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

Copper-catalyzed C-C bond cleavage coupling with CN bond formation toward mild synthesis of lignin-based benzonitriles

N-participated lignin depolymerization is of great importance for the transformation of waste lignin into value-added chemicals. The vast majority of developed strategies employ organic amines as nitrogen source, and considerable methods rely on excessive use of strong base, which suffers severe environmental issues. Herein, benzonitrile derivatives are synthesized from oxidized lignin β-O-4 model compounds in the presence of solid nitrogen source (NH4)2CO3 under mild, base-free conditions over commercially available copper catalyst. Mechanism studies suggest the transformation undergoes a one-pot, highly coupled cascade reaction path involving oxidative C-C bond cleavage and in-situ formation of CN bond. Of which, Cu(OAc)2 catalyzes the transfer of hydrogen from Cβ (Cβ-H) to Cα, leading to the cleavage of Cα-Cβ bonds to offer benzaldehyde derivative, this intermediate then reacts in-situ with (NH4)2CO3 to afford the targeted aromatic nitrile product. Tetrabutylammonium iodide (TBAI), acting as a promoter, plays a key role in breaking the Cα-Cβ bonds to form the intermediate benzaldehyde derivative. With this protocol, the feasibility of the production of value-added syringonitrile from birchwood lignin has been demonstrated. This transformation provides a sustainable approach to benzonitrile chemicals from renewable source of lignin.

Thermally-Stable Single-Site Pd on CeO2 Catalyst for Selective Amination of Phenols to Aromatic Amines without External Hydrogen

Developing a new route to produce aromatic amines as key chemicals from renewable phenols is a benign alternative to current fossil-based routes like nitroaromatic hydrogenation, but is challenging because of the high dissociation energy of the Ar-OH bond and difficulty in controlling side reactions. Herein, an aerosolizing-pyrolysis strategy was developed to prepare high-density single-site cationic Pd species immobilized on CeO2 (Pd1/CeO2) with excellent sintering resistance. The obtained Pd1/CeO2 catalysts achieved remarkable selectivity of important aromatic amines (yield up to 76.2 %) in the phenols amination with amines without external hydrogen sources, while Pd nano-catalysts mainly afforded phenyl-ring-saturation products. The excellent catalytic properties of the Pd1/CeO2 are closely related to high-loading Pd single-site catalysts with abundant surface defect sites and suitable acid-base properties. This report provides a sustainable route for producing aromatic amines from renewable feedstocks.

One-Pot Production of Cinnamonitriles from Lignin β-O-4 Segments Induced by Selective Oxidation of the γ-OH Group

The construction of N-containing aromatic compounds from lignin is of great importance to expanding the boundary of the biorefinery and meeting the demand for value-added biorefinery. However, it remains a huge challenge due to the complex lignin structure and the incompatible catalysis for C-O/C-C bond cleavage and C-N formation. Herein, sustainable synthesis of cinnamonitrile derivatives from lignin β-O-4 model compounds in the presence of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), (diacetoxyiodo)benzene (BAIB), and a strong base has been achieved in a one-pot, two-step fashion under transition-metal-free conditions. Mechanistic studies suggest that this transformation starts from selective oxidation of Cγ-OH of the β-O-4 model compound, followed by retro-aldol condensation, resulting in the cleavage of the Cα-Cβ bond to afford veratraldehyde. Whereafter, the aldol condensation reaction allows coupling of veratraldehyde with nitriles to provide cinnamonitriles. With this protocol, 3,4-dimethoxycinnamonitrile and 3,4-dimethoxyphenyl-2-phenylacrylonitrile were synthesized from lignin β-O-4 model compounds and showed good antibacterial or antifungal activity, showcasing the application potential of lignin in pharmaceutical synthesis.

Phase transition induced hydrogen activation for enhanced furfural reductive amination over a CoCu bimetallic catalyst

The synthesis of primary amines from renewable biomass and its derivatives through reductive amination has garnered significant attention. How to construct efficient non-noble-metal catalysts that enable low-temperature catalysis still remains challenging. Herein, we report a Cu-doped Co@CoO x heterostructure catalyst that features structural Co-CoCuO x bifunctional sites, which enable room temperature reductive amination of various aldehydes with 1.57-45 times higher efficiency than Co@CoO x , outperforming many reported non-noble and even noble metal catalysts. Experiments and DFT calculations indicate that Cu doping leads to a phase transition of Co from hcp to fcc, while electrons are transferred from Cu to Co, forming a dual active site with electron-rich Co closely interacting with CoCuO x . These electron-rich Co sites demonstrate excellent activity in the activation and dissociation of hydrogen, while the CuO x component facilitates hydrogen spillover at the CoCuO x interface, thus resulting in a highly efficient cooperative effect for the furfural (FAL) reductive amination. This work provides general guidance for the rational design of high-performance reductive amination catalysts for biomass upgrading.

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.

The Facile Production of p-Chloroaniline Facilitated by an Efficient and Chemoselective Metal-Free N/S Co-Doped Carbon Catalyst

The catalytic hydrogenation of the toxic and harmful p-chloronitrobenzene to produce the value-added p-chloroaniline is an essential reaction for the sustainable chemical industry. Nevertheless, ensuring satisfactory control of its chemoselectivity is a great challenge. In this work, a N/S co-doped metal-free carbon catalyst has been fabricated by using cysteine as a source of C, N, and S. The presence of calcium citrate (porogen agent) in the mixture subjected to pyrolysis provided the carbon with porosity, which permitted us to overcome the issues associated with the loss of heteroatoms during an otherwise necessary activation thermal treatment. Full characterization was carried out and the catalytic performance of the metal-free carbon material was tested in the hydrogenation reaction of p-chloronitrobenzene to selectively produce p-chloroaniline. Full selectivity was obtained but conversion was highly dependent on the introduction of S due to the synergetic effect of S and N heteroatoms. The N/S co-doped carbon (CYSCIT) exhibits a mesoporous architecture which favors mass transfer and a higher doping level, with more exposed N and S doping atoms which act as catalytic sites for the hydrogenation of p-chloronitrobenzene, resulting in enhanced catalytic performance when compared to the N-doped carbon obtained from melamine and calcium citrate (MELCIT) used as a reference.

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.

Synthesis of 2-benzyl N-substituted anilines via imine condensation-isoaromatization of (E)-2-arylidene-3-cyclohexenones and primary amines

A catalyst- and additive-free synthesis of 2-benzyl N-substituted anilines from (E)-2-arylidene-3-cyclohexenones and primary amines has been reported. The reaction proceeds smoothly through a sequential imine condensation-isoaromatization pathway, affording a series of synthetically useful aniline derivatives in acceptable to high yields. Mild reaction conditions, no requirement of metal catalysts, operational simplicity and the potential for scale-up production are some of the highlighted advantages of this transformation.

Ruthenium/η5-Phenoxo-Catalyzed Amination of Phenols with Amines

Ruthenium(II) complexes are known to form η6-arene complexes with benzene-containing compounds through π-coordination, a property extensively utilized to initiate reactions not typically observed with free arenes. A prime example is nucleophilic aromatic substitution, where ruthenium-complexed aryl halides undergo nucleophilic attack, allowing the direct synthesis of diverse aromatic compounds by displacing halides with nucleophiles. However, this activation relies on the electron-withdrawing effect of the Ru(II) species, as well as is hindered by the resistance of η6-arenes to arene exchange. In the previous pursuit of catalysis, the emphasis of ligand design has centered on promoting arene exchange. In this study, we extended the ruthenium activation strategy to umpolung substitution reactions of phenols. The amination proceeds through a direct condensation between phenols and amines, with a key intermediate identified as [bis(η5-phenoxo)Ru], which is in situ generated from a commercially available ruthenium catalyst. In comparison with the well-studied cyclopentadienyl (Cp) type ligands, we demonstrated that an η5-phenoxo motif, as a superior alternative to Cp, contributes to the amination of phenols in two crucial ways: its less electron-donating nature enhances the withdrawing effect of the ruthenium unit, facilitating substitution on the phenol complex; its distinctive behavior in arene exchange allows for conducting the amination with a catalytic amount of metal. Additionally, hydrogen bonding, wherein the phenoxo serves as the acceptor, was found to be important for the substitution. The versatility of this ruthenium-catalyzed amination was validated by performing reactions with a diverse array of phenols exhibiting various electronic properties, in combination with a wide range of primary amines. This work exemplifies the expansion of the scope of π-coordination activation in catalysis through innovative ligand development.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Production of Oximes Directly from Sustainable Lignocellulose-Derived Aldehydes and Ammonia over HTS-1 Catalyst

Oxime chemicals are the building blocks of many anticancer drugs and widely used in industry and laboratory. A simple but robust hierarchically porous zeolite (HTS-1) catalyst was prepared by hydrothermal methods and used for the preparation of vanillin oxime from vanillin in NH3  ⋅ H2 O/DIO (v/v 1/10) system. The results of the catalyst characterization showed that the larger pore size and more framework Ti were conducive to promote the transformation of the substrates. The conversion of vanillin and the yield of vanillin oxime were both higher than 99 % under optimized reaction conditions. It was found that the reaction proceeded by oxidation of NH3 to hydroxylamine (NH2 OH), and oximation of hydroxylamine with vanillin to obtain vanillin oxime, where the rate-controlling step was the hydroxylamine formation, and the apparent activation energy was 26.22 kJ/mol. The corresponding oximation products could also be obtained by extending this method to other compounds derived from lignin. Furthermore, the catalytic system was used directly to the conversion of birch biomass to obtain oxime products such as vanillin oxime, syringaldehyde oxime, and furfural oxime etc. This work might give insights into the sustainable production of N-containing high-value products from lignocellulose.

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.

Boron Triiodide-Mediated Reduction of Nitroarenes Using Borohydride Reagents

The reduction of nitroarenes using KBH4 and I2 is described. BI3 is generated in situ and was shown to be the active reductant. Conditions were optimized for BI3 generation and then applied to a wide range of nitroarenes, including traditionally challenging substrates. The method constitutes a practical reduction option which produces low-toxicity boric acid and potassium iodide upon workup.

Synthesis of 2,6-Dimethoxy-p-aminophenol from Hardwood Lignin

Although the multiple functional groups in biomass offer notable chances for producing high-value chemicals, most of the current studies focused on the (deep) defunctionalization of biomass and its derivates. Herein, we present a catalytic approach to valorize birch wood lignin with maintaining the methoxy and hydroxy groups in the final product (i. e., 2,6-dimethoxy-p-aminophenol), which has applications in different sectors such as pharmaceuticals. The proved approach involves four steps with a high yield (19.8 wt % on the basis of used lignin) to 2,6-dimethoxy-p-aminophenol. The native lignin in birch wood was first converted using alkaline aerobic oxidation in the presence of copper ions toward high-yield syringaldehyde, which was then selectively oxidized toward 2,6-dimethoxy-1,4-benzoquinone using H2 O2 and V2 O5 . Oximation of 2,6-dimethoxy-1,4-benzoquinone can selectively form 2,6-dimethoxy-1,4-benzoquinone-4-oxime, which can be quantitatively hydrogenated toward 2,6-dimethoxy-p-aminophenol. This work highlights the unique potential of biomass and its derivates for the sustainable production of high-value products with exploring the value of inherent functional groups.

Towards bioresource-based aggregation-induced emission luminogens from lignin β-O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin β-O-4 motifs with γ-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials.

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.

Sustainable production of active pharmaceutical ingredients from lignin-based benzoic acid derivatives via “demand orientation”

Catalytic production of several representative active pharmaceutical ingredients (APIs) from lignin.

Successive Cleavage and Reconstruction of Lignin β-O-4 Models and Polymer to Access Quinoxalines

The construction of N-heterocyclic compounds from lignin remains a great challenge due to the complex lignin structure and the involvement of multiple steps, including the cleavage of lignin C-O linkages and the formation of heterocyclic aromatic rings. Herein, the first example of KOH mediated sustainable synthesis of quinoxaline derivatives from lignin β-O-4 model compounds in a one-pot fashion under transition-metal-free conditions has been achieved. Mechanistic studies suggested that this transformation includes highly coupled cascade steps of cleavage of C-O bonds, dehydrative condensation, sp³ C-H bond oxidative activation, and intramolecular dehydrative coupling reaction. With this protocol, a wide range of functionalized quinoxalines, including an important drug compound AG1295, were synthesized from lignin β-O-4 model compounds and β-O-4 polymer, showcasing the application potential of lignin in pharmaceutical synthesis.

Hydrogen Transfer Coupling with 100% Atom Economy: Synthesis of 2-Indolyltetrahydronaphthyridine Derivatives

An iridium-catalyzed hydrogen transfer strategy, enabling straightforward access to tetrahydropyridine derivatives from aryl-1,8-naphthyridines and indolines was developed. This method has unprecedented advantages, including high step economy. In addition, it does not produce any byproducts or require an external high-pressure H₂ gas source. The method offers an important platform for the transformation of 1,8-naphthyridines and indolines into functionalized products.

Transition-metal-free synthesis of pyrimidines from lignin β-O-4 segments via a one-pot multi-component reaction

Heteroatom-participated lignin depolymerization for heterocyclic aromatic compounds production is of great importance to expanding the product portfolio and meeting value-added biorefinery demand, but it is also particularly challenging. In this work, the synthesis of pyrimidines from lignin β-O-4 model compounds, the most abundant segment in lignin, mediated by NaOH through a one-pot multi-component cascade reaction is reported. Mechanism study suggests that the transformation starts by NaOH-induced deprotonation of Cα-H bond in β-O-4 model compounds, and involves highly coupled sequential cleavage of C-O bonds, alcohol dehydrogenation, aldol condensation, and dehydrogenative aromatization. This strategy features transition-metal free catalysis, a sustainable universal approach, no need of external oxidant/reductant, and an efficient one-pot process, thus providing an unprecedented opportunity for N-containing aromatic heterocyclic compounds synthesis from biorenewable feedstock. With this protocol, an important marine alkaloid meridianin derivative can be synthesized, emphasizing the application feasibility in pharmaceutical synthesis.

Hydride transfer-initiated synthesis of 3-functionalized quinolines by deconstruction of isoquinoline derivatives

Under transition metal catalyst-free conditions, we herein present a hydride transfer-initiated construction of novel 3-(2-aminomethyl)aryl quinolines from N-isoquinolinium salts and 2-aminobenzaldehydes, proceeding with the merits of operational simplicity, high step and atom efficiency, good substrate and functional group compatibility, and mild conditions. The products are formed by reacting with the isoquinolyl motif as a two-carbon synthon along with the cleavage of its C3-N bond. Given the interesting applications of 3-aryl quinolines, the developed chemistry is anticipated to be further applied to develop new functional products.

Bakuchiol - a natural meroterpenoid: structure, isolation, synthesis and functionalization approaches

Bakuchiol is an emblematic meroterpene class of natural product extracted from Psoralea corylifolia. It has been reported to possess a broad range of biological and pharmacological properties and is considered as a leading biomolecule. It is highly desirable to devise an efficient approach to access bakuchiol and its chemical biology applications. In this review we provided structural features, isolation methods, various chemical routes and late-stage functionalization (LSF) approaches for bakuchiol and its derivatives. Moreover, this review encompasses the structure-activity relationships (SAR), value-added contributions and future perspectives of bakuchiol.

Catalytic Amination of Phenols with Amines

Given the wide prevalence and ready availability of both phenols and amines, aniline synthesis through direct coupling between these starting materials would be extremely attractive. Herein, we describe a rhodium-catalyzed amination of phenols, which provides concise access to diverse anilines, with water as the sole byproduct. The arenophilic rhodium catalyst facilitates the inherently difficult keto-enol tautomerization of phenols by means of π-coordination, allowing for the subsequent dehydrative condensation with amines. We demonstrate the generality of this redox-neutral catalysis by carrying out reactions of a large array of phenols with various electronic properties and a wide variety of primary and secondary amines. Several examples of late-stage functionalization of structurally complex bioactive molecules, including pharmaceuticals, further illustrate the potential broad utility of the method.

Synthesis of acridinones via palladium-catalyzed reductive annulation of 2-nitrobenzaldehydes and resorcinols

Through a palladium-catalyzed reductive annulation reaction of resorcinols and 2-nitrobenzaldehydes, reported is a new synthesis of acridinones with the features of operational simplicity, broad substrate scope, and readily available feedstocks.

Nanoparticles and single atoms of cobalt synergistically enabled low-temperature reductive amination of carbonyl compounds

Low-temperature and selective reductive amination of carbonyl compounds is a highly promising approach to access primary amines. However, it remains a great challenge to conduct this attractive route efficiently over earth-abundant metal-based catalysts. Herein, we designed several Co-based catalysts (denoted as Co@C–N(x), where x represents the pyrolysis temperature) by the pyrolysis of the metal–organic framework ZIF-67 at different temperatures. Very interestingly, the prepared Co@C–N(800) could efficiently catalyze the reductive amination of various aldehydes/ketones to synthesize the corresponding primary amines with high yields at 35 °C. Besides non-noble metal and mild temperature, the other unique advantage of the catalyst was that the substrates with different reduction-sensitive groups could be converted into primary amines selectively because the Co-based catalyst was not active for these groups at low temperature. Systematic analysis revealed that the catalyst was composed of graphene encapsulated Co nanoparticles and atomically dispersed Co–N_x sites. The Co particles promoted the hydrogenation step, while the Co–N_x sites acted as acidic sites to activate the intermediate (Schiff base). The synergistic effect of metallic Co particles and Co–N_x sites is crucial for the excellent performance of the catalyst Co@C–N(800). To the best of our knowledge, this is the first study on efficient synthesis of primary amines via reductive amination of carbonyl compounds over earth-abundant metal-based catalysts at low temperature (35 °C).

An earth-abundant Co-based catalyst, Co@C–N(800), could efficiently catalyze the reductive amination of carbonyl compounds into primary amines at 35 °C owing to the synergistic effect of Co nanoparticles and atomically dispersed Co–N_x sites.

Copper-Catalyzed Conjugate Addition of Carbonyls as Carbanion Equivalent via Hydrazones

Copper-catalyzed conjugate addition is a classic method for forming new carbon-carbon bonds. However, copper has never showed catalytic activity for umpolung carbanions in hydrazone chemistry. Herein, we report a facile conjugate addition of hydrazone catalyzed by readily available copper complexes at room temperature. The employment of mesitylcopper(I) and electron-rich phosphine bidentate ligand is a key factor affecting reactivity. The reaction allows various aromatic hydrazones to react with diverse conjugated compounds to produce 1,4-adducts in yields of about 20 to 99%.

Sustainable Production of Benzylamines from Lignin

Catalytic conversion of lignin into heteroatom functionalized chemicals is of great importance to bring the biorefinery concept into reality. Herein, a new strategy was designed for direct transformation of lignin β-O-4 model compounds into benzylamines and phenols in moderate to excellent yields in the presence of organic amines. The transformation involves dehydrogenation of C_α -OH, hydrogenolysis of the C_β -O bond and reductive amination in the presence of Pd/C catalyst. Experimental data suggest that the dehydrogenation reaction proceeds over the other two reactions and secondary amines serve as both reducing agents and amine sources in the transformation. Moreover, the concept of "lignin to benzylamines" was demonstrated by a two-step process. This work represents a first example of synthesis of benzylamines from lignin, thus providing a new opportunity for the sustainable synthesis of benzylamines from renewable biomass, and expanding the products pool of biomass conversion to meet future biorefinery demands.

Synthesis of Substituted Anilines from Cyclohexanones Using Pd/C-Ethylene System and Its Application to Indole Synthesis

The synthesis of anilines and indoles from cyclohexanones using a Pd/C-ethylene system is reported. A simple combination of NH₄OAc and K₂CO₃ under nonaerobic conditions was found to be the most suitable to perform this reaction. Hydrogen transfer between cyclohexanone and ethylene generates the desired products. The reaction tolerates a variety of substitutions on the starting cyclohexanones.

Advances in regioselective functionalization of 9-phenanthrenols

Various approaches are discussed for regioselective functionalization and transformation of 9-phenanthrenol and its ether derivatives, which are readily available from phenanthrene in coal tar.

Dearomatization-Rearomatization Strategy for Synthesizing Carbazoles with 2,2'-Biphenols and Ammonia by Dual C(Ar)-OH Bond Cleavages

Carbazole is an essential building block in various pharmaceuticals, agrochemicals, natural products, and materials. For future sustainability, it is highly desirable to synthesize carbazole derivatives directly from renewable resources or cheap raw materials. Phenolic compounds are a class of degradation products of lignin. On the other hand, ammonia is a very cheap industrial inorganic chemical. Herein, an efficient dearomatization-rearomatization strategy has been developed to directly cross-couple 2,2'-biphenols with ammonia by dual C(Ar)-OH bond cleavages. This strategy provides a greener pathway to synthesize valuable carbazole derivatives from phenols.

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.

Mono/Dual Amination of Phenols with Amines in Water

We herein describe a practical direct amination of phenols through a palladium-catalyzed hydrogen-transfer-mediated activation method to synthesize the secondary and tertiary amines. In this conversion, environmentally friendly water and inexpensive ammonium formate were used as solvent and reductant, respectively. A range of amines, including aliphatic amines, aniline, secondary amines, and diamines, could be coupled effectively by this method to achieve mono/dual amination and cyclization of phenols. This study not only provides a green and mild strategy for the synthesis of secondary and tertiary naphthylamines but also expands the synthesis of chloroquine in organic chemistry.

Amine-Mediated Bond Cleavage in Oxidized Lignin Models

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia-mediated bond cleavage in oxidized lignin β-O-4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative C_α -C_β bond cleavage and C_α -N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between C_α -N and C_β -N bond formation, generating amides and α-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative C_α -C_β bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the C_α or C_β position of the oxidized β-O-4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.

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

Continuous Synthesis of Aryl Amines from Phenols Utilizing Integrated Packed-Bed Flow Systems

Aryl amines are important pharmaceutical intermediates among other numerous applications. Herein, an environmentally benign route and novel approach to aryl amine synthesis using dehydrative amination of phenols with amines and styrene under continuous-flow conditions was developed. Inexpensive and readily available phenols were efficiently converted into the corresponding aryl amines, with small amounts of easily removable co-products (i.e., H₂ O and alkanes), in multistep continuous-flow reactors in the presence of heterogeneous Pd catalysts. The high product selectivity and functional-group tolerance of this method allowed aryl amines with diverse functional groups to be selectively obtained in high yields over a continuous operation time of one week.

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.

Highly Efficient Mesoporous Core-Shell Structured Ag@SiO2 Nanosphere as an Environmentally Friendly Catalyst for Hydrogenation of Nitrobenzene

The size-uniformed mesoporous Ag@SiO₂ nanospheres’ catalysts were prepared in one-pot step via reducing AgNO₃ by different types of aldehyde, which could control the size of Ag@SiO₂ NPs and exhibit excellent catalytic activity for the hydrogenation of nitrobenzene. The results showed that the Ag core size, monitored by different aldehydes with different reducing abilities, together with the ideal monodisperse core-shell mesoporous structure, was quite important to affect its superior catalytic performances. Moreover, the stability of Ag fixed in the core during reaction for 6 h under 2.0 MPa, 140 °C made this type of Ag@SiO₂ catalyst separable and environmentally friendly compared with those conventional homogeneous catalysts and metal NPs catalysts. The best catalyst with smaller Ag cores was prepared by strong reducing agents such as CH₂O. The conversion of nitrobenzene can reach 99.9%, the selectivity was 100% and the catalyst maintained its activity after several cycles, and thus, it is a useful novel candidate for the production of aniline.

Synthesis of unsymmetrically substituted triarylamines via acceptorless dehydrogenative aromatization using a Pd/C and p-toluenesulfonic acid hybrid relay catalyst

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed. A hybrid relay catalyst comprising carbon-supported Pd (Pd/C) and p-toluenesulfonic acid (TsOH) was found to be effective for synthesizing a variety of triarylamines bearing different aryl groups starting from arylamines (diarylamines or anilines), using cyclohexanones as the arylation sources under acceptorless conditions with the release of gaseous H₂. The proposed reaction comprises the following relay steps: condensation of arylamines and cyclohexanones to produce imines or enamines, dehydrogenative aromatization of the imines or enamines over Pd nanoparticles (NPs), and elimination of H₂ from the Pd NPs. In this study, an interesting finding was obtained indicating that TsOH may promote the dehydrogenation.

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed.