Synthesis of ω-functionalized ketones from strained cyclic alcohols by ring-opening and cross-recombination between alkyl and N-oxyl radicals

Radical ring-opening oxyimidation of cyclobutanols and cyclopropanols with the formation of ω-functionalized ketones was discovered. The oxidative C-O coupling proceeds via the interception of a primary alkyl radical generated from a cyclic alcohol with a reactive radical generated in situ, which is an electron-deficient N-oxyl radical. The developed conditions allow for the balanced generation rates of carbon- and N-oxyl radicals, which are necessary for their selective cross-recombination. Thus, typical competitive dimerization processes of carbon-centered radicals, their intermolecular cyclization, and N-oxyl radical self-decay are suppressed. The method is applicable to a wide range of cyclobutanols and results in oxyimidated ketones in yields of up to 82%.

Photochemical direct alkylation of heteroarenes with alkanes, alcohols, amides, and ethers

Direct functionalization of heteroarenes with simple alkanes utilizing anthracene as a photoredox catalyst has been established. This approach provides a sustainable alternative, avoiding costly reagents or peroxides. The method demonstrates a broad substrate scope, enabling regioselective alkylation of various heteroarenes, including azoles, pyridines, quinolines, isoquinolones, and quinoxalinones under mild conditions. A range of alkyl sources, such as alkanes, ethers, dioxane, trioxane, alcohol, and alkylamides were viable substrates. A plausible catalytic cycle was proposed based on the preliminary mechanistic evidence.

Ag-Catalyzed cross-dehydrogenative-coupling for the synthesis of 1,4-dioxan-2-yl substituted quinazoline hybrids in an aqueous medium

We herein developed an effective approach for the construction of 2- or 4-(1,4-dioxan-2-yl) substituted quinazolines under mild conditions. A silver-K2S2O8 catalyzed direct CDC reaction between quinazolines and 1,4-dioxane for the synthesis of a series of 2- or 4-(1,4-dioxan-2-yl) substituted quinazoline hybrids is reported. The reaction proceeded well in water under mild conditions and showed a broad substrate scope and good functional group compatibility.

Visible-Light-Induced PhI(OAc)2-Mediated Alkylation of Heteroarenes with Simple Alkanes and Ethers

The direct alkylation of heteroarenes with alkanes has been successfully achieved through visible-light-induced hypervalent iodine-mediated C-H functionalization of both coupling partners at ambient temperatures. This reaction proceeds via the in situ generation of nucleophilic alkyl radicals from alkanes through hydrogen atom transfer (HAT), followed by a Minisci-type reaction with heteroarenes. These mild reaction conditions have demonstrated their suitability for the alkylation of a wide range of heterocycles, including azoles, pyridines, quinolines, isoquinolines, and quinoxalinones.

α-Functionalisation of Cyclic Sulfides Enabled by Lithiation Trapping

A general and straightforward procedure for the lithiation trapping of cyclic sulfides such as tetrahydrothiophene, tetrahydrothiopyran and a thiomorpholine is described. Trapping with a wide range of electrophiles is demonstrated, leading to more than 50 diverse α-substituted saturated sulfur heterocycles. The methodology provides access to a range of α-substituted cyclic sulfides that are not easily synthesised by the currently available methods.

Photoredox-Catalyzed Radical-Radical Cross-Coupling of α-Ketoesters with Ethers: Access to Sterically Hindered α-Hydroxy Esters

We describe a photoredox catalysis method for synthesizing sterically hindered α-hydroxy esters from α-ketoesters and ethers through a radical-radical cross-coupling reaction. This approach utilizes commercially available Ir[dF(CF3)ppy]2(dtbbpy)PF6 as a photocatalyst and inexpensive and readily available nBu4NBr as a hydrogen atom transfer catalyst. Unactivated tetrahydrofuran and other ethers effectively react with various α-ketoesters to yield the desired products. The efficiency of this reaction is highlighted by its broad substrate scope, good functional group tolerance, and mild reaction conditions.

Ir-Catalyzed B(3)-Amination of o-Carboranes with Amines via Acceptorless Dehydrogenative BH/NH Cross-Coupling

An efficient and convenient strategy for Ir-catalyzed selective B(3)-amination of o-carboranes with amines via acceptorless BH/NH dehydrocoupling was developed, affording a series of B(3)-aminated-o-carboranes in moderate to high isolated yields with H2 gas as a sole by-product. Such an oxidant-free system endues the protocol sustainability, atom-economy and environmental friendliness. A reaction mechanism via an Ir(I)-Ir(III)-Ir(I) catalytic cycle involving oxidative addition, dehydrogenation and reductive elimination was proposed.

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp3)-H to construct C-C bonds

Ether derivatives are widespread as essential building blocks in various drugs, natural products, agrochemicals, and materials. Modern economy requires developing green strategies with improved efficiency and reduction of waste. Due to its atom and step-economy, the cross-dehydrogenative coupling (CDC) reaction has become a major strategy for ether functionalization. This review covers C-H/C-H cross-coupling reactions of ether derivatives with various C-H bond substrates via non-noble metal catalysts (Fe, Cu, Co, Mn, Ni, Zn, Y, Sc, In, Ag). We discuss advances achieved in these CDC reactions and hope to attract interest in developing novel methodologies in this field of organic chemistry.

Visible-light mediated, oxygen-promoted regioselective cross-dehydrogenative coupling of coumarins and dimethylanilines

Herein, we report a regioselective, photocatalytic C3 α-aminoalkylation of coumarins via a cross-dehydrogenative coupling of dimethylanilines and coumarins. Molecular oxygen was utilized as the oxidizing agent in this transformation, which exhibits a wide substrate scope and affords the products in good yields. It was established that 4-amino-substituted coumarin reacts via a different mechanism compared to coumarin derivatives that are unsubstituted at the 4-position.

Synthesis and in Silico Study of Novel Benzisoxazole-Chromene Derivatives as Potent Inhibitors of Acetylcholinesterase: Metal-Free Site-Selective C-N Bond Formation via Aza-Michael Reaction

An efficient metal-free approach for site selective C-N coupling reaction of benzo[d]isoxazole and 2H-chromene derivatives has been designed and developed against AchE. This nitrogen containing organo-base promoted methodology, which is both practical and environmentally friendly, provides an easy and suitable pathway for synthesizing Benzisoxazole-Chromene (BC) possessing poly heteroaryl moieties. The synthesized BC derivatives 4 a-n was docked into the active sites of AChE to obtain more perception into the binding modes of the compounds. Out of them, compound 4 a and 4 l displayed potent activity and high selectivity against the AChE inhibition. Final docking results indicates that compound 4 l showed the lowest binding energy of -11.2260 kcal/mol with AChE. The synthesized BC analogs would be potential candidates for promoting suitable studies in medicinal chemistry research.

Metal- and Base-Free Radical Cascade Cyclization/Hydrolysis of CN-Containing 1,6-Enynes with Ethers to Access Polyheterocycles

A convenient and straightforward approach for the radical cascade cyclization/hydrolysis of CN-containing 1,6-enynes with simple ethers under metal- and base-free conditions is described. This strategy provides a variety of valuable ethers-substituted polyheterocycles via the construction of three C-C bonds, one C=O bond, and two new six-membered rings within a single procedure. The resulting products can smoothly undergo follow-up conversions to various useful scaffolds. The methodology shows excellent functional group tolerance, high step- and atom- economy, and mild reaction conditions, which can be further scaled up to gram quantity in a satisfactory yield.

Visible-Light-Mediated (sp 3)Cα-H Functionalization of Ethers Enabled by Electron Donor-Acceptor Complex

A synthetically beneficial visible-light-mediated protocol has been disclosed to achieve C-H amination of readily available feedstocks cyclic and acyclic ethers. A rarely identified N-bromosuccinamide-tetrahydrofuran electron donor-acceptor complex served as an initiator to functionalize both α-diazoketones and dialkyl azodicarboxylates. This developed methodology gives an alternative and milder way to construct the C-N bond and can be explored for the formation of C-C bond to perform arylation and allylation reactions.

Solvent-free, B(C6 F5 )3 -Catalyzed S-H Insertion of Thiophenols and Thiols with α-Diazoesters

Described herein is a B(C₆ F₅ )₃ -catalyzed S-H insertion reaction of thiophenols and thiols with α-diazoesters to access valuable α-thioesters. With the established protocol, an array of α-thioester products are generated in moderate to good yields with broad scope and functional group tolerance. In addition, this reaction maintains its high efficiency on gram scale and the product can be easily transformed into other useful motifs. This reaction proceeds under solvent-free conditions at room temperature, and generally finishes in twenty minutes upon magnet stirring, which offers an expedient way for synthesis of thioether-containing compounds.

Transition-metal-catalyzed remote C-H functionalization of thioethers

In the last decade, transition-metal-catalyzed direct C-H bond functionalization has been recognized as one of most efficient approaches for the derivatization of thioethers. Within this category, both mono- and bidentate-directing group strategies achieved the remote C(sp2)-H and C(sp3)-H functionalization of thioethers, respectively. This review systematically introduces the major advances and their mechanisms in the field of transition-metal-catalyzed remote C-H functionalization of thioethers from 2010 to 2021.

Cu(ii)-Catalyzed C2-site functionalization of p-aminophenols: an approach for selective cross-dehydrogenative aminations

Site selective cross dehydrogenative aminations from precursors without preactivated C–H and N–H bonds have been challenging.

Metal-free and Selectfluor-mediated diverse transformations of 2-alkylthiobenzamides to access 2,3-dihydrobenzothiazin-4-ones, benzoisothiazol-3-ones and 2-alkylthiobenzonitriles

Diverse transformations of 2-alkylthiobenzamides have been established to synthesize 2,3-dihydrobenzothiazin-4-ones, benzoisothiazol-3-ones and 2-alkylthiobenzonitriles in the presence of Selectfluor.

Hypervalent-iodine promoted selective cleavage of C(sp3)–C(sp3) bonds in ethers

A visible-light-promoted and radical-mediated strategy for the site-specific cleavage of C(sp3)–C(sp3) bonds in ethers is reported.

Cross-dehydrogenative coupling of ethers and amides with the tautomerizable quinazolinones, and mechanistic studies

Cross-dehydrogenative coupling reactions have been utilized to alkylate 4(3H)-quinazolinones with ethers and amides, using catalytic n-Bu4NI and t-BuOOH as oxidant. Reactions with amides represent the first examples under such conditions....

Bio-carbon-layered CuO-catalyzed decarboxylative alkenylation of cyclic ethers

An efficient methodology for the direct decarboxylative functionalization of cinnamic acid derivatives with cyclic ethers has been developed by using biogenic CuO/C nanoparticles. This protocol is compatible with broad range of substrates.

Copper-Catalyzed Direct Allenylation of Inactive Cyclic Ethers

We report here a direct allenylation reaction of inactive cyclic ethers. The reaction proceeds through a copper-catalyzed 1,4-difunctionalization of 1,3-enynes, with cyano group installed at the allenes simultaneously. This methodology shows a broad functional group compatibility to 1,3-enynes. Diversified allene-modified cyclic ether derivatives were synthesized with high regioselectivity under mild conditions.

Combustion-Synthesized Porous CuO-CeO2-SiO2 Composites as Solid Catalysts for the Alkenylation of C(sp3)-H Bonds Adjacent to a Heteroatom via Cross-Dehydrogenative Coupling

A series of mixed oxides of CuO, CeO2, and SiO2 were prepared by gel combustion and employed for the first time as efficient solid catalysts in a solvent-less liquid-phase cross-dehydrogenative coupling. The facile one-pot catalyst synthesis resulted in highly porous materials presenting large specific surface areas and strong metal–support interactions. The interaction with highly dispersed CeO2 enhanced the redox properties of the CuO species. The CuO-CeO2-SiO2 composites exhibited excellent catalytic performance for the selective coupling between 1,1-diphenylethylene and tetrahydrofuran with a yield up to 85% of 2-(2,2-diphenylvinyl)-tetrahydrofuran in the presence of di-tert-butyl peroxide (DTPB) and KI. Albeit both CuO and CeO2 species are proved to be responsible for the catalytic conversion, a great synergistic improvement in the catalytic activity was obtained by extended contact between the oxide phases by high porosity in comparison with the reactions using individual Cu or Ce catalysts. The activity of the composite catalyst was shown to be highly stable after five successive reaction cycles. Furthermore, the study scope was extended to the synthesis of different derivatives via composite-catalyzed coupling of C(sp2)-H with C(sp3-H) adjacent to a heteroatom. The good yields recorded proved the general validity of this composite for the cross-dehydrogenative coupling reaction rarely performed on solid catalysts.

Direct synthetic routes to functionalised crown ethers

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

Aerobic Cross-Dehydrogenative Coupling Reactions for Selective Mono- and Dithiolation of Phenols

A highly efficient strategy for the direct thiolation of phenols under transition metal-free and solvent-free conditions has been developed. These reactions are operationally simple with employing air (molecular oxygen) as an ideal oxidant and can selectively provide mono- and dithiolation products in good to excellent yields under basic conditions. The reaction tolerates a broad range of aryl thiols and arenes and is especially applicable for large-scale synthesis.

Cu-Catalyzed Dehydrogenative C-O Cyclization for the Synthesis of Furan-Fused Thienoacenes

The first Cu-catalyzed dehydrogenative C-O cyclization for the synthesis of furan-fused thienoacenes is described. A variety of heteroacenes including a thieno[3,2-b]furan or a thieno[2,3-b]furan skeleton were synthesized by intramolecular C-H/O-H coupling. The use of a mixed solvent of N-methyl-2-pyrrolidone, ethylene glycol monomethyl ether, and toluene was essential for suppressing side reactions and efficiently promoting the reaction. Double C-O cyclization was also conducted to afford highly π-expanded furan-fused thienoacenes.

Iodine(III) promotes cross-dehydrogenative coupling of N-hydroxyphthalimide and unactivated C(sp3)-H bonds

Cross-dehydrogenative coupling reactions provide a method to construct new chemical bonds by direct C-H activation without any pre-functionalization. Compared to functionalization of a C-H bond α- to ether oxygen, α- to carbonyl, or at a benzylic position, functionalization of unactivated hydrocarbons is difficult and often requires high temperatures, a transition-metal catalyst, or a superstoichiometric quantity of volatile, toxic, and explosive tert-butylhydroperoxide. Here, a cross-dehydrogenative C-O coupling reaction of N-hydroxyphthalimide with unactivated alkanes, nitriles, ethers, and thioethers has been realized by using iodobenzene diacetate as the radical initiator. The current protocol enables efficient functionalization of unactivated hydrocarbons and nitriles through inert C(sp³)-H bond activation under mild reaction conditions. O-substituted NHPI derivatives are generated in good yields under metal-free conditions.

Transition Metal-Free Oxidative Cross-Coupling Reaction of Activated Olefins with N-Alkyl Amides

The K₂S₂O₈-mediated transition metal-free oxidative cross-coupling reaction of activated olefins with N-alkyl amides was developed, and the reaction gave N-allylic amides in moderate to good yield. This reaction protocol was suitable for different kinds of activated olefins.

Recent advances in the application of tetrabromomethane in organic synthesis

This review article covers the use of tetrabromomethane as mediator, catalyst and reagents for organic synthesis for the period from 2007 to 2020.

Catalyst-free, visible-light-promoted S-H insertion reaction between thiols and α-diazoesters

A visible-light-promoted S-H insertion reaction between thiols and α-diazoesters was developed. The reaction proceeded smoothly at room temperature with a broad substrate scope, affording various thioethers in moderate to excellent yields. The catalyst- and additive-free nature, sustainable energy source and mild reaction conditions make this strategy more eco-friendly.

Metal-Free Synthesis of Indolopyrans and 2,3-Dihydrofurans Based on Tandem Oxidative Cycloaddition

The synthesis of versatile scaffold indolopyrans based on C-C radical-radical cross-coupling under metal-free conditions is described. The reaction involving single electron transfer between coupling partners followed by cage collapse allows highly selective cross-coupling while employing only equimolar amounts of coupling partners. Moreover, the mechanistic manifold was expanded for the functionalization of enamines to give the stereoselective synthesis of 2,3-dihydrofurans. This iodine-mediated oxidative coupling features mild conditions and fast reaction kinetics.

New Trends in Enantioselective Cross-Dehydrogenative Coupling

The development of cross-dehydrogenative coupling in recent years has simplified the synthesis of many materials, as a result of facile C–H activation, which, together with its greater atom economy and environmental friendliness, has made an impact on modern organic chemistry. Indeed, many C–C and C–X (X = N, O, P, S, B, or Si) coupling reactions can now be performed directly between two C–H bonds or a C–H and an X–H bond, simply by adding catalytic amounts of a metal salt to a mixture of the two and an oxidant to accept the two hydrogen atoms released. Chiral organocatalysts or chiral ligands have been joined to promote enantioselective processes, resulting in the development of efficient reaction cascades that provide products in high yields and high levels of asymmetric induction through cooperative catalysis. In recent years, photochemical oxidation and electrochemistry have widened even more the scope of cross-dehydrogenative coupling (CDC). In this review, we summarized the recent literature in this subject, hoping that it will inspire many new synthetic strategies.

Oxidative Dearomative Cross-Dehydrogenative Coupling of Indoles with Diverse C-H Nucleophiles: Efficient Approach to 2,2-Disubstituted Indolin-3-ones

The oxidative, dearomative cross-dehydrogenative coupling of indoles with various C-H nucleophiles is developed. This process features a broad substrate scope with respect to both indoles and nucleophiles, affording structurally diverse 2,2-disubstituted indolin-3-ones in high yields (up to 99%). The oxidative dimerization and trimerization of indoles has also been demonstrated under the same conditions.

The regioselective coupling of 2-arylquinazolinone C–H with aldehydes and benzyl alcohols under oxidative conditions

Palladium catalyzed direct and regioselective cross dehydrogenative coupling (CDC) of 2-arylquinazoline-4-one endowed with a quinazolinone nucleus as an inherent directing group with aldehyde and oxidative coupling with benzyl alcohol was developed.

Catalytic enantioselective cross dehydrogenative coupling of sp3 C-H of heterocycles

C-C bond formation in heterocycles via the direct coupling of C-H bonds, under oxidative conditions, classified as cross dehydrogenative coupling (CDC), is without doubt one of the most atom efficient methods for the functionalization of these molecules. The most common is the coupling at the position alpha to the heteroatom, owing to the stabilization of forming carbocation by the heteroatom. The corresponding asymmetric versions, except for a few isolated reports, have been rather evasive for several years. Optically active heterocycles with the chiral center alpha to the heteroatom are widely present in natural products and pharmaceuticals, thus making them an attractive synthetic target. Persistent efforts towards the asymmetric CDC of heterocycles since the beginning of this decade have led to several developments in this challenging area. Particularly, in the last few years, considerable progress has been witnessed in this field. This review summarizes the progress made in the area of asymmetric cross dehydrogenative coupling of heterocycles in recent years. The review covers the catalytic asymmetric CDC of sp³ C-H bonds of the heterocycles with various coupling partners and illustrates the different catalytic systems employed.