Reductive electrophilic C-H alkylation of quinolines by a reusable iridium nanocatalyst

Iridium-catalyzed reductive β-alkylation of (iso)quinoline derivatives by an in situ enone-trapping strategy

By employing [IrCp*Cl2]2/Mg(OMe)2/(CH2O)n as an applicable catalyst system, we report a reductive β-alkylation of (iso)quinolinium salts with cost-effective and readily available β-chloro ketones, proceeding with good chemoselectivity, mild reaction conditions, and without the need for introduction of a substituent at position-3 of the quinolyl skeleton. Mechanistic investigations suggest that the reaction proceeds via a sequence of hydride transfer-initiated dearomatization of (iso)quinolinium salts, in situ enamine-trapping of enone and a second round of hydride transfer to the coupling adducts. The present work offers an important complement to the synthesis of functionalized (iso)tetrahydroquinolines.

Divergent Synthesis of Cyclopropanated Tetrahydroquinolines by Tandem Functionalization of Quinoline Derivatives

Here, reported is a new method for divergent synthesis of functionalized tetrahydroquinolines (THQs), featuring a biomedically interesting azabicyclo[4.1.0]heptane core, proceeding with mild conditions, good substrate and functionality tolerance, and operational simplicity. Mechanistic studies suggest that the products are formed via carbonucleophilic 1,4-addition-induced dearomatization of quinolinium salts and intramolecular cyclopropanation with α-halo ketones followed by α-nucleophilic addition with different nucleophiles. The present work lays a foundation to access new N-heterocycles via the dearomative tandem functionalization of azaarenes.

Reductive Functionalization of Pyridine-Fused N-Heteroarenes

ConspectusThe selective functionalization/transformation of ubiquitous pyridine-fused N-heteroarenes is a practical method to synthesize structurally novel N-heterocycles, which is important for the development of medicines, bioactive agents, agrochemicals, materials, ligands, sensors, pigments, dyes, etc. However, owing to thermodynamic stability, kinetic inertness, and lone electron pair-induced catalyst deactivation of the pyridine-fused N-heteroarenes, limited strategies (e.g., C-H activation/functionalization, electrophilic substitution, and the Minisci reaction) are available to realize the synthetic purpose and maintain the aromaticity of the final products. Moreover, the relevant transformations have limitations such as needing harsh reaction conditions, requiring the preinstallation of specific coupling agents containing transformable functionalities or directing groups, using less environmentally benign oxidants and/or acidic activators, and poor selectivity. Herein, considering that imines, enamines, radicals, and cyclic amines are generated during the reduction of pyridine-fused N-heteroarenes, the precise transformation of these reductive intermediates offers a fundamental basis for developing novel tandem reactions. Our group revealed that a slow reduction rate, synergistic catalysis, and controlled electroreduction are effective strategies for fulfilling the selective reductive functionalization of pyridine-fused N-heteroarenes. Thus, we established a series of new synthetic methods that provide diverse construction modalities for functionalized N-heterocycles. The striking features of these synthetic methods include high efficiency, atom economy, and the use of readily accessible N-heteroarenes as transformable feedstocks in the absence of flammable and pressurized H2 gas, alongside a promising potential of the obtained N-heterocyclic products. The present study would be appealing to the fields of synthetic organic chemistry, catalysis, biomedical chemistry, and functional materials. This Account describes the application of reductive dearomatization as substrate-activating and tandem reaction-initiating modes and summarizes the reductive functionalization of pyridine-fused N-heteroarenes via selective alkylation, arylation, and annulation at nitrogen, α, β, and other remote carbon sites achieved over the past 8 years. Details regarding the development of new reactions and their plausible mechanisms and perspectives are discussed. We hope our contributions to this field will aid in the further development of novel strategies for the functionalization/transformation of pyridine-fused N-heteroarenes and tackle the intractable challenges in this area.

Solvent-controlled halohydroxylation or C3-C2 coupling of pyridinium salts through an interrupted dearomative reduction

Herein, we report an efficient and easily operable method to halohydroxylate pyridiniums through an interrupted dearomative reduction strategy. In this process, we make the most of the halide anion from the pyridinium salts by performing the reaction in DMSO without the need of external HX added. Notably, by changing the solvents from DMSO into Et2O, the bimolecular C3-C2 coupling occurs successfully.

Nickel-catalyzed mild synthesis of functional γ-amino butyric acid esters via direct α-C(sp3)-H allylation of N-alkyl anilines with allyl sulfones

Herein, by employing a readily available Ni(OAc)2·4H2O/TBHP catalyst system, we present a new method for mild synthesis of α-methylene-γ-amino butyric acid esters via direct α-C(sp3)-H allylation of N-alkyl anilines with allyl sulfones under oxidative nickel catalysis. The synthetic protocol proceeds with good substrate and functional group compatibility, operational simplicity, the use of base metal catalysts and easily accessible feedstocks, and no need for pre-functionalization of the α-site of N-alkyl anilines. In addition, the obtained products are applicable for further elaboration of functional molecules.

Highly Selective Cyclization and Isomerization of Propargylamines to Access Functionalized Quinolines and 1-Azadienes

Developing new organic reactions with excellent atom economy and high selectivity is significant and urgent. Herein, by ingeniously regulating the reaction conditions, highly selective transformations of propargylamines have been successfully implemented. The palladium-catalyzed cyclization of propargylamines generates a series of functionalized quinoline heterocycles, while the base-promoted isomerization of propargylamines affords diverse 1-azadienes. Both reactions have good functional group tolerance, mild conditions, excellent atom economy and high yields of up to 93%. More importantly, these quinoline heterocycles and 1-azadienes could be flexibly transformed into valuable compounds, illustrating the validity and practicability of the propargylamine-based highly selective reactions.

Reductive Coupling of Nitroarenes and HCHO for General Synthesis of Functional Ethane-1,2-diamines by a Cobalt Single-Atom Catalyst

Despite the extensive applications, selective and diverse access to N,N'-diarylethane-1,2-diamines remains, to date, a challenge. Here, by developing a bifunctional cobalt single-atom catalyst (CoSA-N/NC), we present a general method for direct synthesis of such compounds via selective reductive coupling of cheap and abundant nitroarenes and formaldehyde, featuring good substrate and functionality compatibility, an easily accessible base metal catalyst with excellent reusability, and high step and atom efficiency. Mechanistic studies reveal that the N-anchored cobalt single atoms (CoN4) serve as the catalytically active sites for the reduction processes, the N-doped carbon support enriches the HCHO to timely trap the in situ formed hydroxyamines and affords the requisite nitrones under weak alkaline conditions, and the subsequent inverse electron demand 1,3-dipolar cycloaddition of the nitrones and imines followed by hydrodeoxygenation of the cycloadducts furnishes the products. In this work, the concept of catalyst-controlled nitroarene reduction to in situ create specific building blocks is anticipated to develop more useful chemical transformations.

Selective access to fused tetrahydroquinolines via a copper-catalysed oxidative three-component annulation reaction

Via a copper-catalyzed three-component annulation reaction, we herein report a new method for the direct and syn-selective construction of cyclic ether-fused tetrahydroquinolines from readily available secondary anilines, saturated five or six-membered cyclic ethers, and paraformaldehyde. The synthesis features operational simplicity, excellent step and atom efficiency, good functionality and substrate compatibility. In comparison with the reported synthetic protocols capable of synthesizing N-alkyl fused tetrahydroquinolines, this newly developed chemistry allows access to both N-alkyl and N-aryl products. The current work complements the preparation of fused tetrahydroquinolines.

Electrochemically enhanced deoxygenative cross-coupling of aryl ketones with heteroarenes through in situ generated benzyl carbocations

Triflic acids/silanes as cooperative reductants enable the convenient transformation of CO bonds through a multistep reaction pathway in one pot. Electrolysis of the acidic reaction mixture significantly improved carbonyl reduction and thus facilitated the generation of benzyl carbocations, which show high reactivity towards electron-rich heteroarenes for C-C bond formation.

Diastereoselective construction of bridged piperidines through an interrupted dearomative reduction

An interrupted dearomative reduction strategy was developed to transform planar chalcone-based pyridinium salts into structurally intriguing bridged piperidines in a completely regio- and diastereoselective manner. This reaction proceeded successfully by using cheap and easily accessible NaBH₄ as the reductant under mild conditions without exclusion of oxygen or use of special equipment.

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.

Construction of Fluorinated Amino Acid Derivatives via Cobalt-Catalyzed Oxidative Difunctionalization of Cyclic Ethers

Via difunctionalization of the α- and β-sites of cyclic ethers, we herein demonstrate a new synthetic method for the efficient construction of novel fluorinated γ-amino acid esters by employing a CoBr₂/m-CPBA catalyst system. Several cyclic ethers were transformed in combination with a vast range of amines and ethyl trifluoropyruvate into the desired products under mild conditions, making this method a practical platform to enrich the library of fluorinated amino acid derivatives from cost-effective and readily available feedstocks.

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.

Copper–cobalt coordination polymers and catalytic applications on borrowing hydrogen reactions

A porous copper–cobalt polymer was synthesized and achieved applications for the N-alkylation of sulfonamides with alcohols, and carboxamides with alcohols.

Copper dual-atom catalyst mediated C3–H amination of indoles at room temperature

An efficient zeolite supported copper dual-atom catalyst for C–H amination of indoles has been developed for the green synthesis of 3-diarylaminoindoles.

Synthesis of functionalized benzimidazoles via oxidative tandem quartic C-H aminations and cleavage of C-N and C-C bonds

Via aerobic copper-catalyzed tandem quartic C-H aminations, we herein present an unprecedented approach for the synthesis of functionalized benzimidazoles from aniline derivatives and 2-substituted cyclic amines. The cyclic amines act as the CN building blocks and are involved in the annulation reaction by cleavage of inert α-C-N and β-C-C bonds. The synthetic protocol features high selectivity, no need for specific aminating agents, mild conditions, and the use of a naturally abundant [Cu]/O₂ catalyst system.