Nickel-based perovskite-catalysed direct phenol-to-aniline liquid-phase transformations

Liquid phase direct amination of phenols to primary anilines with hydrazine was achieved using commercial NiLa-perovskite catalysts as bifunctional Lewis acid/redox-active catalysts without adding any external hydride sources. The amination strategy took place efficiently in the absence of any amount of reducing gasses (H2/NH3) and noble metals under mild conditions.

Atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized β-O-4 segments

This work presents an innovative approach focusing on fine-tuning the coordination environment of atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized lignin model compounds. By meticulously regulating the Co-N coordination environment, the activity of these catalysts in the hydrogenolysis and reductive amination reactions was effectively controlled. Notably, our study demonstrates that, in contrast to cobalt nanoparticle catalysts, atomically dispersed cobalt catalysts exhibit precise control of the sequence of hydrogenolysis and reductive amination reactions. Particularly, the CoN3 catalyst with a triple Co-N coordination number achieved a remarkable 94% yield in the synthesis of primary benzylamine. To our knowledge, there is no previous documentation of the synthesis of primary benzylamines from lignin dimer model compounds. Our study highlights a promising one-pot route for sustainable production of nitrogen-containing aromatic chemicals from lignin.

Aniline Derivatives from Lignin under Mild Conditions Enabled by Electrochemistry

The development of environmentally friendly methods for the valorization of important phenolic platform chemicals originating directly from lignin-first depolymerization into value-added N-chemicals, such as aniline derivatives, is of high industrial interest. In this work, we tackle this challenging transformation by the judicious combination of electrochemical conversion and chemical functionalization steps. In the first step, lignin-derived para-substituted guaiacols and syringols undergo an atom-efficient, room-temperature anodic oxidation using methanol both as solvent and reagent towards the formation of the corresponding cyclohexadienone derivatives, which are subsequently converted to synthetically challenging ortho-methoxy substituted anilines by reaction with ethyl glycinate hydrochloride under mild conditions. The developed method was applied to crude lignin depolymerization bio-oils, derived from reductive catalytic fractionation (RCF) mediated either by copper-doped porous metal oxide (Cu20 PMO) or Ru/C, allowing the selective production of 4-propanol-2-methoxyaniline (1Gb) and 4-propyl-2-methoxyaniline (2Gb), respectively, from pine lignocellulose. Finally, the application of 2Gb was further studied in the synthesis of carbazole 2Gc, a lignin-derived analogue of biologically active alkaloid murrayafoline A.

Heterogeneously Catalyzed Selective Acceptorless Dehydrogenative Aromatization to Primary Anilines from Ammonia via Concerted Catalysis and Adsorption Control

Although catalytic dehydrogenative aromatization from cyclohexanones and NH₃ is an attractive synthetic method for primary anilines, using a hydrogen acceptor was indispensable to achieve satisfactory levels of selectivity in liquid-phase organic synthetic systems without photoirradiation. In this study, we developed a highly selective synthesis of primary anilines from cyclohexanones and NH₃ via efficient acceptorless dehydrogenative aromatization heterogeneously catalyzed by an Mg(OH)₂-supported Pd nanoparticle catalyst in which Mg(OH)₂ species are also deposited on the Pd surface. The basic sites of the Mg(OH)₂ support effectively accelerate the acceptorless dehydrogenative aromatization via concerted catalysis, suppressing the formation of secondary amine byproducts. In addition, the deposition of Mg(OH)₂ species inhibits the adsorption of cyclohexanones on the Pd nanoparticles to suppress phenol formation, achieving the desired primary anilines with high selectivity.

Secondary Amines from Catalytic Amination of Bio-Derived Phenolics over Pd/C and Rh/C: Effect of Operation Parameters

The production of renewable chemicals using lignocellulosic biomass has gained significant attention in green chemistry. Among biomass-derived chemicals, secondary amines have emerged as promising intermediates for synthetic applications. Here, we report a systematic study on the reductive amination of phenolics with cyclohexylamine using Pd/C and Rh/C as catalysts. The catalytic tests were performed in batch reactors under different reaction conditions (various: amine concentration (0.1–0.4 mol/L), hydrogen pressure (0–2.5 bar), temperature (80–160 °C), and substituted phenols (phenol, o-cresol, p-cresol, and methoxyphenol)) and using tert-amyl alcohol as a solvent. The experimental observations were consistent with a multi-step mechanism, where hydrogenation of phenol to cyclohexanone is followed by condensation of the ketone with cyclohexylamine to form an imine, which is finally hydrogenated to produce secondary amines. In addition, there was evidence of parallel self-condensation of the cyclohexylamine. The study also supported a limited dehydrogenation capacity of Rh/C, unlike Pd/C, which increases this capacity at higher temperatures generating a higher yield of cyclohexylaniline (up to 15%). The study of the alkylated phenols demonstrated that the nature and propensity of hydrogenation of the phenolic controls their amination. Kinetic analysis revealed reaction orders between 0.4 and 0.7 for H2, indicating its dissociative adsorption. Meanwhile, phenol’s order (between 1–1.8) suggests a single participation of this compound in the hydrogenation step. The order of 0.4 for cyclohexylamine suggests its participation as a surface-abundant species. The apparent activation energies derived from a power law approximation were of 37 kJ/mol and 10 kJ/mol on Pd/C and Rh/C, respectively.

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Reductive amination of phenol over Pd-based catalysts: elucidating the role of the support and metal nanoparticle size

The Pd-catalyzed reductive amination of phenol is sensitive to the support's nature, and to the atoms' coordination in palladium clusters.

C–N coupling reactions with arenes through C–H activation: the state-of-the-art versus the principles of green chemistry

Herein, we discuss the state-of-the-art in arene C–N coupling through C–H activation and to what extent it complies with the principles of green chemistry, with a focus on heterogeneously catalysed systems.

Techno-economic analysis of aniline production via amination of phenol

The purpose of this research is to demonstrate through a techno-economic assessment that aniline can be industrially produced using a profitable and inherently safer process than the ones currently employed. The aniline production process was designed using process simulation software. From this, the mass and energy balances were determined, the equipment sizing was performed and the net present value (NPV) was calculated to be USD 93.5 million. Additionally, a heat integration analysis was carried out in order to improve process profitability, obtaining a new NPV of USD 97.5 million. The economic sensitivity analysis showed that the process could withstand fixed capital investment changes of up to +89%, weighted average cost of capital changes between 16–24% and a decrease in cyclohexylamine demand of up to 44%. The conceptual design is still profitable when aniline price is varied in a range of 1224–1840 $/t and phenol cost in a range of 815–1178 $/t.

Improved H2 utilization by Pd doping in cobalt catalysts for reductive amination of polypropylene glycol

Cobalt based catalysts having enhanced H2 dissociation and desorption were synthesized by inserting a trace amount of palladium. These catalysts were used for the reductive amination of polypropylene glycol (PPG) to polyetheramine (PEA). The catalytic activity toward PEA was significantly increased by incorporating an extremely low content of palladium (around 0.01 wt%) into cobalt based catalysts. The Pd inserted cobalt catalysts promoted reduction of cobalt oxide to cobalt metal and inhibited formation of cobalt nitride in the reductive amination. The Pd inserted cobalt catalysts not only enhanced hydrogen dissociation but also accelerated hydrogen desorption by increasing the electron density of cobalt through interaction between cobalt and palladium. These play a critical role in reducing cobalt oxide or cobalt nitride to cobalt metal as an active site for the reductive amination. Thus, the Pd inserted cobalt catalysts provide improved catalytic performance toward PEA production by maintaining the cobalt metal state.

Coupling without Coupling Reactions: En Route to Developing Phenols as Sustainable Coupling Partners via Dearomatization-Rearomatization Processes

Transition-metal-catalyzed cross-coupling reactions represent one of the most straightforward and efficient protocols to assemble two different molecular motifs for the construction of carbon-carbon or carbon-heteroatom bonds. Because of their importance and wide applications in pharmaceuticals, agrochemicals, materials, etc., cross-coupling reactions have been well recognized in the 2010 Nobel Prize in chemistry. However, in the classical transition-metal-catalyzed cross-coupling reactions (e.g., the Suzuki-Miyaura, the Buchwald-Hartwig, and the Ullmann cross-coupling reactions), organohalides, which mainly stem from the nonrenewable fossil resources, are often utilized as coupling partners with halide wastes being generated after the reactions. To make cross-coupling reactions more sustainable, we initiated a general research program by employing phenols and cyclohexa(e)nones (the reduced forms of phenols) as pivotal feedstocks (coupling partners), instead of the commonly used fossil-derived organohalides, for cross-coupling reactions to build C-O, C-N, and C-C bonds. Phenols (cyclohexa(e)nones) are widely available and can be obtained from lignin biomass, highlighting their renewable and sustainable features. Moreover, water is expected to be the only stoichiometric byproduct, thus avoiding halide wastes.Notably, the cross-coupling reactions utilizing phenols/cyclohexa(e)nones are not based on the traditional transition-metal-catalyzed "oxidative-addition and reductive-elimination" mechanism, but via a novel "phenol-cyclohexanone" redox couple. This new working mechanism opens up new horizons of designing cross-coupling reactions via simple nucleophilic addition of cyclohexanones along with aromatization processes, thereby simplifying the design and avoiding laborious optimization of transition-metal precursors (e.g., Pd, Ni, Cu, etc.), as well as ligands in classical transition-metal-catalyzed cross-coupling reactions. Specifically, in this Account, we will summarize and discuss our related research work in the following three categories: "formal oxidative couplings of cyclohexa(e)nones", "formal reductive couplings of phenols", and "formal redox-neutral couplings of phenols". The successes of these research projects clearly demonstrated our initial inspirations and rational designs to develop cross-coupling reactions without the "conventional cross-coupling conditions" by pushing the reaction frontiers from initial cyclohexanones, ultimately, to the sustainable phenol targets.

Downstream Processing Strategies for Lignin-First Biorefinery

The lignin-first strategy has emerged as one of the most powerful approaches for generating novel platform chemicals from lignin by efficient depolymerization of native lignin. Because of the emergence of this novel depolymerization method and the definition of viable platform chemicals, future focus will soon shift towards innovative downstream processing strategies. Very recently, many interesting approaches have emerged that describe the production of valuable products across the whole value chain, including bulk and fine chemical building blocks, and several concrete examples have been developed for the production of polymers, pharmaceutically relevant compounds, or fuels. This Minireview provides an overview of these recent advances. After a short summary of catalytic systems for obtaining aromatic monomers, a comprehensive discussion on their separation and applications is given. This Minireview will fill the gap in biorefinery between deriving high yields of lignin monomers and tapping into their potential for making valuable consumer products.

Dearomatization–Rearomatization Strategy for Palladium-Catalyzed C–N Cross-Coupling Reactions

Substituted aromatic compounds play important roles in materials, biological agents, dyes, etc. Thus, the synthesis of substituted aromatic compounds has been a hot topic throughout the history of organic chemistry. Traditionally, the Friedel–Crafts reaction was a powerful tool for synthesizing substituted aromatic compounds. In recent decades, metal-catalyzed cross-coupling reactions were well developed via carbon–heteroatom bond cleavage, however, having difficulties towards some strong bonds, such as C(Ar)–OH. To overcome such challenges, newer strategies are needed. In this review, we summarize the recent efforts in the development of dearomatization–rearomatization strategy for cross-coupling reactions via C(Ar)–O bond cleavage.

1 Introduction

2 Dearomatization–Rearomatization Strategy for Cross-Coupling of Phenols

3 Dearomatization–Rearomatization Strategy for Cross-Coupling of Biphenols

4 Dearomatization–Rearomatization Strategy for Cross-Coupling of Diphenyl Ethers

5 Dearomatization–Rearomatization Strategy for Cross-Coupling of Indoles

6 Summary

Rhodium catalysed C-3/5 methylation of pyridines using temporary dearomatisation

Pyridines are ubiquitous aromatic rings used in organic chemistry and are crucial elements of the drug discovery process. Herein we describe a new catalytic method that directly introduces a methyl group onto the aromatic ring; this new reaction is related to hydrogen borrowing, and is notable for its use of the feedstock chemicals methanol and formaldehyde as the key reagents. Conceptually, the C-3/5 methylation of pyridines was accomplished by exploiting the interface between aromatic and non-aromatic compounds, and this allows an oscillating reactivity pattern to emerge whereby normally electrophilic aromatic compounds become nucleophilic in the reaction after activation by reduction. Thus, a set of C-4 functionalised pyridines can be mono or doubly methylated at the C-3/5 positions.

Electron poor pyridines can be activated by reduction and then methylated at C3/5 using formaldehyde.

Construction of Spirocyclic Tetrahydro-β-carbolines via Cross-Annulation of Phenols with Tryptamines in Water

Phenols are readily available by degradation of lignin resource. Palladium-catalyzed conversion of phenols to tetrahydro-β-carboline skeletons bearing a spirocycle at the C-1 position in water is reported. Various substituted phenols are successfully cross-annulated with different tryptamines via sequential C(Ar)-O bond cleavage of phenols, C-H bond activation of tryptamines, and C-N/C-C bond formations. This method provides a new protocol of converting lignin phenols into high-value-added compounds, such as natural product Komavine.

Direct conversion of phenols into primary anilines with hydrazine catalyzed by palladium

Primary anilines are essential building blocks to synthesize various pharmaceuticals, agrochemicals, pigments, electronic materials, and others. To date, the syntheses of primary anilines mostly rely on the reduction of nitroarenes or the transition-metal-catalyzed Ullmann, Buchwald-Hartwig and Chan-Lam cross-coupling reactions with ammonia, in which non-renewable petroleum-based chemicals are typically used as feedstocks via multiple step syntheses. A long-standing scientific challenge is to synthesize various primary anilines directly from renewable sources. Herein, we report a general method to directly convert a broad range of phenols into the corresponding primary anilines with the cheap and widely available hydrazine as both amine and hydride sources with simple Pd/C as the catalyst.