Synthesis of Polysubstituted Quinolines from α-2-Aminoaryl Alcohols Via Nickel-Catalyzed Dehydrogenative Coupling

Molecular oxygen-promoted sustainable synthesis of functionalized quinolines using catalytic glucose-derived ionic liquids and copper

An expedient one-pot sustainable synthesis of quinoline analogues was developed via protection-free chemo-selective oxidation of 2-aminobenzyl alcohols to form aldehydes, followed by annulation with various 1,3-dicarbonyl compounds or nitriles under mild reaction conditions in an acetonitrile-water medium using a copper catalyst and new hydrogen bond-rich glucose-based ionic liquids (GSILs). Overall, 40 functionalized quinolines were synthesized with up to 93% yields following significant green chemistry parameters. The developed GSILs were recyclable with not much decrease in the yields of the products and the reaction rate.

Nitroso-azomethine(ene) reaction enabled annulations of nitrosoarenes, azomethines and alkenes

An unprecedented example of a nitroso-azomethine(ene) reaction is reported. Nitroso-azomethine(ene) reaction-mediated unprecedented annulation of nitrosoarenes, azomethines, and alkenes to furnish arylquinolines via arene functionalization of nitrosoarene has been developed. DFT studies provided mechanistic insights into the newly developed nitroso-azomethine(ene) reaction.

Recent advances in cross-coupling of alcohols via borrowing hydrogen catalysis

Application of the borrowing hydrogen strategy facilitates utilization of abundantly available alcohols for linear or branched long-chain alcohols. Selective synthesis of such alcohols is highly challenging and involves the utilization of transition metal catalysts towards the desired cross-coupled product. Herein, we have highlighted recent advances (from 2015 to 2023) towards the synthesis of higher alcohols. Major focus has been given to the development of ligands, including transition metal catalysts. Judicious catalyst design plays a key role in the alkylation process and is summarised in this review.

Multicomponent synthesis via acceptorless alcohol dehydrogenation: an easy access to tri-substituted pyridines

Herein, we report palladium supported on a hydroxyapatite catalyst for synthesizing tri-substituted pyridines using ammonium acetate as the nitrogen source via acceptorless alcohol dehydrogenation strategy. The strategy offers a broad substrate scope using inexpensive and readily available alcohols as the starting material. The catalyst was prepared using a simple method and analyzed by several techniques, including FE-SEM, EDS, HR-TEM, BET, XRD, FT-IR, UV-visible spectroscopy, and XPS, demonstrating the anchoring of Pd nanoparticles on hydroxyapatite in the zero oxidation state. Moreover, several controlled experiments were carried out to understand the reaction pathway and a suitable mechanism has been proposed.

Nickel Pincer Complexes Catalyzed Sustainable Synthesis of 3,4-Dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides via Acceptorless Dehydrogenative Coupling of Primary Alcohols

We report the atom-economic and sustainable synthesis of biologically important 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide (DHBD) derivatives from readily available aromatic primary alcohols and 2-aminobenzenesulfonamide catalyzed by nickel(II)-N∧N∧S pincer-type complexes. The synthesized nickel complexes have been well-studied by elemental and spectroscopic (FT-IR, NMR, and HRMS) analyses. The solid-state molecular structure of complex 2 has been authenticated by a single-crystal X-ray diffraction study. Furthermore, a series of 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide derivatives have been synthesized (24 examples) utilizing a 3 mol % Ni(II) catalyst through acceptorless dehydrogenative coupling of benzyl alcohols with benzenesulfonamide. Gratifyingly, the catalytic protocol is highly selective with the yield up to 93% and produces eco-friendly water/hydrogen gas as byproducts. The control experiments and plausible mechanistic investigations indicate that the coupling of the in situ generated aldehyde with benzenesulfonamide leads to the desired product. In addition, a large-scale synthesis of one of the thiadiazine derivatives unveils the synthetic usefulness of the current methodology.

Nickel-Catalyzed Chemodivergent Coupling of Alcohols: Efficient Routes to Access α,α-Disubstituted Ketones and α-Substituted Chalcones

Chemodivergent (de)hydrogenative coupling of primary and secondary alcohols is achieved utilizing an inexpensive nickel catalyst, (6-OH-bpy)NiCl2 . This protocol demonstrates the synthesis of branched carbonyl compounds, α,α-disubstituted ketones, and α-substituted chalcones via borrowing hydrogen strategy and acceptorless dehydrogenative coupling, respectively. A wide range of aryl-based secondary alcohols are coupled with various primary alcohols in this tandem dehydrogenation/hydrogenation reaction. The nickel catalyst, along with KOt Bu or K2 CO3 , governed the selectivity for the formation of branched saturated ketones or chalcones. A preliminary mechanistic investigation confirms the reversible dehydrogenation of alcohols to carbonyls via metal-ligand cooperation (MLC) and the involvement of radical intermediates during the reaction.

Dehydrogenative Synthesis of Quinolines and Quinazolines via Ligand-Free Cobalt-Catalyzed Cyclization of 2-Aminoaryl Alcohols with Ketones or Nitriles

We present a convenient and efficient protocol to synthesize quinolines and quinazolines in one pot under mild conditions. A variety of substituted quinolines were synthesized in good to excellent yields (up to 97% yield) from the dehydrogenative cyclizations of 2-aminoaryl alcohols and ketones catalyzed by readily available Co(OAc)₂·4H₂O. This cobalt catalytic system also showed high activity in the reactions of 2-aminobenzyl alcohols with nitriles, affording various quinazoline derivatives (up to 95% yield). The present protocol offers an environmentally benign approach for the synthesis of N-heterocycles by employing an earth-abundant cobalt salt under ligand-free conditions.

Transition-metal-free approach to quinolines via direct oxidative cyclocondensation reaction of N,N-dimethyl enaminones with o-aminobenzyl alcohols

A transition-metal-free method for the construction of 3-substituted or 3,4-disubstituted quinolines from readily available N,N-dimethyl enaminones and o-aminobenzyl alcohols is reported. The direct oxidative cyclocondensation reaction tolerates broad functional groups, allowing the efficient synthesis of various quinolines in moderate to excellent yields. The reaction involves a C (sp3)-O bond cleavage and a C=N bind and a C=C bond formation during the oxidative cyclization process, and the mechanism was proposed.

tert-Butyl Hydroperoxide Promoted the Reaction of Quinazoline-3-oxides with Primary Amines Affording Quinazolin-4(3H)-ones

An efficient and facile approach for the synthesis of quinazolin-4(3H)-ones via the reaction of quinazoline-3-oxides with primary amines is described. This approach is demonstrated to be applicable for a broad range of substrates and proceeds efficiently under metal-free and mild reaction conditions employing easily available tert-butyl hydroperoxide as the oxidant. Remarkably, 3-(2-(1H-indol-3-yl) ethyl)quinazolin-4(3H)-one 3w, which was conveniently obtained by this process in 70% yield, was an excellent precursor for the synthesis of bioactive evodiamine and rutaempine.

Well-Defined Phosphine-Free Manganese(II)-Complex-Catalyzed Synthesis of Quinolines, Pyrroles, and Pyridines

Herein, we report a simple, phosphine-free, and inexpensive catalytic system based on a manganese(II) complex for synthesizing different important N-heterocycles such as quinolines, pyrroles, and pyridines from amino alcohols and ketones. Several control experiments, kinetic studies, and DFT calculations were carried out to support the plausible reaction mechanism. We also detected two potential intermediates in the catalytic cycle using ESI-MS analysis. Based on these studies, a metal-ligand cooperative mechanism was proposed.

Ketones as strategic building blocks for the synthesis of natural product-inspired compounds

Natural product-inspired compound collections serve as excellent sources for the identification of new bioactive compounds to treat disease. However, such compounds must necessarily be more structurally-enriched than traditional screening compounds, therefore inventive synthetic strategies and reliable methods are needed to prepare them. Amongst the various possible starting materials that could be considered for the synthesis of natural product-inspired compounds, ketones can be especially valuable due to the vast variety of complexity-building synthetic transformations that they can take part in, their high prevalence as commercial building blocks, and relative ease of synthesis. With a view towards developing a unified synthetic strategy for the preparation of next generation bioactive compound collections, this review considers whether ketones could serve as general precursors in this regard, and summarises the opulence of synthetic transformations available for the annulation of natural product ring-systems to ketone starting materials.

Microwave-assisted, [Bmim]HSO4-catalyzed the Friedländer quinoline synthesis of quinoline under solvent-free conditions

Abstact:

An efficient and green method for the Friedländer quinoline synthesis is described. The synthesis was performed under microwave irradiation using ionic liquid [Bmim]HSO4 as a catalyst. A diverse range of quinoline derivatives was obtained in high yields from 2‐aminoaryl aldehydes and ketones under solvent-free conditions.

Iron-catalysed quinoline synthesis via acceptorless dehydrogenative coupling

An iron-catalysed atom-economical and straightforward methodology for the synthesis of quinolines from α-2-aminoaryl alcohols and secondary alcohols is presented.

A Domino Heck Coupling–Cyclization–Dehydrogenative Strategy for the One-Pot Synthesis of Quinolines

An efficient, one-pot, domino synthesis of quinolines via the coupling of iodoanilines with allylic alcohols facilitated by palladium catalysis is described. The overall synthetic process involves an intermolecular Heck coupling between 2-iodoanilines and allylic alcohols, intramolecular condensation of in situ generated ketones with an internal amine functional group, and a dehydrogenation sequence. Notably, this protocol occurs in water as a green solvent. Significantly, the method exhibits broad substrate scope and is applied for the synthesis of deuterated quinolines through a deuterium-exchange process.

Ruthenium pincer complex catalyzed efficient synthesis of quinoline, 2-styrylquinoline and quinazoline derivatives via acceptorless dehydrogenative coupling reactions

The synthesis of N-heterocycles has been considered an emerging area of chemical research due to their extensive utilization in pharmaceuticals, materials science, and natural product synthesis.

Designed pincer ligand supported Co(II)-based catalysts for dehydrogenative activation of alcohols: Studies on N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines

Base-metal catalysts Co1, Co2 and Co3 were synthesized from designed pincer ligands L1, L2 and L3 having NNN donor atoms respectively. Co1, Co2 and Co3 were characterized by IR, UV-Vis. and ESI-MS spectroscopic studies. Single crystal X-ray diffraction studies were investigated to authenticate the molecular structures of Co1 and Co3. Catalysts Co1, Co2 and Co3 were utilized to study the dehydrogenative activation of alcohols for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines. Under optimized reaction conditions, a broad range of substrates including alcohols, anilines and ketones were exploited. A series of control experiments for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines were examined to understand the reaction pathway. ESI-MS spectral studies were investigated to characterize cobalt-alkoxide and cobalt-hydride intermediates. Reduction of styrene by evolved hydrogen gas during the reaction was investigated to authenticate the dehydrogenative nature of the catalysts. Probable reaction pathways were proposed for N-alkylation of amines, α-alkylation of ketones and synthesis of quinolines on the basis of control experiments and detection of reaction intermediates.

Recent advances in nickel-catalyzed C-C and C-N bond formation via HA and ADC reactions

In recent times, earth-abundant 3d-transition-metal catalysts have attracted much attention in contemporary catalysis. They have been widely employed as suitable alternatives to their counterparts noble metals. In particular, nickel catalysts provide distinctive redox properties; thus, their efficiency in sustainable organic transformations is manifold. In this review article, recent advances in nickel-catalyzed hydrogen auto-transfer (HA) and acceptorless dehydrogenative coupling (ADC) reactions for the construction of C-C and C-N bonds have been discussed.

[(PPh3)2NiCl2]-Catalyzed C-N Bond Formation Reaction via Borrowing Hydrogen Strategy: Access to Diverse Secondary Amines and Quinolines

Commercially available [(PPh₃)₂NiCl₂] was found to be an efficient catalyst for the mono-N-alkylation of (hetero)aromatic amines, employing alcohols to deliver diverse secondary amines, including the drug intermediates chloropyramine (5b) and mepyramine (5c), in excellent yields (up to 97%) via the borrowing hydrogen strategy. This method shows a superior activity (TON up to 10000) with a broad substrate scope at a low catalyst loading of 1 mol % and a short reaction time. Further, this strategy is also successful in accessing various quinoline derivatives following the acceptorless dehydrogenation pathway.

Synthetic and medicinal perspective of quinolines as antiviral agents

In current scenario, various heterocycles have come up exhibiting crucial role in various medicinal agents which are valuable for mankind. Out of diverse range of heterocycle, quinoline scaffold have been proved to play an important role in broad range of biological activities. Several drug molecules bearing a quinoline molecule with useful anticancer, antibacterial activities etc have been marketed such as chloroquine, saquinavir etc. Owing to their broad spectrum biological role, various synthetic strategies such as Skraup reaction, Combes reaction etc. has been developed by the researchers all over the world. But still the synthetic methods are associated with various limitations as formation of side products, use of expensive metal catalysts. Thus, several efforts to develop an efficient and cost effective synthetic protocol are still carried out till date. Moreover, quinoline scaffold displays remarkable antiviral activity. Therefore, in this review we have made an attempt to describe recent synthetic protocols developed by various research groups along with giving a complete explanation about the role of quinoline derivatives as antiviral agent. Quinoline derivatives were found potent against various strains of viruses like zika virus, enterovirus, herpes virus, human immunodeficiency virus, ebola virus, hepatitis C virus, SARS virus and MERS virus etc.

One-pot intramolecular cyclization of 5-hydroxymethylcytosine for sequencing DNA hydroxymethylation at single-base resolution

Here we establish a one-pot reaction to directly convert the DNA base 5-hydroxymethylcytosine (5hmC) to an intramolecular cyclization nucleobase, which loses both protons of the exocyclic N4-amino group and thus is recognized as thymine (T) by DNA polymerase. Based on this 5hmC-specific reaction, a prospective bisulfite-free strategy for 5hmC sequencing is proposed. This is also the first example to show modified DNA labeling in non-water solvent-dominant media for DNA sequencing.

Mild and efficient copper-catalyzed oxidative cyclization of oximes with 2-aminobenzyl alcohols at room temperature: synthesis of polysubstituted quinolines

A simple and efficient ligand-free Cu-catalyzed protocol for the synthesis of polysubstituted quinolines via oxidative cyclization of oxime acetates with 2-aminobenzyl alcohols at room temperature has been developed. The presented approach provides a new synthetic pathway leading to polysubstituted quinolines with good functional group tolerance under mild conditions. Moreover, this transformation can be applied for the preparation of quinolines on a gram scale. Oxime acetates serve as the internal oxidants in the reactions, thus making this method very attractive.

Direct synthesis of ring-fused quinolines and pyridines catalyzed by NNHY-ligated manganese complexes (Y = NR2 or SR)

A robust NNS-manganese(i) complex has been developed that can efficiently catalyze the dehydrogenation coupling of γ-amino alcohols with ketones or secondary alcohols in a one-step synthesis of quinolines and pyridines with high yields.

Alcohols as Fluoroalkyl Synthons: Ni-catalyzed Dehydrogenative Approach to Access Polyfluoroalkyl Bis-indoles

An acceptorless dehydrogenative strategy for the synthesis of polyfluoroalkylated bis-indoles is described by employing an earth-abundant nickel-based catalytic system under air. The notable feature of the present transformation is the use of bench stable and easily affordable polyfluorinated alcohols without any pre-functionalization for the introduction of precious polyfluoroalkyl groups. The developed straightforward protocol accomplished biologically relevant fluoroalkyl bis-indoles in a sustainable fashion. Extensive DFT study predicts the unique role of indole molecules which stabilizes the transition states during the dehydrogenation process of polyfluorinated alcohols, presumably through non-covalent π⋅⋅⋅π and H-bonding interactions.

Lewis acid catalyzed reactivity switch: pseudo three-component annulation of nitrosoarenes and (epoxy)styrenes

A Lewis acid catalyzed annulation reaction via arene functionalization of nitrosoarenes and C-C cleavage of (epoxy)styrene to provide arylquinolines is reported. The Lewis acid catalyst altered the annulation pattern providing arylquinolines instead of oxazolidines. The reaction with styrene resulted in a mixture of 2,4-diarylquinoline and 4-arylquinoline, while only 3-arylquinoline was formed from the reaction of epoxystyrene.

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.

Synthesis of Tetrahydroquinolines via Borrowing Hydrogen Methodology Using a Manganese PN3 Pincer Catalyst

A straightforward and selective synthesis of 1,2,3,4-tetrahydroquinolines starting from 2-aminobenzyl alcohols and simple secondary alcohols is reported. This one-pot cascade reaction is based on the borrowing hydrogen methodology promoted by a manganese(I) PN³ pincer complex. The reaction selectively leads to 1,2,3,4-tetrahydroquinolines thanks to a targeted choice of base. This strategy provides an atom-efficient pathway with water as the only byproduct. In addition, no further reducing agents are required.

Recent advances in the synthesis of biologically and pharmaceutically active quinoline and its analogues: a review

Recently, quinoline has become an essential heterocyclic compound due to its versatile applications in the fields of industrial and synthetic organic chemistry. It is a vital scaffold for leads in drug discovery and plays a major role in the field of medicinal chemistry. Nowadays there are plenty of articles reporting syntheses of the main scaffold and its functionalization for biological and pharmaceutical activities. So far, a wide range of synthesis protocols have been reported in the literature for the construction of this scaffold. For example, Gould-Jacob, Friedländer, Pfitzinger, Skraup, Doebner-von Miller and Conrad-Limpach are well-known classical synthesis protocols used up to now for the construction of the principal quinoline scaffold. Transition metal catalysed reactions, metal-free ionic liquid mediated reactions, ultrasound irradiation reactions and green reaction protocols are also useful for the construction and functionalization of this compound. The main part of this review focuses on and highlights the above-mentioned synthesis procedures and findings to tackle the drawbacks of the syntheses and side effects on the environment. Furthermore, various selected quinolines and derivatives with potential biological and pharmaceutical activities will be presented.