Functionalization of remote C(sp3)-H bonds enabled by copper-catalyzed coupling of O-acyloximes with terminal alkynes

Photoinduced copper catalyzed nitrogen-to-alkyl radical relay Sonogashira-type coupling of o-alkylbenzamides with alkynes

This report describes a copper-catalyzed, photoinduced N-to-alkyl radical relay Sonogashira-type reactions at benzylic sites in o-alkylbenzamides with alkynes. The process employs an N-to-alkyl radical mechanism, initiated through the copper-catalyzed reductive generation of nitrogen radicals. Radical translocation is facilitated by a 1,5-hydrogen atom transfer (1,5-HAT), leading to the formation of translocated carbon radicals. These radicals are then subjected to copper-catalyzed alkynylation. The methodology exhibits broad sub-strate scope and applicability to the synthesis of complex natural products.

Copper-Catalyzed [3 + 2] Annulation of O-Acyl Oximes with 4-Sulfonamidophenols for the Synthesis of 5-Sulfonamidoindoles and 2-Amido-5-sulfonamidobenzofuran-3(2H)-ones

A copper-catalyzed [3 + 2] annulation of O-acyl oximes with 4-sulfonamidophenols is developed. The advantage of this method lies in the concurrent double activation of two substrates to form nucleophilic enamines and electrophilic quinone monoimines. The substituent on the α-carbon of O-acyl oxime determines two different reaction pathways, thereby leading to the selective generation of 5-sulfonamidoindoles and 2-amido-5-sulfonamidobenzofuran-3(2H)-ones.

Visible-light-driven EDA complex-promoted cascade cyclization to construct 4-cyanoalkyl isoquinoline-1,3-diones

Visible-light-induced EDA complex-promoted ring-opening of cycloketone oxime esters to synthesise various cyanoalkylated products with N-methacryloyl benzamides was developed. Various radical receptors were compatible with the current reaction system to furnish diverse heterocyclic compounds. Mechanistic analysis shows that the formation of an EDA complex was crucial to the photocatalytic strategy. Importantly, 4-cyanoalkyl isoquinoline-1,3-diones were obtained in high yields by using a catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DABCO) through prolonging the reaction time, which provided a practical approach to give a variety of isoquinoline-1,3-dione derivatives.

Copper-Mediated Radical-Induced Ring-Opening Relay Cascade Carboannulation Reaction of [60]Fullerene with Cyclobutanone Oxime Esters: Access to [60]Fullerene-Fused Cyclopentanes

An unexpected copper-mediated radical-induced ring-opening relay cascade carboannulation reaction of [60]fullerene with cyclobutanone oxime esters is presented for the preparation of various Cl-/Br-incorporated [60]fullerene-fused cyclopentanes. The unique relay cascade transformation uses inexpensive copper salts as promoters and halogen sources and features simple redox-neutral conditions and a broad substrate scope, providing a practical access to a class of novel five-membered carbocycle-fused fullerenes.

Formal γ-C(sp3)-H Activation of Ketones via Microwave-Promoted and Iminyl-Radical-Mediated 1,5-Hydrogen Atom Transfer

Microwave irradiation of O-phenyloximes triggers N-O homolysis and 1,5-hydrogen atom transfer (HAT), resulting in formal γ-C-H functionalization of ketones after trapping of the radical intermediate and in situ imine hydrolysis. The Lewis acid InCl3·H2O facilitated HAT, enabling functionalization of benzylic and nonbenzylic secondary carbon atoms. Functionalization of primary carbons was feasible but afforded low yields, requiring ClCH2CO2H instead of InCl3·H2O as an additive. C-O and C-C bond formation could both be accomplished by this method.

Transition-metal-catalyzed synthesis of quinazolines: A review

Quinazolines are a class of nitrogen-containing heterocyclic compounds with broad-spectrum of pharmacological activities. Transition-metal-catalyzed reactions have emerged as reliable and indispensable tools for the synthesis of pharmaceuticals. These reactions provide new entries into pharmaceutical ingredients of continuously increasing complexity, and catalysis with these metals has streamlined the synthesis of several marketed drugs. The last few decades have witnessed a tremendous outburst of transition-metal-catalyzed reactions for the construction of quinazoline scaffolds. In this review, the progress achieved in the synthesis of quinazolines under transition metal-catalyzed conditions are summarized and reports from 2010 to date are covered. This is presented along with the mechanistic insights of each representative methodology. The advantages, limitations, and future perspectives of synthesis of quinazolines through such reactions are also discussed.

Copper-Catalyzed Annulation of O-Acyl Oximes with Cyclic 1,3-Diones for the Synthesis of 7,8-Dihydroindolizin-5(6H)-ones and Cyclohexanone-Fused Furans

A copper-catalyzed annulation of O-acyl oximes with cyclic 1,3-diones has been developed for the concise synthesis of 7,8-dihydroindolizin-5(6H)-ones and cyclohexanone-fused furans through the substituent-controlled selective radical coupling process. 2-Alkyl cyclic 1,3-diones undergo C-C radical coupling, while 2-unsubstituted cyclic 1,3-diones undergo C-O radical coupling.

Electroreductive Ring-Opening Carboxylation of Cycloketone Oxime Esters with Carbon Dioxide

Electroreductive ring-opening carboxylation of cycloketone oxime esters with atmospheric carbon dioxide is reported. This reaction proceeded under simple constant current conditions in an undivided cell using glassy carbon as the cathode and magnesium as the sacrificial anode, providing substituted γ- and δ-cyanocarboxylic acids in moderate to good yields. Electrochemically generated cyanoalkyl radicals and cyanoalkyl anion are proposed as the key intermediates.

PdII-Catalyzed γ-C(sp3)-H (Hetero)Arylation of Ketones Enabled by Transient Directing Groups

Pd(II)-catalyzed γ-C(sp3)-H (hetero)arylation of aliphatic ketones is developed using α-amino acid as transient directing groups (TDG). A variety of aliphatic ketones were (hetero)arylated at the γ-position via a 5,6-membered fused cyclopalladation intermediate to afford the remotely arylated products in up to 88% yield. The crucial ligand effect of 2-pyridone is further enhanced by reducing the loading of acid additives. Consequentially, the improved reactivity of this catalytic system has also made possible the cyclic γ-methylene C(sp3)-H arylation of ketones. Mechanistic investigtigation and comparison to the γ-C-H arylation of aldehydes revealed a structural insight for designing site selective TDG.

Divergent Synthesis of 2-Cyanoaryl Carbamate and 2-Cyanoaryl Urea Derivatives via Hypervalent Iodine-Induced C-C Bond Cleavage

Divergent synthetic methods for transforming isatins to 2-cyanoaryl carbamate and 2-cyanoaryl urea derivatives were developed using ammonium carbamate as the nitrogen source and iodobenzene diacetate as the oxidant. This reaction features mild conditions, broad substrate scope, and moreover, the use of toxic cyano-containing compounds is avoided.

Photoredox/copper cocatalyzed domino cyclization of oxime esters with TMSCN: access to antifungal active tetrasubstituted pyrazines

A photoredox/copper cocatalyzed domino cyclization of oxime esters with TMSCN has been developed. A range of structurally novel tetrasubstituted pyrazines have been obtained. This method features high bond-forming efficiency, high step economy, broad substrate scope, and gram-scale synthesis. Moreover, preliminary bioactivity evaluation of pyrazine products shows their promising antifungal activities.

Synthesis of Stable N-H Imines with a Benzo[7,8]indolizine Core and Benzo[7,8]indolizino[1,2-c]quinolines via Copper-Catalyzed Annulation of α,β-Unsaturated O-Acyl Ketoximes with Isoquinolinium N-Ylides

A copper-catalyzed annulation of α,β-unsaturated O-acyl ketoximes with isoquinolinium N-ylides has been developed for the concise synthesis of stable N-H imines with a benzo[7,8]indolizine core. When β-(2-bromoaryl)-α,β-unsaturated O-acyl ketoximes are used as the starting materials, a cascade cyclization occurs to afford the benzo[7,8]indolizino[1,2-c]quinolines.

Copper-catalyzed radical relay in C(sp3)-H functionalization

Radical-involved transition metal (TM) catalysis has greatly enabled new reactivities in recent decades. Copper-catalyzed radical relay offers enormous potential in C(sp³)-H functionalization which combines the unique regioselectivity of hydrogen atom transfer (HAT) and the versatility of copper-catalyzed cross-coupling. More importantly, significant progress has been achieved in asymmetric C-H functionalization through judicious ligand design. This tutorial review will highlight the recent advances in this rapidly growing area, and we hope this survey will inspire future strategic developments for selective C(sp³)-H functionalization.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Polysubstituted Indole Synthesis via Palladium/Norbornene Cooperative Catalysis of Oxime Esters

Polysubstituted indoles are prevalent in pharmaceuticals, agrochemicals, and organic materials. Presented herein is the fact that polyfunctionalized indoles can be efficiently constructed from easily accessible oxime esters and aryl iodides, involving a palladium/norbornene synergistic synthesis. The reaction is enabled by a unique class of electrophiles in palladium/norbornene cooperative catalysis, which are oxime esters derived from simple ketone. The broad substrate scope and high functional group tolerance could make this method attractive for the synthesis of polysubstituted indoles.

Facile synthesis of fully substituted 1H-imidazoles from oxime esters via dual photoredox/copper catalyzed multicomponent reactions

A novel and efficient photoredox/copper cocatalyzed domino cyclization of oxime esters, aldehydes, and amines has been achieved, affording a broad range of fully substituted 1H-imidazoles in good yields.

Iminyl radical-triggered relay annulation for the construction of bridged aza-tetracycles bearing four contiguous stereogenic centers

Bridged tetracyclic nitrogen scaffolds are found in numerous biologically active molecules and medicinally relevant structures. Traditional methods usually require tedious reaction steps, and/or the use of structurally specific starting materials. We report an unprecedented, iminyl radical-triggered relay annulation from oxime-derived peresters and azadienes, which shows good substrate scope and functional group compatibility, and can deliver various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation. This transformation represents the first example of trifunctionalization of iminyl radicals through simultaneous formation of one C–N and two C–C bonds. DFT calculation studies were conducted to obtain an in-depth insight into the reaction pathways, which revealed that the reactions involved an interesting 1,6-hydrogen atom transfer process.

A novel radical relay annulation is realized for the construction of various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation.

Cyanomethylative cyclization of unactivated alkenes with nitriles for the synthesis of cyano-containing ring-fused quinazolin-4(3H)-ones

The synthesis of cyano-containing pyrrolo- and piperidino-quinazolinones was developed using alkyl nitriles through radical cascade addition/cyclization under metal-free conditions.

Divergent copper-catalyzed syntheses of 3-carboxylpyrroles and 3-cyanofurans from O-acetyl oximes and β-ketoesters/nitriles

The reaction between O-acetyl oximes and β-ketoesters/nitriles catalyzed by copper generated polysubstituted pyrroles and furans, respectively, under redox–neutral reaction conditions. Using this protocol, pyrroles or furans could be obtained simply by choosing an appropriate active methylene compound. Although both transformations occur essentially under the same reaction conditions, control experiments indicated that they follow different mechanistic pathways.

A copper-catalyzed heteroannulation of O-acetyl oximes and β-ketoesters/nitriles to afford polysubstituted pyrroles and furans was successfully achieved.

Cyclic Oxime Esters as Deconstructive Bifunctional Reagents for Cyanoalkyl Esterification of 1,6-Enynes

A concise copper catalysis strategy for the addition-cyclization of cyclic oxime esters across 1,6-enynes with high stereoselectivity to generate 1-indanones bearing an all-carbon quaternary center is reported. In this process, single-electron reduction of cyclic oxime esters enables deconstructive carbon-carbon cleavage to provide a key cyanopropyl radical poised for the addition-cyclization. This reaction is redox-neutral, exhibits good functional group compatibility, and features 100% atomic utilization. This process driven by copper catalyst makes readily available cyclic oxime esters as bifunctional reagents to demonstrate convergent synthesis.

Copper-Catalyzed Aza-Sonogashira Cross-Coupling To Form Ynimines: Development and Application to the Synthesis of Heterocycles

Nitrogen‐substituted alkynes, such as ynamines and ynamides, are versatile synthetic building blocks. Ynimines bearing additional nucleophilic and electrophilic centers relative to ynamines and ynamides are expected to have high synthetic potential. However, their chemical reactivity remains unexplored owing mainly to the lack of synthetic accessibility. We report herein a versatile copper‐catalyzed synthesis of ynimines from readily available O‐acetyl ketoximes and terminal alkynes. A wide range of O‐acetyl ketoximes derived from diaryl ketones, aryl alkyl ketones and dialkyl ketones underwent cross‐coupling with a diverse set of terminal alkynes to afford the ynimines in good to excellent yields. An unprecedented [5+1] heteroannulation reaction exploiting the reactivity of the ynimine generated in situ was subsequently developed for the synthesis of medicinally important heterocycles, including isoquinolines, azaindoles, azabenzofurans, azabenzothiophenes and carbolines.

Copper-Catalyzed Regioselective 1,4-Selenosulfonylation of 1,3-Enynes to Access Cyanoalkylsulfonylated Allenes

By employing CuOAc as the catalyst, we realize a four-component reaction of 1,3-enynes, diselenides, DABCO·(SO₂)₂, and cycloketone oxime esters, providing facile access to diverse cyanoalkylsulfonylated allenyl selenides in moderate to good yields. This reaction features high functional group tolerance and a broad substrate scope, enabling the regioselective, sequential formation of C-SO₂ and C-Se bonds under mild reaction conditions. Moreover, the utility of this methodology is further illustrated through the late-stage functionalization of drug-based molecules.

C3-Arylation of indoles with aryl ketones via C-C/C-H activations

C3-Arylation of indoles with aryl ketones is accomplished via palladium-catalyzed ligand-promoted Ar-C(O) cleavage and subsequent C-H arylation of indole. Various (hetero)aryl ketones are compatible in this reaction, affording the corresponding 3-arylindoles in moderate to good yields. Further introduction of an indole moiety into the natural products desoxyestrone and evodiamine demonstrate the synthetic utility of this protocol.

Enabling the Use of Alkyl Thianthrenium Salts in Cross-Coupling Reactions by Copper Catalysis

Alkyl groups are one of the most widely used groups in organic synthesis. Here, a a series of thianthrenium salts have been synthesized that act as reliable alkylation reagents and readily engage in copper-catalyzed Sonogashira reactions to build C(sp³ )-C(sp) bonds under mild photochemical conditions. Diverse alkyl thianthrenium salts, including methyl and disubstituted thianthrenium salts, are employed with great functional breadth, since sensitive Cl, Br, and I atoms, which are poorly tolerated in conventional approaches, are compatible. The generality of the developed alkyl reagents has also been demonstrated in copper-catalyzed Kumada reactions.

Visible Light Driven and Copper-Catalyzed C(sp3)-H Functionalization of O-Pentafluorobenzoyl Ketone Oximes

The C(sp³)-H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer as the key step, with the iminyl radicals being generated via copper-effected N-O cleavage. The reaction afforded 3,4-dihydro-2H-pyrroles under the conditions of [Cu(DPEphos)(bcp)]PF₆ and DABCO, while γ-pentafluorobenzoyloxy ketones were produced predominantly when [Cu(dpp)₂]PF₆ and InCl₃·4H₂O were used as catalysts.

C-C Bond Acylation of Oxime Ethers via NHC Catalysis

An N-heterocyclic carbene (NHC)-catalyzed strategy has been developed to address the issue of using toxic transitional metals in the field of C-C bond activation. The novel reaction mode enables an efficient docking between the cyanoalkyl from the cycloketone oxime derivative and the acyl group from the aldehyde, affording ketonitrile in moderate to good yields, which is one kind of useful building block for synthesizing nitrogen-containing pharmacophores.

Recent advances in the transition metal catalyzed synthesis of quinoxalines: a review

This review summarizes the recent developments in the synthesis of a variety of substituted quinoxalines using transition metal catalysts.

Copper-catalyzed regio- and chemoselective selenosulfonylation of 1,6-enynes from sulfur dioxide

An efficient copper-catalyzed multicomponent reaction of 1,6-enynes, diselenides, DABCO·(SO2)2, and cycloketone oxime esters was achieved, providing cyanoalkylsulfonated pyrrolidines in moderate to good yields.

Recent Advances in Radical-Involved Alkynylation of Unactivated C(sp3)–H Bonds by Hydrogen Atom Abstraction

The direct C(sp3)–H functionalization is one of the major research topics in synthetic chemistry since C(sp3)–H bonds are ubiquitous in every aspect of chemistry. Despite impressive advances in transition-metal-catalyzed C(sp3)–H activation, the radical-initiated process via hydrogen atom abstraction (HAA) of C(sp3)–H bonds represents a more appealing strategy owing to the mild reaction conditions and good regioselectivity. Given the importance of alkynes as versatile synthons in organic synthesis and key structural motifs in drug discovery, great efforts have been made toward their synthesis via the combination of HAA and alkynylation process in recent years. This review summarizes the recent progress in radical-initiated C(sp3)–H alkynylation reactions with emphasis on the alkynylating reagents and mechanistic discussion.

1 Introduction

2 Alkynylation of C(sp3)–H via Intermolecular Hydrogen Atom Abstraction

3 Alkynylation of C(sp3)–H via Intramolecular Hydrogen Atom Abstraction

4 Conclusion

Organocatalytic Three-Component 1,2-Cyanoalkylacylation of Alkenes via Radical Relay

Here, we report an unprecedented regioselective, intermolecular 1,2-cyanoalkylacylation of feedstock alkenes with readily available oxime esters and aldehydes by N-heterocyclic carbene (NHC) organocatalysis. The crux of this success is the exquisite control over the radical relay process by an NHC organocatalyst. This protocol offers a general platform for diversity-oriented synthesis of valuable ketonitriles under mild, transition-metal-free, and redox-neutral conditions and highlights its potential in the late-stage functionalization of pharmaceutical architectures and natural products.

Visible-Light Photoredox-Catalyzed Ring-Opening Carboxylation of Cyclic Oxime Esters with CO2

The carboxylation of cyclic oxime esters with carbon dioxide via visible-light photoredox catalysis is demonstrated for the first time. A variety of cyclic oxime esters undergo ring-opening C-C bond cleavage and carboxylation to give cyanoalkyl-containing carboxylic acids in moderate to good yields. Moreover, this methodology features mild reaction conditions (room temperature, 1 atm), wide substrate scope, good functional group tolerance as well as facile derivations of products. Mechanistic studies indicate that the benzylic radicals and anions might be the key intermediates.

Cross-Dehydrogenative Alkynylation: A Powerful Tool for the Synthesis of Internal Alkynes

Alkynes are among the most fundamentally important organic compounds and are widely used in synthetic chemistry, biochemistry, and materials science. Thus, the development of an efficient and sustainable method for the preparation of alkynes has been a central concern in organic synthesis. Cross-dehydrogenative coupling utilizing E-H and Z-H bonds in two different molecules can avoid the need for prefunctionalization of starting materials and has become one of the most straightforward methods for the construction of E-Z chemical bonds. This Review summarizes recent progress in the preparation of internal alkynes by cross-dehydrogenative coupling with terminal alkynes.

Progress and prospects in copper-catalyzed C-H functionalization

Copper-catalyzed C-H functionalization is becoming a significant area in organic chemistry. Copper is now widely used as a catalyst in organic synthesis as it is inexpensive and not very toxic. Functionalization of C-H bonds to construct wide varieties of organic compounds has received much attention in recent times. This review focuses on the recent advances in Cu-catalyzed C-H functionalization and covers literature from 2018-2020.