Diboron-Assisted Palladium-Catalyzed Transfer Hydrogenation of N-Heteroaromatics with Water as Hydrogen Donor and Solvent

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

Electrocatalytic hydrogenation of 1-octene as non-activated model substrate with neutral water as H-donor is reported, using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a kobs of 87 s-1. Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl- and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]- (2), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [(tBuPCP)Ir(H)2] (3) in 88 % FE from the reaction of 2 with H2O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro-hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H2 formation. Even in strongly alkaline conditions (Bu4NOH added), the electro-hydrogenation of 1-octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H2O activation, reminiscent to transfer hydrogenation pathways, instead of classical H+ reduction. DFT calculations confirmed oxidative addition of H2O as a key step in this context.

Co(II)-Catalyzed Additive-Free Transfer Hydrogenation of N-Heteroarenes at Room Temperature

Traditional catalyst development relies on multistep synthesis and isolation of ligand and precatalyst. Designing a catalytic system that can be assembled in situ from easily accessible starting materials can decrease the reaction complexity and enhance the synthetic utility. Herein, we report an inexpensive and commercially available CoBr2·H2O/terpyridine-catalyzed effective and straightforward transfer hydrogenation (TH) protocol for N-heteroarenes, utilizing NH3·BH3 (AB) under ambient conditions. Synthesis of diverse substrates and bioactive molecules demonstrated a practical applicability. Control experiments and DFT studies elucidate the mechanism.

Methyl Formate, an Alternative Transfer Hydrogenating Agent for Chemoselective Reduction of N-Heteroarenes and Azoarenes

The search for efficient molecular hydrogen precursors and their catalytic exploration is necessary for the evolution of catalytic transfer hydrogenation. Methyl formate (MF) having high hydrogen content still remains unexplored for such transformations. Herein, we disclosed a bifunctional Ir(III)-complex catalyzed chemoselective TH protocol for N-heteroarenes and azoarenes using MF. A variety of substrates including ten bioactive molecules have been synthesized under mild reaction conditions. A probable mechanistic pathway was proposed based on control experiments and mechanistic studies.

Facile Transfer Hydrogenation of N-Heteroarenes and Nitroarenes Using Magnetically Recoverable Pd@SPIONs Catalyst

Catalysts with active, selective, and reusable features are desirable for sustainable development. The present investigation involved the synthesis and characterization of bear-surfaced ultrasmall Pd particles (<1 nm) loaded onto the surface of magnetic nanoparticles (8-10 nm). The amount of Pd loading onto the surface of magnetite is recorded as 2.8 wt %. The characterization process covered the utilization of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) methods. The Pd@Fe3O4 catalyst has shown remarkable efficacy in the hydrogenation of quinoline, resulting in the production of >99% N-ring hydrogenated (py-THQ) product. Additionally, the catalyst facilitated the conversion of nitroarenes into their corresponding aniline derivatives, where hydrogen was achieved by H2O molecules with the aid of tetrahydroxydiboron (THDB) as an equilibrium supportive at 80 °C in 1 h. The high efficiency of a transfer hydrogenation catalyst is closely related to the metal-support synergistic effect. The broader scope of functional group tolerance is evaluated. The potential mechanism underlying the hydrogenation process has been elucidated through the utilization of isotopic labeling investigations. The application of the heterocyclic compound hydrogenation reaction is extended to formulate the medicinally important tubular polymerization inhibitor drug synthesis. The investigation of the recyclability of Pd@Fe3O4 has been conducted.

BICAAC-Derived Covalent and Cationic Ir(I) Complexes: Application of Ir(BICAAC)Cl(COD) Complexes as Catalysts for Transfer Hydrogenation and Hydrosilylation Reactions

The ambiphilic bicyclic (alkyl)(amino)carbenes (Me/iPrBICAAC) upon reaction with [IrCl(COD)]2 smoothly afford mononuclear Ir(I) complexes that have been spectroscopically and structurally characterized. These complexes exhibit good catalytic activity for transfer hydrogenation (TH) of 4-chlorobenzaldehyde using isopropyl alcohol (iPrOH), with turnover frequency values ranging between 6269 and 8093 h-1. Choosing the covalent complex Ir(MeBICAAC)Cl(COD) as a catalyst, a wide array of carbonyls and imines functionalized with electron-withdrawing and electron-donating substituents have been surveyed and afforded their reduced products in moderate-to-good yields. No detachment of the BICAAC unit from the Ir center was observed upon prolonged heating of Ir(MeBICAAC)Cl(COD) in toluene-d8 or isopropyl alcohol-d8, which evidenced good thermal stability of the catalyst. Complex Ir(MeBICAAC)Cl(COD) was also found to be catalytically active for the hydrosilylation of a variety of aldehydes using triethylsilane (Et3SiH).

Lanthanide/B(C6F5)3-Promoted Hydroboration Reduction of Indoles and Quinolines with Pinacolborane

We have developed a lanthanide/B(C6F5)3-promoted hydroboration reduction of indoles and quinolines with pinacolborane (HBpin). This reaction provides streamlined access to a range of nitrogen-containing compounds in moderate to excellent yields. Large-scale synthesis and further transformations to bioactive compounds indicate that the method has potential practical applications. Preliminary mechanistic studies suggest that amine additives promote the formation of indole-borane intermediates, and the lanthanide/B(C6F5)3-promoted hydroboration reduction proceeds via hydroboration of indole-borane intermediates with HBpin and in situ-formed BH3 species, followed by the protodeborylation process.

Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water

A base-assisted metal species modulation mechanism enables Ni-catalyzed stereodivergent transfer semihydrogenation of alkynes with water, delivering both olefinic isomers smoothly using cheap and nontoxic catalysts and additives. Different from most precedents, in which E-alkenes derive from the isomerization of Z-alkene products, the isomers were formed in orthogonal catalytic pathways. Mechanistic studies suggest base as a key early element in modulation of the reaction pathways: by adding different bases, nickel species with disparate valence states could be accessed to initiate two catalytic cycles toward different stereoisomers. The practicability of the method is showcased with nearly 70 examples, including internal and terminal triple bonds, enynes and diynes, affording semi-hydrogenated products in high yields and selectivity.

Synthesis of N-alkoxy amines and hydroxylamines via the iridium-catalyzed transfer hydrogenation of oximes

Cationic iridium (Ir) complexes were found to catalyze the transfer hydrogenation of oximes to access N-alkoxy amines and hydroxylamines, and the reaction was accelerated by trifluoroacetic acid. The practical application of this protocol was demonstrated by a gram-scale transformation and two-step synthesis of the fungicide furmecyclox (BAS 389F) in overall yields of 92 and 85%, respectively. An asymmetric protocol using chiral Ir complexes to afford chiral N-alkoxy amines was demonstrated, but the low yields/ee obtained indicated that further development was required.

Mechanistic insights into H2 evolution via water splitting at the expense of B2(OH)4: a theoretical study

H₂ has been comprehensively deemed a promising potential candidate to replace traditional fossil fuel-based energy. Typically, the hydrolysis of most hydrogen-rich boron hydrides (e.g. NaBH₄, NH₃BH₃ and Me₂NHBH₃) catalyzed by nanomaterials generates H₂ with only one H atom supplied by water and the other one by a hydrogen-rich boron hydride. Interestingly, both H atoms of produced H₂ are provided by water upon hydrolysis of B₂(OH)₄. Herein, the catalytic mechanisms of H₂ evolution upon water splitting at the expense of B₂(OH)₄ in its hydrolysis reactions catalyzed by acid, base or metal nanoparticles have been investigated by density functional theory (DFT) calculations. By computational studies, the mechanisms of catalysis by base and metal nanoparticles are basically the same as those speculated from our previous experiments. The previously proposed acid catalytic mechanism has been overturned, however. This study not only provides important insights into the catalytic mechanism for water splitting at the expense of B₂(OH)₄, but also opens up an exciting opportunity to use water to store H₂.

Chemoselective reduction of α,β-unsaturated ketones to allylic alcohols under catalyst-free conditions

A strategy of chemoselective reduction of α, β-unsaturated ketones was developed in our group. H3N·BH3 would prefer to coordinate with CO bond, forming six-membered ring, and ketones were hydrogenated via concerted double-hydrogen-transfer process.

Tetrabutylammonium Bromide-Catalyzed Transfer Hydrogenation of Quinoxaline with HBpin as a Hydrogen Source

A metal-free environmentally benign, simple, and efficient transfer hydrogenation process of quinoxaline has been developed using the HBpin reagent as a hydrogen source. This reaction is compatible with a variety of quinoxalines offering the desired tetrahydroquinoxalines in moderate-to-excellent yields with Bu₄NBr as a noncorrosive and low-cost catalyst.

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of Quinolines in Biphasic Systems or Water

An asymmetric transfer hydrogenation (ATH) of quinolines in water or biphasic systems was developed. This ATH reaction proceeds smoothly without the need for inert atmosphere protection in the presence of a water-soluble iridium catalyst, which bears an easily available aminobenzimidazole ligand. This ATH system can work at a catalyst loading of 0.001 mol % (S/C = 100 000, turnover number (TON) of up to 33 000) under mild reaction conditions. The turnover frequency (TOF) value can reach as high as 90 000 h-1. A variety of quinoline and N-heteroaryl compounds are transformed into the desired products in high yield and up to 99% enantiomeric excess (ee).

Pd-Catalyzed Tandem Isomerization/Cyclization for the Synthesis of Aromatic Oxazaheterocycles and Pyrido[3,4-b]indoles

An effient tandem process consisting of palladium-catalyzed double-bond isomerization of long-chain olefins and subsequent intramolecular cyclization promoted by B₂(OH)₂ for the synthesis of aromatic oxazaheterocycles is disclosed. This strategy can also provide rapid access to pyrido[3,4-b]indoles, trans-2-olefins, and eneamides bearing various functional groups with high regio- and stereoselectivity.

Present Status and Future Prospects of Jute in Nanotechnology: A Review

Nanotechnology has transformed the world with its diverse applications, ranging from industrial developments to impacting our daily lives. It has multiple applications throughout financial sectors and enables the development of facilitating scientific endeavors with extensive commercial potentials. Nanomaterials, especially the ones which have shown biomedical and other health-related properties, have added new dimensions to the field of nanotechnology. Recently, the use of bioresources in nanotechnology has gained significant attention from the scientific community due to its 100 % eco-friendly features, availability, and low costs. In this context, jute offers a considerable potential. Globally, its plant produces the second most common natural cellulose fibers and a large amount of jute sticks as a byproduct. The main chemical compositions of jute fibers and sticks, which have a trace amount of ash content, are cellulose, hemicellulose, and lignin. This makes jute as an ideal source of pure nanocellulose, nano-lignin, and nanocarbon preparation. It has also been used as a source in the evolution of nanomaterials used in various applications. In addition, hemicellulose and lignin, which are extractable from jute fibers and sticks, could be utilized as a reductant/stabilizer for preparing other nanomaterials. This review highlights the status and prospects of jute in nanotechnology. Different research areas in which jute can be applied, such as in nanocellulose preparation, as scaffolds for other nanomaterials, catalysis, carbon preparation, life sciences, coatings, polymers, energy storage, drug delivery, fertilizer delivery, electrochemistry, reductant, and stabilizer for synthesizing other nanomaterials, petroleum industry, paper industry, polymeric nanocomposites, sensors, coatings, and electronics, have been summarized in detail. We hope that these prospects will serve as a precursor of jute-based nanotechnology research in the future.

Diastereoselective Synthesis of Tetracyclic Tetrahydroquinoline Derivative Enabled by Multicomponent Reaction of Isocyanide, Allenoate, and 2-Aminochalcone

We report here a multicomponent protocol to assemble several polycyclic dihydropyran-fused tetrahydroquinoline structures with excellent diastereoselectivity. This procedure employs simple feedstocks to accomplish a series of diverse structures, which is difficult to attain by traditional sequences.

Metal-free reduction of unsaturated carbonyls, quinones, and pyridinium salts with tetrahydroxydiboron/water

A series of unsaturated carbonyls, quinones, and pyridinium salts have been effectively reduced to the corresponding saturated carbonyls, dihydroxybenzenes, and hydropyridines in moderate to high yields with tetrahydroxydiboron/water as a mild, convenient, and metal-free reduction system. Deuterium-labeling experiments have revealed this protocol to be an exclusive transfer hydrogenation process from water.

Asymmetric Reduction of Aromatic α-Dehydroamino Acid Esters with Water as Hydrogen Source

The asymmetric reduction of aromatic α-dehydroamino acid esters with water as the hydrogen source was developed by a Rh/Cu co-catalytic system. The reaction tolerates various functional groups, providing a valuable synthetic tool to access chiral α-amino acid esters readily. Moreover, the present methodology also was applied in the cost-effective and easy to handle preparation of chiral deuterated α-amino esters by using D₂O.

Mild and Selective Rhodium-Catalyzed Transfer Hydrogenation of Functionalized Arenes

Diboron-mediated rhodium-catalyzed transfer hydrogenation of functionalized arenes is reported. In addition to good functional group tolerance, the reaction features operational simplicity and controllable chemoselectivity. The general applicability of this procedure is demonstrated by the selective hydrogenation of a range of arenes, including functionalized benzenes, biphenyls, and polyaromatics.

Chemoselective hydroborative reduction of nitro motifs using a transition-metal-free catalyst

The first transition-metal-free catalysts for deoxygenative reduction of nitroarenes, nitro heteroarenes and even notoriously challenging nitroalkanes using pinacolborane have been reported.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Bis-NHC–Ag/Pd(OAc)2 Catalytic System Catalyzed Transfer Hydrogenation Reaction

The bis-NHC–Ag/Pd(OAc)2 catalytic system (NHC = N-heterocyclic carbene), a combination of bis-NHC–Ag complex and Pd(OAc)2, was found to be a smart catalyst in the Pd-catalyzed transfer hydrogenation of various functionalized arenes and internal/terminal alkynes. The catalytic system demonstrated high efficiency for the reduction of a wide range of various functional groups such as carbonyls, alkynes, olefins, and nitro groups in good to excellent yields and high chemoselectivity for the reduction of functional groups. In addition, the protocol was successfully exploited to stereoselectivity for the transformation of alkynes to alkenes in aqueous media under air. This methodology successfully provided an alternative useful protocol for reducing various functional groups and a simple operational protocol for transfer hydrogenation.

Synthetic Strategies, Reactivity and Applications of 1,5-Naphthyridines

This review covers the synthesis and reactivity of 1,5-naphthyridine derivatives published in the last 18 years. These heterocycles present a significant importance in the field of medicinal chemistry because many of them exhibit a great variety of biological activities. First, the published strategies related to the synthesis of 1,5-naphthyridines are presented followed by the reactivity of these compounds with electrophilic or nucleophilic reagents, in oxidations, reductions, cross-coupling reactions, modification of side chains or formation of metal complexes. Finally, some properties and applications of these heterocycles studied during this period are examined.

Diastereoselective Construction of Cyclopropane-Fused Tetrahydroquinolines via a Sequential [4 + 2]/[2 + 1] Annulation Reaction

A sequential [4 + 2]/[2 + 1] annulation of α-aryl vinylsulfoniums with 2-aminochalcones and 2-(2-aminobenzylidene)-1H-indene-1,3(2H)-dione is reported that affords a series of cyclopropane-fused tetrahydroquinolines. The salient features of this novel and practical transformation include high efficiency, transition-metal-free nature, operational simplicity, and outstanding functional group tolerance.

Triflyl-assisted reductive Pd-catalyzed Tsuji–Trost type reaction

A palladium-catalyzed hydrodetriflylation reaction using water as a convenient hydrogen source has allowed the preparation of novel (triflyl)cyclobutenes from readily available precursors.

Copper-catalyzed mild desulfonylation of vinyl sulfonyl molecules

The first Cu-catalyzed chemical selective desulfonylation of vinyl sulfonyl molecules to olefins was developed using B₂pin₂–water as the reductant.

Catalytic Reductions Without External Hydrogen Gas: Broad Scope Hydrogenations with Tetrahydroxydiboron and a Tertiary Amine

Facile reduction of aryl halides with a combination of 5% Pd/C, B₂(OH)₄, and 4-methylmorpholine is reported. Aryl bromides, iodides, and chlorides were efficiently reduced. Aryl dihalides containing two different halogen atoms underwent selective reduction: I over Br and Cl, and Br over Cl. Beyond these, aryl triflates were efficiently reduced. This combination was broadly general, effectuating reductions of benzylic halides and ethers, alkenes, alkynes, aldehydes, and azides, as well as for N-Cbz deprotection. A cyano group was unaffected, but a nitro group and a ketone underwent reduction to a low extent. When B₂(OD)₄ was used for aryl halide reduction, a significant amount of deuteriation occurred. However, H atom incorporation competed and increased in slower reactions. 4-Methylmorpholine was identified as a possible source of H atoms in this, but a combination of only 4-methylmorpholine and Pd/C did not result in reduction. Hydrogen gas has been observed to form with this reagent combination. Experiments aimed at understanding the chemistry led to the proposal of a plausible mechanism and to the identification of N,N-bis(methyl-d ₃)pyridine-4-amine (DMAP-d ₆) and B₂(OD)₄ as an effective combination for full aromatic deuteriation.

Copper-Catalyzed Transfer Hydrogenation of N-Heteroaromatics with an Oxazaborolidine Complex

The first-time use of the oxazaborolidine complex in transfer hydrogenation was accomplished. It was prepared without difficulty from cheap materials: ethanolamine and BH₃·THF. A general and efficient method for copper-catalyzed transfer hydrogenation of a variety of N-heteroaromatics with an oxazaborolidine-BH₃ complex under mild reaction conditions afforded the corresponding hydrogenated products in up to 96% yield. Mechanistic studies indicate that the hydrogen source originated from water and borane that coordinate with the nitrogen atom of oxazaborolidine. Accordingly, a plausible mechanism for this reaction was proposed. This method was successfully used in the key step synthesis of natural products (±)-angustureine and (±)-galipinine in three steps.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

HMPA-Catalyzed Transfer Hydrogenation of 3-Carbonyl Pyridines and Other N-Heteroarenes with Trichlorosilane

A method for the HMPA (hexamethylphosphoric triamide)-catalyzed metal-free transfer hydrogenation of pyridines has been developed. The functional group tolerance of the existing reaction conditions provides easy access to various piperidines with ester or ketone groups at the C-3 site. The suitability of this method for the reduction of other N-heteroarenes has also been demonstrated. Thirty-three examples of different substrates have been reduced to designed products with 45⁻96% yields.