Catalytic condensation for the formation of polycyclic heteroaromatic compounds

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.

Single-Atom Fe-Catalyzed Acceptorless Dehydrogenative Coupling to Quinolines

A single-atom iron catalyst was found to exhibit exceptional reactivity in acceptorless dehydrogenative coupling for quinoline synthesis, outperforming known homogeneous and nanocatalyst systems. Detailed characterizations, including aberration-corrected HAADF-STEM, XANES, and EXAFS, jointly confirmed the presence of atomically dispersed iron centers. Various functionalized quinolines were efficiently synthesized from different amino alcohols and a range of ketones or alcohols. The iron single-atom catalyst achieved a turnover number (TON) of up to 105, far exceeding the results of current homogeneous and nanocatalyst systems. Detailed mechanistic studies verified the significance of single-atom Fe sites in the dehydrogenation process.

N-Alkylation of Amines by C1-C10 Aliphatic Alcohols Using A Well-Defined Ru(II)-Catalyst. A Metal-Ligand Cooperative Approach

A Ru(II)-catalyzed efficient and selective N-alkylation of amines by C1-C10 aliphatic alcohols is reported. The catalyst [Ru(L^1a)(PPh₃)Cl₂] (1a) bearing a tridentate redox-active azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^1a) is air-stable, easy to prepare, and showed wide functional group tolerance requiring only 1.0 mol % (for N-methylation and N-ethylation) and 0.1 mol % of catalyst loading for N-alkylation with C3-C10 alcohols. A wide array of N-methylated, N-ethylated, and N-alkylated amines were prepared in moderate to good yields via direct coupling of amines and alcohols. 1a efficiently catalyzes the N-alkylation of diamines selectively. It is even suitable for synthesizing N-alkylated diamines using (aliphatic) diols producing the tumor-active drug molecule MSX-122 in moderate yield. 1a showed excellent chemo-selectivity during the N-alkylation using oleyl alcohol and monoterpenoid β-citronellol. Control experiments and mechanistic investigations revealed that the 1a-catalyzed N-alkylation reactions proceed via a borrowing hydrogen transfer pathway where the hydrogen removed from the alcohol during the dehydrogenation step is stored in the ligand backbone of 1a, which in the subsequent steps transferred to the in situ formed imine intermediate to produce the N-alkylated amines.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Low temperature in situ immobilization of nanoscale fcc and hcp polymorphic nickel particles in polymer-derived Si-C-O-N(H) to promote electrocatalytic water oxidation in alkaline media

We synthesized nickel (Ni) nanoparticles (NPs) in a high specific surface area (SSA) p-block element-containing inorganic compound prepared via the polymer-derived ceramics (PDC) route to dispatch the obtained nanocomposite towards oxygen evolution reaction (OER). The in situ formation of Ni NPs in an amorphous silicon carboxynitride (Si-C-O-N(H)) matrix is allowed by the reactive blending of a polysilazane, NiCl2 and DMF followed by the subsequent thermolysis of the Ni : organosilicon polymer coordination complex at a temperature as low as 500 °C in flowing argon. The final nanocomposite displays a BET SSA as high as 311 m2 g-1 while the structure of the NPs corresponds to face-centred cubic (fcc) Ni along with interstitial-atom free (IAF) hexagonal close-packed (hcp) Ni as revealed by XRD. A closer look into the compound through FEG-SEM microscopy confirms the formation of pure metallic Ni, while HR-TEM imaging reveals the occurrence of Ni particles featuring a fcc phase and surrounded by carbon layers; thus, forming core-shell structures, along with Ni NPs in an IAF hcp phase. By considering that this newly synthesized material contains only Ni without doping (e.g., Fe) with a low mass loading (0.15 mg cm-2), it shows promising OER performances with an overpotential as low as 360 mV at 10 mA cm-2 according to the high SSA matrix, the presence of the IAF hcp Ni NPs and the development of core-shell structures. Given the simplicity, the flexibility, and the low cost of the proposed synthesis approach, this work opens the doors towards a new family of very active and stable high SSA nanocomposites made by the PDC route containing well dispersed and accessible non-noble transition metals for electrocatalysis applications.

Cu-Catalyzed Ullmann-Type Double C-N Coupling Approach to 5-Aryl-5H-indolizino[3,2-b]indoles

A modular approach to a polyfunctionalized 5H-indolizino[3,2-b]indole, an indolizine-indole fused system, was achieved from readily available pyridine-2-acetonitrile, 2-bromobenzaldehyde, and TMSCN via the strategic combination of a one-pot three-component assembly and Cu-catalyzed Ullmann-type double C-N coupling reactions through which five new bonds (two C-C and three C-N) were formed in two steps.

Porous Silicon Oxycarbonitride Ceramics with Palladium and Pd2Si Nanoparticles for Dry Reforming of Methane

Pd-containing precursor has been synthesized from palladium acetate and poly(vinly)silazane (Durazane 1800) in an ice bath under an argon atmosphere. The results of ATR-FTIR and NMR characterizations reveal the chemical reaction between palladium acetate and vinyl groups in poly(vinyl)silazane and the hydrolyzation reaction between –Si–H and –Si–CH=CH₂ groups in poly(vinyl)silazane. The palladium nanoparticles are in situ formed in the synthesized precursors as confirmed by XRD, XPS, and TEM. Pd- and Pd₂Si-containing SiOCN ceramic nanocomposites are obtained by pyrolysis of the synthesized precursors at 700 °C, 900 °C–1100 °C in an argon atmosphere. The pyrolyzed nanocomposites display good catalytic activity towards the dry reforming of methane. The sample pyrolyzed at 700 °C possesses the best catalytic performance, which can be attributed to the in situ formed palladium nanoparticles and high BET surface area of about 233 m² g⁻¹.

Mechanistic Investigation of the Formation of Nickel Nanocrystallites Embedded in Amorphous Silicon Nitride Nanocomposites

Herein, we report the mechanistic investigation of the formation of nickel (Ni) nanocrystallites during the formation of amorphous silicon nitride at a temperature as low as 400 °C, using perhydropolysilazane (PHPS) as a preformed precursor and further coordinated by nickel chloride (NiCl₂); thus, forming the non-noble transition metal (TM) as a potential catalyst and the support in an one-step process. It was demonstrated that NiCl₂ catalyzed dehydrocoupling reactions between Si-H and N-H bonds in PHPS to afford ternary silylamino groups, which resulted in the formation of a nanocomposite precursor via complex formation: Ni(II) cation of NiCl₂ coordinated the ternary silylamino ligands formed in situ. By monitoring intrinsic chemical reactions during the precursor pyrolysis under inert gas atmosphere, it was revealed that the Ni-N bond formed by a nucleophilic attack of the N atom on the Ni(II) cation center, followed by Ni nucleation below 300 °C, which was promoted by the decomposition of Ni nitride species. The latter was facilitated under the hydrogen-containing atmosphere generated by the NiCl₂-catalyzed dehydrocoupling reaction. The increase of the temperature to 400 °C led to the formation of a covalently-bonded amorphous Si₃N₄ matrix surrounding Ni nanocrystallites.

Recent progress on selective hydrogenation of phenol toward cyclohexanone or cyclohexanol

Phenol is considered as an important platform molecule for synthesizing value-added chemical intermediates and products. To date, various strategies for phenol transformation have been developed, and among them, selective hydrogenation of phenol toward cyclohexanone (K), cyclohexanol (A) or the mixture KA oil has been attracted great interest because they are both the key raw materials for the synthesis of nylon 6 and 66, as well as many other chemical products, including polyamides. However, until now it is still challengeable to realize the industrilized application of phenol hydrogenation toward KA oils. To better understand the selective hydrogenation of phenol and fabricate the enabled nanocatalysts, it is necessary to summarize the recent progress on selective hydrogenation of phenol with different catalysts. In this review, we first summarize the selective hydrogenation of phenol toward cyclohexanone or cyclohexanol by different nanocatalysts, and simultaneously discuss the relationship among the active components, type of supports and their performances. Then, the possible reaction mechanism of phenol hydrogenation with the typical metal nanocatalysts is summarized. Subsequently, the possible ways for scale-up hydrogenation of phenol are discussed. Finally, the potential challenges and future developments of metal nanocatalysts for the selective hydrogenation of phenol are proposed.

Iron-Catalyzed Alkyne-Based Multicomponent Synthesis of Pyrimidines under Air

An iron-catalyzed sustainable, economically affordable, and eco-friendly synthetic protocol for the construction of various trisubstituted pyrimidines is described. A wide range of trisubstituted pyrimidines were prepared using a well-defined, easy to prepare, bench-stable, and phosphine-free iron catalyst featuring a redox-noninnocent tridentate arylazo pincer under comparatively mild aerobic conditions via dehydrogenative functionalization of alcohols with alkynes and amidines.

Low temperature in situ formation of cobalt in silicon nitride toward functional nitride nanocomposites

This work highlights the first demonstration of a low-temperature in situ formation of Co nanocrystallites embedded within an amorphous silicon nitride matrix through careful control of the chemistry behind material design using perhydropolysilazane (PHPS) as a Si3N4 precursor further coordinated with CoCl2 and ammonia as a pyrolysis atmosphere. The Co nucleation was allowed to proceed at temperatures as low as 400 °C via thermal decomposition of Co2N pre-formed in situ by the reaction of CoCl2 with the Si centers of PHPS at the early stage of pyrolysis (220-350 °C).

Superparamagnetic Silicon Carbonitride Ceramic Fibers Through In Situ Generation of Iron Silicide Nanoparticles During Pyrolysis of an Iron-Modified Polysilazane

Ceramic fibers are high-tech structural key components of ceramic matrix composites (CMCs), which are a very promising class of materials for applications in next-generation turbines, especially nonoxide ceramic fibers, usually produced by the polymer-derived ceramics (PDC) route, which possess the enhanced mechanical and thermostructural properties necessary to withstand the harsh conditions (temperature and atmosphere) imposed on CMCs. However, recycling composite materials, such as fiber-reinforced polymers and CMCs, is still a big challenge. Here, we present for the first time the processing of superparamagnetic iron-containing ceramic fibers, which, due to their magnetic properties, can be separated from the matrix material of a composite. The synthesis strategy of the novel functional ceramic fibers is based on a tailored reaction of polyorganosilazane with an iron complex, resulting in a suitable, meltable polymer. After melt-spinning and curing, subsequent pyrolysis leads to superparamagnetic ceramic fibers with a saturation magnetization of 1.54 emu g^-1 because of in situ-formed iron silicide nanoparticles of an average size of 7.5 nm, homogeneously dispersed in an amorphous SiCNO matrix. Moreover, the ceramic fibers exhibit a tensile strength of 1.24 GPa and appropriate oxidation resistance. The developed versatile reaction strategy allows also for the incorporation of other elements to implement further functionalities for processing of multifunctional composites.

The ammoxidation of alcohols over heterogeneous catalysts for the green synthesis of nitriles

This is the first review on the ammoxidation of alcohols over heterogeneous catalysts, in which issues and potential solutions are demonstrated.

Homogeneous Nickel-Catalyzed Sustainable Synthesis of Quinoline and Quinoxaline under Aerobic Conditions

Dehydrogenative coupling-based reactions have emerged as an efficient route toward the synthesis of a plethora of heterocyclic rings. Herein, we report an efficacious, nickel-catalyzed synthesis of two important heterocycles such as quinoline and quinoxaline. The catalyst is molecularly defined, is phosphine-free, and can operate at a mild reaction temperature of 80 °C. Both the heterocycles can be easily assembled via double dehydrogenative coupling, starting from 2-aminobenzyl alcohol/1-phenylethanol and diamine/diol, respectively, in a shorter span of reaction time. This environmentally benign synthetic protocol employing an inexpensive catalyst can rival many other transition-metal systems that have been developed for the fabrication of two putative heterocycles. Mechanistically, the dehydrogenation of secondary alcohol follows clean pseudo-first-order kinetics and exhibits a sizable kinetic isotope effect. Intriguingly, this catalyst provides an example of storing the trapped hydrogen in the ligand backbone, avoiding metal-hydride formation. Easy regeneration of the oxidized form of the catalyst under aerobic/O₂ oxidation makes this protocol eco-friendly and easy to handle.

The Synthesis of Primary Amines through Reductive Amination Employing an Iron Catalyst

The reductive amination of ketones and aldehydes by ammonia is a highly attractive method for the synthesis of primary amines. The use of catalysts, especially reusable catalysts, based on earth-abundant metals is similarly appealing. Here, the iron-catalyzed synthesis of primary amines through reductive amination was realized. A broad scope and a very good tolerance of functional groups were observed. Ketones, including purely aliphatic ones, aryl-alkyl, dialkyl, and heterocyclic, as well as aldehydes could be converted smoothly into their corresponding primary amines. In addition, the amination of pharmaceuticals, bioactive compounds, and natural products was demonstrated. Many functional groups, such as hydroxy, methoxy, dioxol, sulfonyl, and boronate ester substituents, were tolerated. The catalyst is easy to handle, selective, and reusable and ammonia dissolved in water could be employed as the nitrogen source. The key is the use of a specific Fe complex for the catalyst synthesis and an N-doped SiC material as catalyst support.

Aryl-Nickel-Catalyzed Benzylic Dehydrogenation of Electron-Deficient Heteroarenes

This manuscript describes the first practical benzylic dehydrogenation of electron-deficient heteroarenes, including pyridines, pyrazines, pyrimidines, pyridazines, and triazines. This transformation allows for the efficient benzylic oxidation of heteroarenes to afford heterocyclic styrenes by the action of nickel catalysis paired with an unconventional bromothiophene oxidant.

General and selective deoxygenation by hydrogen using a reusable earth-abundant metal catalyst

Chemoselective deoxygenation by hydrogen is particularly challenging but crucial for an efficient late-stage modification of functionality-laden fine chemicals, natural products, or pharmaceuticals and the economic upgrading of biomass-derived molecules into fuels and chemicals. We report here on a reusable earth-abundant metal catalyst that permits highly chemoselective deoxygenation using inexpensive hydrogen gas. Primary, secondary, and tertiary alcohols as well as alkyl and aryl ketones and aldehydes can be selectively deoxygenated, even when part of complex natural products, pharmaceuticals, or biomass-derived platform molecules. The catalyst tolerates many functional groups including hydrogenation-sensitive examples. It is efficient, easy to handle, and conveniently synthesized from a specific bimetallic coordination compound and commercially available charcoal. Selective, sustainable, and cost-efficient deoxygenation under industrially viable conditions seems feasible.

Iridium Catalyzed Synthesis of Tetrahydro-1H-Indoles by Dehydrogenative Condensation

Novel synthetic routes to the commonly encountered indole motif are highly sought after. Tetrahydro-1H-indoles were synthesized for the first time from secondary alcohols and 2aminocyclohexanol in the presence of a well-established iridium catalyst using a modified synthetic procedure recently developed for the synthesis of hydrocarbazoles. The catalyst is stabilized by an inexpensive and easy-to-synthesize triazine based PN5P pincer ligand. The reaction proceeds through acceptorless dehydrogenative condensation (ADC) and yields the title compound, dihydrogen, and water and can thus be classified as sustainable synthesis. Overall, five examples, three of which were previously unknown compounds, were prepared. The propitious isolated yields and the mild reaction conditions show the synthetic value of this approach. These tetrahydroindoles can be quantitatively dehydrogenated over a heterogeneous Pd catalyst to yield the corresponding indoles.

Reversible Hydrogen Uptake/Release over a Sodium Phenoxide-Cyclohexanolate Pair

Hydrogen uptake and release in arene-cycloalkane pairs provide an attractive opportunity for on-board and off-board hydrogen storage. However, the efficiency of arene-cycloalkane pairs currently is limited by unfavorable thermodynamics for hydrogen release. It is shown here that the thermodynamics can be optimized by replacement of H in the -OH group of cyclohexanol and phenol with alkali or alkaline earth metals. The enthalpy change upon dehydrogenation decreases substantially, which correlates with the delocalization of the oxygen electron to the benzene ring in phenoxides. Theoretical calculations reveal that replacement of H with a metal leads to a reduction of the HOMO-LUMO energy gap and elongation of the C-H bond in the α site in cyclohexanolate, which indicates that the cyclohexanol is activated upon metal substitution. The experimental results demonstrate that sodium phenoxide-cyclohexanolate, an air- and water-stable pair, can desorb hydrogen at ca. 413 K and 373 K in the solid form and in an aqueous solution, respectively. Hydrogenation, on the other hand, is accomplished at temperatures as low as 303 K.

Phosphine free Mn-complex catalysed dehydrogenative C–C and C–heteroatom bond formation: a sustainable approach to synthesize quinoxaline, pyrazine, benzothiazole and quinoline derivatives

Sustainable synthesis of quinoxalines, pyrazines, benzothiazoles and quinolines catalysed by a phosphine free Mn(i) complex.