A radical addition/cyclization of diverse ethers to 2-isocyanobiaryls under mildly basic aqueous conditions

Addition of ethers and acetals to 2-isocyanobiaryls: a radical cascade promoted under mildly basic aqueous conditions.

Palladium-Catalyzed Directed C(sp3)-H Arylation of Saturated Heterocycles at C-3 Using a Concise Optimization Approach

Saturated heterocycles, such as THFs, pyrrolidines, piperidines and THPs, are essential components of many biologically active compounds. Examples of C-H functionalization on these important ring systems remain scarce, especially at unactivated positions. Here we report the development of conditions for the palladium-catalyzed stereoselective C(sp3)-H arylation at unactivated 3-positions of 5- and 6-membered N- and O-heterocycles with aminoquinoline directing groups. Subtle differences in substrate structures altered their reactivity significantly; and different conditions were required to achieve high yields in each case. Successful conditions were developed using a short empirical optimization approach to cover reaction space with a limited set of variables. Excellent cis-selectivity was achieved in all cases, except for the THP substrate where minor trans-products were formed through a different palladacyclic intermediate. Here, differences in reactivity and selectivity with other directing groups were examined.

Tuning the Reactivity of Radical through a Triplet Diradical Cu(II) Intermediate in Radical Oxidative Cross-Coupling

Highly selective radical/radical cross-coupling is paid more attention in bond formations. However, due to their intrinsic active properties, radical species are apt to achieve homo-coupling instead of cross-coupling, which makes the selective cross-coupling as a great challenge and almost untouched. Herein a notable strategy to accomplish direct radical/radical oxidative cross-coupling has been demonstrated, that is metal tuning a transient radical to a persistent radical intermediate followed by coupling with another transient radical. Here, a transient nitrogen-centered radical is tuned to a persistent radical complex by copper catalyst, followed by coupling with a transient allylic carbon-centered radical. Firstly, nitrogen-centered radical generated from N-methoxybenzamide stabilized by copper catalyst was successfully observed by EPR. Then DFT calculations revealed that a triplet diradical Cu(II) complex formed from the chelation N-methoxybenzamide nitrogen-centered radical to Cu(II) is a persistent radical species. Moreover, conceivable nitrogen-centered radical Cu(II) complex was observed by high-resolution electrospray ionization mass spectrometry (ESI-MS). Ultimately, various allylic amides derivatives were obtained in good yields by adopting this strategy, which might inspire a novel and promising landscape in radical chemistry.

Copper-Catalyzed Oxidative C-H Amination of Tetrahydrofuran with Indole/Carbazole Derivatives

A simple α-C-H amination of cyclic ether with indole/carbazole derivatives has been accomplished by employing copper(II) chloride/bipy as the catalyst system. In the presence of the di-tert-butyl peroxide oxidant, cyclic ethers such as tetrahydrofuran, 1,4-dioxane, and tetrahydropyran successfully undergo C-H/N-H cross dehydrogenative coupling (CDC) with various carbazole or indole derivatives in good-to-excellent yields.

Coordinating activation strategy for C(sp(3))-H/C(sp(3))-H cross-coupling to access β-aromatic α-amino acids

The past decade has witnessed significant advances in C–H bond functionalizations with the discovery of new mechanisms. Non-precious transition-metal-catalysed radical oxidative coupling for C(sp³)–H bond transformations is an appealing strategy for C–C bond formations. The radical oxidative C(sp³)–H/C(sp³)–H cross-coupling reactions of α-C(sp³)–H bonds of amines with free radicals represent a conceptual and practical challenge. We herein develop the coordinating activation strategy to illustrate the nickel-catalysed radical oxidative cross-coupling between C(sp³)–H bonds and (hetero)arylmethyl free radicals. The protocol can tolerate a rich variety of α-amino acids and (hetero)arylmethanes as well as arylmethylenes and arylmethines, affording a large library of α-tertiary and α-quaternary β-aromatic α-amino acids. This process also features low-cost metal catalyst, readily handled and easily removable coordinating group, synthetic simplicity and gram-scale production, which would enable the potential for economical production at commercial scale in the future.

Oxidative coupling of two C(sp3)-H groups is an attractive approach for the formation of carbon-carbon bonds, building structural complexity while avoiding the formation of waste salts. Here, the authors report such a transformation, using a nickel catalyst to functionalise amino acids with benzylic radicals.

α-Arylation of Saturated Azacycles and N-Methylamines via Palladium(II)-Catalyzed C(sp(3))-H Coupling

Pd(II)-catalyzed α-C(sp(3))-H arylation of pyrrolidines, piperidines, azepanes, and N-methylamines with arylboronic acids has been developed for the first time. This transformation is applicable to wide arrays of pyrrolidines and boronic acids, including heteroaromatic boronic acids. A diastereoselective one-pot heterodiarylation of pyrrolidines has also been achieved.

Iminoiodane- and Brønsted base-mediated cross dehydrogenative coupling of cyclic ethers with 1,3-dicarbonyl compounds

A one-pot, two-step approach to prepare 2-tetrahydrofuran and -pyran substituted 1,3-dicarbonyl compounds by PhI=NTs-mediated amination/Brønsted base-catalyzed cross dehydrogenative coupling (CDC) reaction of the cyclic ether and 1,3-dicarbonyl derivative under mild conditions is reported. The reaction is compatible with a variety of cyclic ethers and 1,3-dicarbonyl compounds, affording the corresponding coupled products in moderate to good yields of up to 80% over two steps.

External Oxidant-Free Oxidative Cross-Coupling: A Photoredox Cobalt-Catalyzed Aromatic C-H Thiolation for Constructing C-S Bonds

An external oxidant-free oxidative coupling for aromatic C-H thiolation by visible-light photoredox cobalt-catalysis has been developed. Various substrates could afford benzothiazoles in good to excellent yields, and only H2 is generated as a side product. When catalytic TBAOH was used as the base, not only 2-aryl but also 2-alkylbenzothiazoles could be obtained through this novel dehydrogenative coupling reaction. This method could be scaled up and applied to the synthesis of biologically active molecules bearing benzothiazole structural scaffolds (potent antitumor agents). Furthermore, the unexpected oxidation byproduct amides, which are often generated in oxidative cyclization of thiobenzanilides, can be completely avoided. Mechanistic studies showed that the H2 originates from the substrates. The kinetic studies indicate that the interaction between the cobalt catalyst and proton might be involved in the rate-limiting process.

Copper-catalyzed oxidative alkenylation of thioethers via Csp(3)-H functionalization

A novel copper-catalyzed oxidative alkenylation of thioethers via Csp(3)-H functionalization to construct allylic thioethers is first demonstrated. Different 1,1-disubstituted olefins could cross-couple with thioethers to generate the corresponding alkenylation products in moderate to excellent yields. This reaction is supposed to proceed via a radical process.

Organocatalytic amination of alkyl ethers via n-Bu4NI/t-BuOOH-mediated intermolecular oxidative C(sp(3))-N bond formation: novel synthesis of hemiaminal ethers

A novel method for constructing the hemiaminal ether framework under metal-free conditions has been developed. It involves direct organocatalytic amination of alkyl ethers through intermolecular oxidative C(sp(3))-N bond formation, with t-BuOOH being the oxidant and n-Bu4NI as the catalyst.

Nickel-catalyzed monofluoromethylation of aryl boronic acids

Aryl boronic acids can be monofluoromethylated under nickel catalysis. The utility of this method is demonstrated by the monofluoromethylation of a borylated and acyl-protected derivative of the statin drug ezetimibe. Mechanistic investigations indicate that a fluoromethyl radical is involved in the Ni(I)/Ni(III) catalytic cycle.

Metal-free oxidative functionalization of C(sp(3))-H bond adjacent to oxygen and radical addition to olefins

A DTBP-promoted oxidative functionalization of a C(sp(3))-H bond adjacent to oxygen and intermolecular radical addition to olefins without use of any metal catalyst or photoredox catalysis is reported. The reaction has a wide scope of olefin, alcohol, and cycloether substrates, which provides an easy way for direct preparation of α,ω-amino alcohols.

Nickel-catalyzed dehydrogenative cross-coupling: direct transformation of aldehydes into esters and amides

By exploring a new mode of nickel-catalyzed cross-coupling, a method to directly transform both aromatic and aliphatic aldehydes into either esters or amides has been developed. The success of this oxidative coupling depends on the appropriate choice of catalyst and organic oxidant, including the use of either α,α,α-trifluoroacetophenone or excess aldehyde. Mechanistic data that supports a catalytic cycle involving oxidative addition into the aldehyde C-H bond is also presented.

Metal-free radical oxidative decarboxylation/cyclization of acyl peroxides and 2-isocyanobiphenyls

A radical oxidative decarboxylation/cyclization of acyl peroxides and 2-isocyanobiphenyls was achieved, leading to 6-substituted phenanthridines under metal-free and base-free conditions.

Copper-catalyzed oxidative alkenylation of C(sp3)–H bonds via benzyl or alkyl radical addition to β-nitrostyrenes

A new method for the preparation of (E)-β-alkylstyrene derivatives has been developed via the addition of benzyl or alkyl radicals to β-nitrostyrenes using di-tert-butyl peroxide (DTBP) as the oxidant in the presence of Cu powder catalyst.

Metal-free direct difunctionalization of alkenes with I2O5 and P(O)–H compounds leading to β-iodophosphates

A novel and efficient procedure for direct difunctionalization of alkenes with I₂O₅ and P(O)–H compounds has been developed under metal-free conditions.

Direct oxidative coupling of thiols and benzylic ethers via C(sp3)–H activation and C–O cleavage to lead thioesters

An unprecedented protocol for the synthesis of thioesters via C–H thioesterification and C–O cleavage was developed. This CDC protocol was found convenient and easy-to-handle involving the use of DTBP as the only green oxidant.

Oxidative C(sp3)–H functionalization of acetonitrile and alkanes with allylic alcohols under metal-free conditions

Direct oxidative C(sp³)–H functionalization of acetonitrile and alkanes with α,α-diaryl allylic alcohols has been developed.

Cross-dehydrogenative coupling of α-C(sp3)–H of ethers/alkanes with C(sp2)–H of heteroarenes under metal-free conditions

A metal-free cross-dehydrogenative coupling method between α-oxyalkyl/alkyl radicals and electron deficient heteroarenes is reported.

Selective formation of C–N and CN bonds via C(sp3)–H activation of isochroman in the presence of DTBP

An organocatalytic approach for the synthesis of isochroman derivatives via direct C(sp³)–H bond and N–H bond coupling is described. The C–N (amine or amide) and CN (imidate) products can be selectively achieved by controlling the amount of oxidants.

DBU-promoted cyclization of vinyl isocyanides with ethers via the functionalization of a C(sp3)–H bond for the synthesis of isoquinolines

A DBU-promoted cascade functionalization of a C(sp³)–H bond adjacent to oxygen and a radical cyclization reaction of vinyl isocyanides was developed to easily access multi-functionalized isoquinolines.

Cu-catalyzed oxidative Povarov reactions between N-alkyl N-methylanilines and saturated oxa- and thiacycles

Cu-catalyzed oxidative Povarov reactions between N,N-dialkylanilines and saturated oxa- or thiacycles with tert-butyl hydroperoxide (TBHP) are described.

Metal-free oxidative C(sp3)–H bond functionalization of alkanes and alkylation-initiated radical 1,2-aryl migration in α,α-diaryl allylic alcohols

A C(sp³)–H bond functionalization of alkanes and 1,2-aryl migration of allylic alcohols was reported with up to 93% yields.

Nickel N-heterocyclic carbene-catalyzed cross-coupling reaction of aryl aldehydes with organozinc reagents to produce aryl ketones

The transformation of aromatic aldehydes into aryl ketones by nickel-catalyzed cross-coupling has been developed. This transformation represents an efficient and attractive synthetic utilization of organozinc reagents.

Bond and small-molecule activation with low-valent nickel complexes

The use of nickel compounds in low oxidation states allowed a variety of useful transformations of interest for academia, industry and in the solution of environmental issues.

A nickel-mediated oxidative α-C(sp3)–H functionalization of amides with allylic alcohols terminated by radical 1,2-aryl migration

A Ni-mediated oxidative C(sp³)–H functionalization of N,N-substituted amides with α,α-diaryl allylic alcohols through a radical 1,2-aryl migration process has been developed. This process features a broad substrate scope and excellent functional group tolerance.

Iron-catalyzed direct α-arylation of ethers with azoles

The direct α-arylation of cyclic and acyclic ethers with azoles has been achieved, which features a novel iron-catalyzed cross-dehydrogenative coupling (CDC) process.

Vanadyl species-catalyzed complementary β-oxidative carbonylation of styrene derivatives with aldehydes

By judicious choice of the counter anions in the vanadyl catalysts, we can achieve β-hydroxylated and t-butyl peroxylated carbonylation of styrenes by aromatic 1° and 2° alkyl aldehydes in a complementary manner.

Recent advances of transition-metal catalyzed radical oxidative cross-couplings

CONSPECTUS: Oxidative cross-coupling reactions between two nucleophiles are a powerful synthetic strategy to synthesize various kinds of functional molecules. Along with the development of transition-metal-catalyzed oxidative cross-coupling reactions, chemists are applying more and more first-row transition metal salts (Fe, Co, etc.) as catalysts. Since first-row transition metals often can go through multiple chemical valence changes, those oxidative cross-couplings can involve single electron transfer processes. In the meantime, chemists have developed diverse mechanistic hypotheses of these types of reactions. However, none of these hypotheses have led to conclusive reaction pathways until now. From studying both our own work and that of others in this field, we believe that radical oxidative cross-coupling reactions can be classified into four models based on the final bond formations. In this Account, we categorize and summarize these models. In model I, one of the starting nucleophiles initially loses one electron to generate its corresponding radical under oxidative conditions. Then, bond formations between this radical and another nucleophile create a new radical, [Nu(1)-Nu(2)](•), followed by a further radical oxidation step to generate the cross-coupling product. The radical oxidative alkenylation with olefin, radical oxidative arylative-annulation, and radical oxidative amidation are examples of this model. In model II, one of the starting nucleophiles loses its two electrons via two steps of single-electron-transfer to generate an electrophilic intermediate, followed by a direct bond formation with the other nucleophile. For example, the oxidative C-O coupling of benzylic sp(3) C-H bonds with carboxylic acids and oxidative C-N coupling of aldehydes with amides are members of this model group. For model III, both nucleophiles are oxidized to their corresponding radicals. Then, the radicals combine to form the final coupling product. The dioxygen-involved radical oxidative cross-couplings between sulfinic acids and olefins or alkynes belong to this bond formation model. Lastly, in model IV, one nucleophile loses two electrons to generate an electrophilic intermediate, while the other nucleophile loses one electron to generate a radical. Then, a bond forms between the cation and the radical to generate a cationic radical, followed by a one-electron reduction to afford the final coupling product. The oxidative coupling between arylboronic acids and simple ethers was classified in this model. At the current stage, there are only a few examples presented for models III and IV, but they represent two types of potentially important transformations. More and more examples of these two models will be developed in the future.

Selective nickel- and manganese-catalyzed decarboxylative cross coupling of some α,β-unsaturated carboxylic acids with cyclic ethers

A nickel- and manganese-catalyzed decarboxylative cross coupling of α, β-unsaturated carboxylic acids with cyclic ethers such as tetrahydrofuran and 1, 4-dioxane was developed. Oxyalkylation was achieved when nickel acetate was used as catalyst, while manganese acetate promoted the reaction of alkenylation.

Structurally diverse α-substituted benzopyran synthesis through a practical metal-free C(sp3)-H functionalization

A trityl ion-mediated practical C-H functionalization of a variety of benzopyrans with a wide range of nucleophiles (organoboranes and C-H molecules) at ambient temperature has been disclosed. The metal-free reaction has an excellent functional group tolerance and high chemoselectivity and displays a broad scope with respect to both benzopyran and nucleophile partners, efficiently affording a collection of benzopyrans bearing diverse skeletons and α-functionalities in one step.

Nickel-Mediated Synthesis of Isoindolinones at Room Temperature

This communication describes a method for the Ni(cod) 2-mediated intramolecular arylation of alkyl C-H bonds adjacent to the nitrogen atom in benzamide substrates. The transformation proceeds at room temperature and exhibits selectivity for functionalization of more substituted C-H bonds. The yields of the desired isoindolinone products are higher with benzamide substrates containing tertiary alkyl groups on the nitrogen atom than with those bearing primary or secondary alkyls. The results described herein suggest a mechanism involving radical intermediates for these reactions.