Ruthenium-Catalyzed Dehydrogenative β-Benzylation of 1,2,3,4-Tetrahydroquinolines with Aryl Aldehydes: Access to Functionalized Quinolines

Halogen bonding accelerated aerobic dehydrogenative aromatization for 4-aminoquinoline preparation

This study presents a highly efficient method for 4-aminoquinoline derivative preparation under transition metal-free conditions. The process involves an aerobic oxidative dehydrative coupling of 2,3-dihydroquinolin-4(1H)-ones with various amines, including ammonia, resulting in high yields of the desired products. The method is also applicable to substituted 4-aminoquinoline derivative construction through a cyclization/dehydrative coupling cascade process starting from 2'-amino chalcones. Mechanistic studies reveal that iodine (I2) is consumed to produce 3-iodoquinolin-4-ol, which acts as a true catalyst with high catalytic efficacy (as low as 0.5 mol%). The presence of halogen bonding is critical in the inter-molecular transfer hydrogenation process to generate inactive quinolin-4-ol. Subsequently, using air/oxygen as the terminal oxidant, the iodine anion was oxidized to I2 to regenerate the 3-iodoquinolin-4-ol from quinolin-4-ol in the catalytic cycle. Key benefits of this methodology include its simplicity, transition metal-free conditions, environmentally-benign oxidant, and high atom economy, making it a valuable approach for synthesizing medicinally significant 4-aminoquinoline derivatives.

Three-Component Synthesis of 2-Substituted Quinolines and Benzo[f]quinolines Using Tertiary Amines as the Vinyl Source

A metal-free method for the construction of 2-substituted quinolines and benzo[f]quinolines from aromatic amines, aldehydes, and tertiary amines has been demonstrated. Cheap and readily available tertiary amines acted as the vinyl source. A new pyridine ring was selectively formed via [4 + 2] condensation that was promoted by ammonium salt under neutral conditions and an oxygen atmosphere. This strategy provided a new route for the preparation of various quinoline derivatives with different substituents at the pyridine ring, which provides the possibility of further modification.

Metal-free three-component assemblies of anilines, α-keto acids and alkyl lactates for quinoline synthesis and their anti-inflammatory activity

A new and metal-free three-component method for the synthesis of 2,4-disubstituted quinolines via the reactions of anilines, α-keto acids and alkyl lactates is reported. The reactions proceed in the presence of p-toluene sulfonic acid (p-TSA) and tert-butyl peroxybenzoate (TBPB) to provide diverse quinoline products via the construction of new CC double, C-C single and CN double bonds without producing any organic mass-based side product. Notably, the anti-inflammatory activity of the quinolines has been investigated by measuring their ability to inhibit NO release by lipopolysaccharide (LPS) induced RAW264.7 cells, leading to the identification of 4i, 4t and 4x as potent anti-inflammatory compounds in vitro.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts

Merging hydrogen (H₂) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H₂ fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H₂ can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H₂ production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H₂ production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H₂ production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Vanadium-doped molybdenum carbides as a promising catalyst for C–N/C–C coupling reactions

Vanadium-doped molybdenum carbides encapsulated in nitrogen-doped carbon exhibit significantly enhanced catalytic efficiency for both the oxidative C–N coupling of amines and C–C bond couplings of alcohols and ketones.

Retracted: Synthesis of 2-Arylisoindoline Derivatives Catalyzed by Reusable 1,2,4-Triazole Iridium on Mesoporous Silica through a Cascade Borrowing Hydrogen Strategy

Covalent attachment of a 1,2,4-triazole iridium complex to mesoporous MCM-41 generated a heterogeneous catalyst that was found to be effective in the synthesis of 2-aryl isoindolines, quinolines, cyclic amines, and symmetrical secondary amines through a cascade borrowing hydrogen strategy. Interestingly, the supported heterogeneous iridium catalyst prepared from the 1,2,4-triazole iridium complex and mesoporous MCM-41 exhibited high catalytic activity in the preparation of 2-aryl isoindoline derivatives and symmetrical secondary amines. The catalyst system is highly recyclable for at least five times. Besides the important effect of the triazole, iridium sites grafted on siliceous supports can act as multifunctional catalytic centers and thus greatly enhance the catalytic activity of the catalysts. Furthermore, mechanistic experiments revealed that the reaction is initiated by an initial alcohol dehydrogenation and promoted by an iridium hydride intermediate. Importantly, the direct detection of a diagnostic iridium hydride signal confirmed that the synthesis of 2-aryl isoindolines occurs by a borrowing hydrogen process. This work provides an efficient example of isoindolines synthesis through a borrowing hydrogen strategy.

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.

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

Thienylbenzotriazole promoted highly active gold nanoparticles supported on N-doped graphene as efficient catalysts in water and a mechanism exploration

Gold nanocomposite catalyzed alkylation of ketone with alcohol, alcohol and alcohol, particularly for amine and amine in water.

Immobilization of manganese dioxide nanoparticles on modified poly 2,4-dichlorostyrene microspheres: a highly efficient and recyclable catalyst for borrowing hydrogen reactions

Modified poly 2,4-dichlorostyrene microspheres were designed and synthesized, and were proved to be an effective carrier to synthesize supported manganese dioxide nanoparticles.

Copper-catalysed dehydrogenative α-C(sp3)–H amination of tetrahydroquinolines with O-benzoyl hydroxylamines

A copper-catalysed dehydrogenative α-C(sp³)–H amination of tetrahydroquinolines with O-benzoyl hydroxylamines has been demonstrated, which enables direct access to 2-alkylaminoquinolines in efficient manner.

Transfer hydrogenative para-selective aminoalkylation of aniline derivatives with N-heteroarenes via ruthenium/acid dual catalysis

By ruthenium/acid dual catalysis, a novel transfer hydrogenative para-selective aminoalkylation of aniline derivatives with N-heteroarenes has been demonstrated.

Preparation of pyridyltriazole ruthenium complexes as effective catalysts for the selective alkylation and one-pot C–H hydroxylation of 2-oxindole with alcohols and mechanism exploration

Pyridyltriazole-ligated ruthenium(ii) complexes have been designed and synthesized, which were characterized by X-ray crystallography.

Hydrogen-Transfer-Mediated α-Functionalization of 1,8-Naphthyridines by a Strategy Overcoming the Over-Hydrogenation Barrier

A general catalytic hydrogen transfer-mediated α-functionalization of 1,8-naphthyridines is reported for the first time that benefits from a hydrogen transfer-mediated activation mode for non-activated pyridyl cores. The pyridyl α-site selectively couples with the C8-site of various tetrahydroquinolines (THQs) to afford novel α-functionalized tetrahydro 1,8-naphthyridines, a class of synthetically useful building blocks and potential candidates for the discovery of therapeutic and bio-active products. The utilization of THQs as inactive hydrogen donors (HDs) appears to be a key strategy to overcome the over-hydrogenation barrier and address the chemoselectivity issue. The developed chemistry features operational simplicity, readily available catalyst and good functional group tolerance, and offers a significant basis for further development of new protocols to directly transform or functionalize inert N-heterocycles.

Selective synthesis of nitrogen bi-heteroarenes by a hydrogen transfer-mediated direct α,β-coupling reaction

By an external hydrogen transfer-mediated activation mode, we herein demonstrate a new palladium-catalyzed direct α,β-coupling of different types of N-heteroarenes. Such a selective coupling reaction proceeds with the advantages of operational simplicity, high atom-economical efficiency, and use of safe and abundant i-propanol as the activating agent, offering a practical way to access nitrogen bi-heteroarenes. Preliminary exploration has revealed that the obtained bis-1,10-phenanthroline 2qq' as a ligand is capable of improving a copper catalyst for C-C bond formation. The work reported in this paper has built an important basis for the creation of extended π-conjugated systems that are of high significance in biological, medicinal, materials and synthetic organic chemistry as well as catalysis.

Silver-catalyzed [3 + 2] domino reaction: an efficient strategy to synthesize imidazole-5-carbaldehydes

An unprecedented regioselective silver-catalyzed [3 + 2] domino reaction of amidines and ynals for the formation of C–N bonds has been developed.

Ruthenium catalyzed β-C(sp3)–H functionalization on the ‘privileged’ piperazine nucleus

Ruthenium catalysed β-C(sp³)–H functionalization of piperazines has been revealed.

A novel iridium/acid co-catalyzed transfer hydrogenative C(sp3)–H bond alkylation to access functionalized N-heteroaromatics

A transfer hydrogenative coupling strategy, enabling direct alkylation of C(sp³)–H bonds and atom-economic access to functionalized N-heteroaromatics, has been demonstrated.

An annulative transfer hydrogenation strategy enables straightforward access to tetrahydro fused-pyrazine derivatives

A ruthenium-catalyzed annulative transfer hydrogenation strategy, enabling straightforward access to tetrahydro fused-pyrazine derivatives from N-heteroaryl diamines and vicinal diols, is demonstrated.