First C−H Activation Route to Oxindoles using Copper Catalysis

Visible-light-induced radical-cascade alkylation/cyclization of acrylamides: access to 3,3-dialkylated oxindoles

A visible-light-induced deoxygenative alkylation/cyclization of acrylamides with alcohols activated by CS2 has been developed by using xanthate salts as alkyl radical precursors in the presence of tricyclohexylphosphine. It proceeds through a tandem radical addition/cyclization process, and this protocol provides a reliable and practical approach to building the skeleton of 3,3-disubstituted oxindoles in moderate to good yields. Notable features of this reaction include readily available starting reagents, broad substrate scope and mild reaction conditions.

Redox-Neutral Radical Cascade Cyclization of N-Arylacrylamides with Unactivated Alkyl and Aryl Chlorides

Herein, we report the first metal-free, redox-neutral strategy for radical cascade alkylative radical addition, cyclization of N-arylacrylamides with unactivated alkyl chlorides to give corresponding 3,3-disubstituted oxindoles in moderate to good yields. This transformation's salient features are the utilization of an organo photocatalyst, mild reaction conditions, and broad substrate scope. Moreover, this methodology is suitable for hetero cycle derived acrylamides and further allowed to utilize aryl chlorides for radical cyclization reaction. Finally, DFT studies allow us to shed light on the reaction mechanism.

DTBP-mediated cross-dehydrogenative coupling of 3-aryl benzofuran-2(3H)-ones with toluenes/phenols for all-carbon quaternary centers

We have developed a transition-metal free protocol for efficient cross-dehydrogenative coupling of 3-aryl benzofuran-2(3H)-ones and toluenes/phenols using DTBP as an oxidant. A diverse range of 3-aryl benzofuran-2(3H)-ones, toluenes, and phenols undergo C-H bond cleavage to generate all-carbon quaternary centers in good yields, making this protocol useful for the synthesis of complex molecules. A gram scale experiment was performed in good yield.

Silver-catalysed double decarboxylative addition-cyclisation-elimination cascade sequence for the synthesis of quinolin-2-ones

An atom-efficient silver-catalysed double carboxylative strategy for the one-step synthesis of quinolin-2-ones via an addition-cyclisation-elimination cascade sequence of oxamic acids to acrylic acids, mediated either thermally or photochemically, is reported. The reaction was applicable to the synthesis of a broad range of quinolin-2-ones and featured a double-disconnection approach that constructed the quinolin-2-one core via the formal and direct addition of a C(sp²)-H/C(sp²)-H olefin moiety to a phenylformamide precursor.

Oxindole synthesis via polar-radical crossover of ketene-derived amide enolates in a formal [3 + 2] cycloaddition

Herein we introduce a simple, efficient and transition-metal free method for the preparation of valuable and sterically hindered 3,3-disubstituted oxindoles via polar-radical crossover of ketene derived amide enolates. Various easily accessible N-alkyl and N-arylanilines are added to disubstituted ketenes and the resulting amide enolates undergo upon single electron transfer oxidation a homolytic aromatic substitution (HAS) to provide 3,3-disubstituted oxindoles in good to excellent yields. A variety of substituted anilines and a 3-amino pyridine engage in this oxidative formal [3 + 2] cycloaddition and cyclic ketenes provide spirooxindoles. Both substrates and reagents are readily available and tolerance to functional groups is broad.

Transition-metal-free oxindole synthesis: quinone-K2CO3 catalyzed intramolecular radical cyclization

A novel and highly efficient transition-metal-free approach for the conversion of α-bromoanilides to 3,3-disubstituted oxindoles is described. This transformation is promoted by catalytic amount of 9,10-phenanthrenequinone (PQ) together with K2CO3,...

Palladium-Catalyzed Sequential Heck Coupling/C-C Bond Activation Approach to Oxindoles with All-Carbon-Quaternary Centers

Catalytic construction of oxindoles bearing all-carbon-quaternary centers draws wide attentions from synthetic community. Herein, we report a Palladium-catalyzed sequential Heck coupling/C-C bond activation of aryl halide-tethered alkenes with benzocyclobutenols affording...

Lambert Salt-Initiated Development of Friedel-Crafts Reaction on Isatin to Access Distinct Derivatives of Oxindoles

Herein, a mild metal-free and efficacious route for the synthesis of biologically important 3-aryl oxindole derivatives is described. Using Lambert salt-initiated hydroarylation of isatin, a diverse array of monoarylated products, symmetrical/unsymmetrical double-arylated products, and deoxygenated hydroarylated products could be synthesized from the single starting substrate in good to excellent yields. A preliminary mechanistic study revealed that the reaction proceeds via a monoarylated product followed by a nucleophilic attack by another electron-rich arene nucleophile under mild conditions. The potential of newly synthesized symmetric/unsymmetric 3,3-disubstituted oxindole, 3-substituted 3-hydroxy oxindoles, 3,3-di(indolyl)indolin-2-ones, and α-aryl oxindoles as valuable building blocks is further illustrated.

Cross-Dehydrogenative Cyclization-Dimerization Cascade Sequence for the Synthesis of Symmetrical 3,3'-Bisoxindoles

The synthesis of symmetrical 3,3'-bisoxindoles from simple acyclic β-oxoanilides is reported. The described method forges three new C-C bonds in a single step via a sequential Mn(OAc)₃·2H₂O mediated oxidative radical cyclization-fragmentation-dimerization process. The scope of this reaction is demonstrated in the preparation of a variety of 3,3'-bisoxindoles, as well as its application toward the formal synthesis of the Calycanthaceae alkaloid, (±)-folicanthine.

NHC-catalyzed oxindole synthesis via single electron transfer

A NHC-catalyzed metal-free oxindole synthesis method is developed.

Recent advances in organocatalytic asymmetric multicomponent cascade reactions for enantioselective synthesis of spirooxindoles

Catalytic asymmetric MCCRs for enantioselective synthesis of spirooxindoles by using chiral phosphoric acids, amines, bifunctional thiourea/squaramides and metal-based reagents as catalysts.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Copper-Catalyzed Electrochemical C-H Amination of Arenes with Secondary Amines

Electrochemical oxidation represents an environmentally friendly solution to conventional methods that require caustic stoichiometric chemical oxidants. However, C-H functionalizations merging transition-metal catalysis and electrochemical techniques are, to date, largely confined to the use of precious metals and divided cells. Herein, we report the first examples of copper-catalyzed electrochemical C-H aminations of arenes at room temperature using undivided electrochemical cells, thereby providing a practical solution for the construction of arylamines. The use of n-Bu₄NI as a redox mediator is crucial for this transformation. On the basis of mechanistic studies including kinetic profiles, isotope effects, cyclic voltammetric analyses, and radical inhibition experiments, the reaction appears to proceed via a single-electron-transfer (SET) process, and a high valent Cu(III) species is likely involved. These findings provide a new avenue for transition-metal-catalyzed electrochemical C-H functionalization reactions using redox mediators.

Electrochemical dehydrogenative cyclization of 1,3-dicarbonyl compounds

The intramolecular C(sp3)-H/C(sp2)-H cross-coupling of 1,3-dicarbonyl compounds has been achieved through Cp2Fe-catalyzed electrochemical oxidation. The key to the success of these dehydrogenative cyclization reactions is the selective activation of the acidic α-C-H bond of the 1,3-dicarbonyl moiety to generate a carbon-centered radical.

Cobalt-catalyzed C–H activation: recent progress in heterocyclic chemistry

Cobalt-catalyzed C–H activation has gone through some major advancements in the past couple of decades.

Low-Temperature, Transition-Metal-Free Cross-Dehydrogenative Coupling Protocol for the Synthesis of 3,3-Disubstituted Oxindoles

A new low-temperature procedure for the synthesis of 3,3-disubstituted 2-oxindoles via cross-dehydrogenative coupling (CDC) is reported. The use of a strong, nonreversible base in these reactions has been found to effect a dramatic drop in reaction temperature (to room temperature) relative to the current state-of-the-art (>100 °C) procedure. When employing iodine as an "oxidant", new evidence suggests that this transformation may occur via a transiently stable iodinated intermediate rather than by direct single-electron oxidation.

Photoredox-catalyzed procedure for carbamoyl radical generation: 3,4-dihydroquinolin-2-one and quinolin-2-one synthesis

A reductive approach for carbamoyl radical generation from N-hydroxyphthalimido oxamides under photoredox catalysis is outlined. This strategy was applied to the synthesis of 3,4-dihydroquinolin-2-ones via the intermolecular addition/cyclization of carbamoyl radicals with electron deficient olefins in a mild, redox-neutral manner. Under a general set of reaction conditions, diversely substituted 3,4-dihydroquinolin-2-ones, including spirocyclic systems can be prepared. By using chlorine-substituted olefins, aromatic quinolin-2-ones can also be accessed.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

A Cu-Catalysed Radical Cross-Dehydrogenative Coupling Approach to Acridanes and Related Heterocycles

The synthesis of acridanes and related compounds through a Cu-catalysed radical cross-dehydrogenative coupling of simple 2-[2-(arylamino)aryl]malonates is reported. This method can be further streamlined to a one-pot protocol involving the in situ fomation of the 2-[2-(arylamino)aryl]malonate by α-arylation of diethyl malonate with 2-bromodiarylamines under Pd catalysis, followed by Cu-catalysed cyclisation.

Enantioselective synthesis of sterically hindered α-allyl–α-aryl oxindoles via palladium-catalysed decarboxylative asymmetric allylic alkylation

The highly enantioselective synthesis of sterically hindered α-allyl–α-aryl oxindoles possessing an all-carbon quaternary stereocenter at the oxindole 3-position has been developed.

Synthesis of oxindole from acetanilide via Ir(iii)-catalyzed C–H carbenoid functionalization

The Ir(iii)-catalyzed intermolecular C–H annulations of acetanilide with diazotized Meldrum's acid furnished the oxindole derivative. Brief synthetic applications of the synthesized oxindole were also demonstrated.

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp(2) C-H and sp(3) C-H bonds

The synthesis of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudo benzylic position has been achieved via a ‘transition-metal-free’ intramolecular dehydrogenative coupling (IDC). The construction of 2-oxindole moieties was carried out through formation of carbon–carbon bonds using KOt-Bu-catalyzed one pot C-alkylation of β-N-arylamido esters with alkyl halides followed by a dehydrogenative coupling. Experimental evidences indicated toward a radical-mediated path for this reaction.

Synthesis of 1,3,5-triazines via Cu(OAc)2-catalyzed aerobic oxidative coupling of alcohols and amidine hydrochlorides

Cu(OAc)2 was found to be an efficient catalyst for dehydrogenative synthesis of 1,3,5-triazine derivatives via oxidative coupling reaction of amidine hydrochlorides and alcohols in air. Both aromatic and aliphatic alcohols can be involved in the reaction and thirty-three products were obtained with good to excellent yields. Moreover, the use of a ligand, strong base and organic oxidant is unnecessary.