and chemoselective hydrogenation of nitroarenes

Ligand-assisted synthesis of noble metal nanoparticles has gained significant recent interest in the field of heterogeneous catalysis. In line with this, herein, we report the synthesis of a nanostructured iridium catalyst supported on silica via a one-pot wet chemical approach by treating IrCl3 with commercially available phosphine-functionalized silica gel, without the use of an external reducing or stabilizing agent. The phosphine ligand attached to silica plays the dual role of stabilizing and reducing agents. High-resolution transmission electron microscopy (HR-TEM) analysis revealed the formation of uniformly sized iridium nanoparticles (Ir NPs ∼2 nm), consistently distributed throughout the silica surface without showing any signs of agglomeration. The nanomaterial was also characterized by other techniques such as SEM-EDX, XRD, ICP-AES, and XPS. To test the catalytic potential of the iridium-based material, oxidant-free dehydrogenation of primary alcohols to carboxylic acids, with concomitant liberation of two equivalents of molecular hydrogen, was chosen as the benchmark reaction. A range of primary alcohols, including usually less reactive aliphatic alcohols, can be converted to their corresponding carboxylates/acids in high yields, demonstrating excellent efficacy and a broad substrate scope, better than the state-of-the-art Ir-based heterogeneous catalysts reported to date for this transformation. In addition, the Ir-based nanomaterial was also explored as a catalyst for chemoselective hydrogenation of nitroarenes to aminoarenes. Moderate to excellent yields of the desired products were obtained with a diverse range of substrates. For both catalytic reactions, the catalyst was found to be recyclable for at least eight consecutive runs without significantly compromising yields.

Ru-CNP Complex-Catalyzed Hydrogen Transfer/Annulation Reaction of 2-Nitrobenzylalcohol via an Outer-Sphere Mechanism

The development of efficient catalysts plays a central role in advancing chemical reactions. In this study, a ruthenium complex modified with a N-heterocyclic carbene-imine-phosphine ligand (CNP) was employed to enhance the hydrogen transfer/annulation process of 2-nitrobenzyl alcohols with various primary or secondary alcohols. The NMR and HRMS analyses of the reaction solution revealed the in situ formation of [fac-RuH(CO)(PPh3)(κ3-CN(H)P)]Cl through the transfer hydrogenation of the imine moiety within the CNP ligand under the reaction conditions. This species, a bifunctional Noyori-type ruthenium complex featuring facial coordination with a CN(H)P ligand, served as a key catalytic intermediate. By leveraging the outer-sphere mechanism facilitated by [fac-RuH(CO)(PPh3)(κ3-CN(H)P)]Cl, the synthesis of 75 quinolines from 2-nitrobenzyl alcohols and a wide range of alcohols has been achieved with yields as high as 95%.

Ruthenium-Catalyzed Synthesis of 2-Pyrazolines via Acceptorless Dehydrogenative Coupling of Allylic Alcohols with Hydrazines

Described herein is the synthesis of 2-pyrazolines via acceptorless dehydrogenative coupling of allylic alcohols with hydrazines based on a Ru3(CO)12/NHC-phosphine-phosphine ligand L catalytic system. The reaction not only exhibits low catalyst loading (only 0.3 mol %), wide substrate scope, good to excellent yields, and high selectivity but also omits the use of sacrificial hydrogen acceptor with only H2 and H2O as byproducts, making the reaction green and atom-economical.

Efficient co-upcycling of glycerol and CO2 into valuable products enabled by a bifunctional Ru-complex catalyst

Glycerol and CO2 are largely produced as by-products in modern industry. Herein, three Ru complexes bearing air- and water-stable NHC-nitrogen-nitrogen ligands are designed as bifunctional catalysts to upcycle glycerol and CO2 simultaneously. Among them, Ru complex 2 featuring an N-H structure shows the highest efficiency with TONs up to 300 000 and 387 000 for formate and lactate, respectively. 13C labelling experiments clearly manifest that formate is primarily derived from CO2. Furthermore, in situ FTIR spectra suggest that glyceraldehyde from glycerol might serve as a key intermediate to form lactate, while DFT calculations indicate that Ru complex 2 possesses the lowest reaction barriers and can form the RuN intermediate, contributing to its higher activity.

Half-Sandwich Iridium Complexes: A Recyclable and Stable Catalyst for Dehydrogenation of Alcohols to Carboxylic Acids

A series of acylhydrazone-based N,N-chelate half-sandwich iridium complexes have been synthesized through a facile route in good yields. The dehydrogenation of a series of aromatic and aliphatic primary alcohols to corresponding carboxylic acids has been accomplished catalyzed by the prepared air stable iridium complexes under mild reaction conditions. Carboxylic acids were obtained in high yields under open flask condition with broad substrates and good tolerance to sensitive functional groups. Such a half-sandwich iridium catalyst system exhibited high catalytic activity and stability, and a high TOF of 316.7 h-1 could be achieved with a catalyst loading as low as 0.05 mol %. Furthermore, the sustainable catalyst could be reused at least five times without obviously losing its activity, highlighting its potential application in industry. Molecular structure of iridium complex 1 was confirmed by single-crystal X-ray analysis.

Highly selective and additive-free Pd(OAc)2/CPP catalyzed hydroaminocarbonylation of alkynes

Herein, the synthesis of branched α,β-unsaturated amides by a hydroaminocarbonylation reaction of alkynes with various amine substrates such as aromatic amines, aliphatic amines, solid amine sources like NH4HCO3, and even strongly basic piperidines is reported, using a Pd(OAc)2/hybrid N-heterocyclic carbene-phosphine-phosphine (CPP) catalytic system. The reactions feature no additives, wide substrate scope, high selectivity (b/l > 99 : 1) and excellent yields. Mechanistic studies have disclosed that the reaction takes place via a palladium hydride pathway. CPP adopts a hybrid bidentate ligand conformation with a carbene-phosphine coordination mode, wherein one phosphorus atom remains externally accessible, potentially serving as a stabilizing auxiliary during catalytic cycles.

Synthesis of novel solid scale inhibitors based on silver tungstate loaded KIT-6 for scale removal from produced water: static and modeling evaluation

Oilfield scaling is a major problem in the oil and gas industry. Scaling issues cost the industry millions of dollars in damage and lost production every year as scaling is one of the main causes of global production decline. In this study, solid scale inhibitors based on silver tungstate loaded KIT-6 were synthesized and evaluated using a static scale inhibition test. The synthesized materials were characterized using wide and low XRD, N2 adsorption-desorption, TGA, and FTIR, SEM, and XPS analyses. Small-angle XRD patterns showed that KIT-6 had 3D-mesopore diffraction peaks with a cubic Ia3d space group. Wide-angle XRD patterns of silver tungstate loaded KIT-6 confirmed the crystallinity. The prepared catalysts are characterized by higher surface areas (394-918 m2 g-1), large pore volumes (0.63-0.98 cm3 g-1), narrow pore size distributions (5.3 nm), and high thermal stability up to 1000 °C. The results of this study demonstrate that the inhibition efficiency of the scale inhibitor increases and that of the calcite scale inhibitor decreases with increasing pH (2 to 8). This proposes that the scale inhibitor is more effective under alkaline conditions. An inhibition efficiency of 99% on calcium carbonate can be achieved at an optimal dosage of 7.5 ppm at 55 °C, indicating that the scale inhibitor exhibits a relatively good inhibition performance on calcium carbonate. The use of these materials can potentially lead to more efficient and cost-effective solutions for scale inhibition in various industrial processes.

Pyrazole-pyridine-pyrazole (NNN) ruthenium(II) complex catalyzed acceptorless dehydrogenation of alcohols to aldehydes

A new series of cationic ruthenium(II) complexes, [RuCl(PPh3)2(κ3-NNN-L1)]Cl (1), [RuH(PPh3)2(κ3-NNN-L2)]Cl (2), and [RuH(PPh3)2(κ3-NNN-L3)]Cl (3), bearing a 2,6-bis (1-R-5-methyl-pyrazol-3-yl) pyridine ligand (L1: R = H, L2: R = C6H4-p-CF3, L3: R = C6H4-p-OCH3), were synthesized and characterized with NMR, HRMS, and single-crystal X-ray diffraction. Their catalytic application was investigated in the acceptorless dehydrogenation (AD) of primary alcohols. Complex 1 outperformed 2 and 3 in terms of the selectivity towards the aldehydes, and provided the aldehydes with a yield of up to 99%, with good functional group tolerance under mild conditions. In addition, the only by-product of the reaction was dihydrogen, which can be collected as clean energy, and the reaction meets the requirements of environment-friendly chemistry. Complex 1 also proved to be a promising catalyst in the sequential AD/condensation reaction between the primary and secondary alcohols, affording α,β-unsaturated ketones in moderate to good selectivity.

Oxidation of Organic Compounds Using Water as the Oxidant with H2 Liberation Catalyzed by Molecular Metal Complexes

Oxidation reactions of organic compounds play a central role in both industrial chemical and material synthesis as well as in fine chemical and pharmaceutical synthesis. While traditional laboratory-scale oxidative syntheses have relied on the use of strong oxidizers, modern large-scale oxidation processes preferentially utilize air or pure O₂ as an oxidant, with other oxidants such as hydrogen peroxide, nitric acid, and aqueous chlorine solution also being used in some processes. The use of molecular oxygen or air as an oxidant has been very attractive in recent decades because of the abundance of air and the lack of wasteful byproduct generation. Nevertheless, the use of high-pressure air or, in particular, pure oxygen can lead to serious safety concerns with improper handling and also necessitates the use of sophisticated high-pressure reactors for the processes.Several research groups, including ours, have investigated in recent times the possibility of carrying out catalytic oxidation reactions using water as the formal oxidant, with no added conventional oxidants. Along with the abundant availability of water, these processes also generate dihydrogen gas as the reaction coproduct, which is a highly valuable fuel. Several well-defined molecular metal complexes have been reported in recent years to catalyze these unusual oxidative reactions with water. A ruthenium bipyridine-based PNN pincer complex was reported by us to catalyze the oxidation of primary alcohols to carboxylate salts with alkaline water along with H₂ liberation, followed by reports by other groups using other complexes as catalysts. At the same time, ruthenium-, iridium-, and rhodium-based complexes have been reported to catalyze aldehyde oxidation to carboxylic acids using water. Our group has combined the catalytic aqueous alcohol and aldehyde oxidation activity of a ruthenium complex to achieve the oxidation of biomass-derived renewable aldehydes such as furfural and 5-hydroxymethylfurfural (HMF) to furoic acid and furandicarboxylic acid (FDCA), respectively, using alkaline water as the oxidant, liberating H₂. Ruthenium complexes with an acridine-based PNP ligand have also been employed by our group for the catalytic oxidation of amines to the corresponding lactams, or to carboxylic acids via a deaminative route, using water. Similarly, we also reported molecular complexes for the catalytic Markovnikov oxidation of alkenes to ketones using water, similar to Wacker-type oxidation, which, however, does not require any terminal oxidant and produces H₂ as the coproduct. At the same time, the oxidation of enol ethers to the corresponding esters with water has also been reported. This account will highlight these recent advances where water was used as an oxidant to carry out selective oxidation reactions of organic compounds, catalyzed by well-defined molecular complexes, with H₂ liberation. The oxidation of alcohols, aldehydes, amines, alkenes, and enol ethers will be discussed to provide an outlook toward other functional groups' oxidation. We hope that this will aid researchers in devising other oxidative dehydrogenative catalytic systems using water, complementing traditional oxidative processes involving strong oxidants and molecular oxygen.

Tuning the selectivity in iridium-catalyzed acceptorless dehydrogenative coupling of primary alcohols

An acceptorless dehydrogenative coupling of primary alcohols to carboxylic acids/carboxylates, esters, and Guerbet alcohols (via both homo- and cross-β-alkylation of the alcohols) in the presence of an N-heterocyclic carbene iridium(I) catalyst was developed under aerobic conditions. The product selectivity can be easily tuned among the products with a single catalyst through simple modification of the reaction conditions, such as the catalyst and base amounts, the choice of base, and the reaction temperature.

Acceptorless dehydrogenation of alcohols to carboxylic acids by palladium nanoparticles supported on NiO: delving into metal-support cooperation in catalysis

In this work, we have developed a simple NiO-supported Pd nanocatalyst (Pd@NiO) for oxidant-free dehydrogenative oxidation of primary alcohols to carboxylic acids along with hydrogen gas as a byproduct. The catalyst has been characterized by techniques like XRD, HRTEM, SEM-EDX, XPS and ICP-AES. The nanostructured Pd@NiO material showed excellent dehydrogenative oxidation activity and outperformed the activity of free NiO or Pd nanoparticles supported on silica/carbon as a catalyst, which could be attributed to synergistic effect of Pd and NiO. A diverse range of aromatic and aliphatic primary alcohols could be efficiently converted to their corresponding carboxylates in high yields with a catalyst loading as low as 0.08 mol%. Notably, highly challenging biomass derived heterocyclic alcohols such as furfuryl alcohol and piperonyl alcohol can also be efficiently converted to their corresponding acids. Moreover, our catalyst can convert benzyl alcohol to benzoic acid on a gram scale with 89% yield. Interestingly, the H₂ gas liberated in the reaction can also be used as a substrate for the hydrogenation of 3a to 4a in 65% yield. The nanostructured catalyst is highly reusable and no significant decrease in activity was observed after six reaction cycles. A kinetic study revealed that the reaction followed first-order kinetics with a rate constant of k = 1.47 × 10^-4 s^-1, under optimized conditions. The extent of reactivity of different functionalities towards dehydrogenation was also investigated using a Hammett plot showing good linearity.

Ruthenium-Catalyzed Divergent Acceptorless Dehydrogenative Coupling of 1,3-Diols with Arylhydrazines: Synthesis of Pyrazoles and 2-Pyrazolines

Herein, the divergent transformations of 1,3-diols with arylhydrazines via acceptorless dehydrogenative coupling reactions to selectively synthesize pyrazoles and 2-pyrazolines were reported, which were based on Ru₃(CO)₁₂/NHC-phosphine-phosphine catalytic systems. The reactions featured low catalyst loading, high selectivity, wide substrate scope, and good yields, with only water and hydrogen gas (H₂) as the byproducts.

Acceptorless Dehydrogenative Cross-Coupling of Primary Alcohols Catalyzed by an N-Heterocyclic Carbene-Nitrogen-Phosphine Chelated Ruthenium(II) Complex

The acceptorless dehydrogenative cross-coupling of primary alcohols to form cross-esters with the liberation of H₂ gas was enabled using a [RuCl(η⁶-C₆H₆)(κ²-CNP)][PF₆]Cl complex as the catalyst. This sustainable protocol is applicable to a broad range of primary alcohols, particularly for the sterically demanding ones, featuring good functional group tolerance and high selectivity. The good catalytic performance can be attributed to the nitrogen-phosphine-functionalized N-heterocyclic carbene (CNP) ligand, which adopts a facial coordination mode as well as the facile dissociation of coordinated benzene.

Dehydrogenative ester synthesis from enol ethers and water with a ruthenium complex catalyzing two reactions in synergy

We report the dehydrogenative synthesis of esters from enol ethers using water as the formal oxidant, catalyzed by a newly developed ruthenium acridine-based PNP(Ph)-type complex. Mechanistic experiments and density functional theory (DFT) studies suggest that an inner-sphere stepwise coupled reaction pathway is operational instead of a more intuitive outer-sphere tandem hydration-dehydrogenation pathway.

MOF-stabilized perfluorinated palladium cages catalyze the additive-free aerobic oxidation of aliphatic alcohols to acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd 2+ coordinate cages within the channels of an anionic tridimensional metal organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd 2+ to dehydrogenate primary alcohols, without any base, and also to activate O 2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.

Bulky PNP ligands blocking metal-ligand cooperation allow for isolation of Ru(0), and lead to catalytically active Ru complexes in acceptorless alcohol dehydrogenation

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H 2 allows for stabilization and extensive characterization of bis CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2)-H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Catalytic hydrogenation of CO2 with unsymmetric N-heterocyclic carbene–nitrogen–phosphine ruthenium complexes

Unsymmetric Ru-CNP and Ru-CN(H)P complexes are synthesized and applied in the hydrogenation of CO2 to formate for the first time.

Direct Oxidation of Primary Alcohols to Carboxylic Acids

Oxidation of primary alcohols to carboxylic acids is a fundamental transformation in organic chemistry, yet despite its simplicity, extensive use, and relationship to pH, it remains a subject of active research for synthetic organic chemists. Since 2013, a great number of new methods have emerged that utilize transition-metal compounds as catalysts for acceptorless dehydrogenation of alcohols to carboxylates. The interest in this reaction is explained by its atom economy, which is in accord with the principles of sustainability and green chemistry. Therefore, the methods for the direct synthesis of carboxylic acids from alcohols is ripe for a modern survey, which we provide in this review.

1 Introduction

2 Thermodynamics of Primary Alcohol Oxidation

3 Oxometalate Oxidation

4 Transfer Dehydrogenation

5 Acceptorless Dehydrogenation

6 Electrochemical Methods

7 Outlook

A cyclometalated Ir(III)-NHC complex as a recyclable catalyst for acceptorless dehydrogenation of alcohols to carboxylic acids

In this work, we have synthesized two new [C, C] cyclometalated Ir(iii)-NHC complexes, [IrCp*(C∧C:NHC)Br](1a,b), [Cp* = pentamethylcyclopentadienyl; NHC = (2-flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazoline-2-ylidene (a); (2-flurobenzyl)-1-(4-formylphenyl)-1H-imidazoline-2-ylidene (b)] via intramolecular C-H bond activation. The molecular structure of complex 1a was determined by X-ray single crystal analysis. The catalytic potentials of the complexes were explored for acceptorless dehydrogenation of alcohols to carboxylic acids with concomitant hydrogen gas evolution. Under similar experimental conditions, complex 1a was found to be slightly more efficient than complex 1b. Using 0.1 mol% of complex 1a, good-to-excellent yields of carboxylic acids/carboxylates have been obtained for a wide range of alcohols, both aliphatic and aromatic, including those involving heterocycles, in a short reaction time with a low loading of catalyst. Remarkably, our method can produce benzoic acid from benzyl alcohol on a gram scale with a catalyst-to-substrate ratio as low as 1 : 5000 and exhibit a TON of 4550. Furthermore, the catalyst could be recycled at least three times without losing its activity. A mechanism has been proposed based on controlled experiments and in situ NMR study.

Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands

The transition-metal-catalyzed alcohol dehydrogenation to carboxylic acids has been identified as an atom-economical and attractive process. Among various catalytic systems, Ru-based systems have been the most accessed and investigated ones. With our growing interest in the discovery of new Ru catalysts comprising N-heterocyclic carbene (NHC) ligands for the dehydrogenative reactions of alcohols, we designed and prepared five NHC/Ru complexes ([Ru]-1–[Ru]-5) bearing different ancillary NHC ligands. Moreover, the effects of ancillary and additional ligands on the alcohol dehydrogenation with KOH were thoroughly explored, followed by the screening of other parameters. Accordingly, a highly active catalytic system, which is composed of [Ru]-5 combined with an additional NHC precursor L5, was discovered, affording a variety of acid products in a highly efficient manner. Gratifyingly, an extremely low Ru loading (125 ppm) and the maximum TOF value until now (4800) were obtained.

Ruthenium-catalyzed synthesis of N-substituted lactams by acceptorless dehydrogenative coupling of diols with primary amines

Herein, we report the first example of synthesis of N-substituted lactams via an acceptorless dehydrogenative coupling of diols with primary amines in one step, which was enabled by combining Ru3(CO)12 with a hybrid N-heterocyclic carbene-phosphine-phosphine ligand as the catalyst.