Visible Light Mediated Synthesis of Oxindoles

Synthesis of ɑ-Aryl-oxindoles via Grignard Reaction with Isatin in the Presence of Diphenyl Phosphite

A protocol has been developed for the synthesis of α-aryl-oxindoles from isatin and Grignard reagents in the presence of diphenyl phosphite for the first time. This reaction was conveniently carried out under mild conditions in a one-pot fashion with moderate to excellent yields.

Electrochemical and photochemical reaction of isatins: a decade update

This review presents the latest progress in photochemical and electrochemical reactions involving isatins. Isatin and its functionalized scaffolds e.g., oxindoles, spirooxindoles, and quinolines are privileged heterocycles as they are largely present in several agrochemical, natural products, and pharmaceuticals. Thus, the functionalization of isatins using sustainable approaches, i.e., electro- and photochemical methods is of recent research interest worldwide. In this review, we have discussed most of the important reactions of isatins based on types of bond formation involved under electro- and photochemical conditions over the last decade. The reaction mechanism for each reaction has been discussed in detail to offer an inclusive guide to readers. Lastly, a summary of current challenges and the future outlook toward the development of effective electrochemical and photochemical methods for the reaction of isatins is also presented.

Oxidant-Assisted Sulfonylation/Cyclization Cascade Synthesis of Alkylsulfonylated Oxindoles via the Insertion of SO2

An oxidant-assisted tandem sulfonylation/cyclization of electron-deficient alkenes with 4-alkyl-substituted Hantzsch esters and Na2S2O5 for the preparation of 3-alkylsulfonylated oxindoles under mild conditions in the absence of a photocatalyst and transition metal catalyst is established. The mechanism studies show that the alkyl radicals, which come from the cleavage of the C-C bond in 4-substituted Hantzsch esters under oxidant conditions, subsequently undergo the in situ insertion of sulfur dioxide to generate the crucial alkylsulfonyl radical intermediates. This three-component reaction provides an efficient and facile route for the construction of alkylsulfonylated oxindoles and avoids the use of highly toxic alkylsulfonyl chlorides or alkylsulfonyl hydrazines as alkylsulfonyl sources.

Visible-light-promoted difluoroamidated oxindole synthesis via electron donor-acceptor complexes

A method involving a metal-free visible-light-promoted synthesis was developed for the construction of difluoroalkylated oxindoles with N-phenylacrylamides and bromodifluoroacetamides as starting materials in the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA). Twenty-four examples of the photochemical reaction were successfully performed, with good yields (44-99%) and excellent substrate adaptability. Mechanistic studies showed that the visible-light-promoted reaction involved a radical addition to N-phenylacrylamide, intramolecular cyclization, dehydrogenation, and rearomatization. The difluoroacetamide radical was produced as a result of electron transfer to bromodifluoroacetamides from the electron donor TMEDA in their electron-donor-acceptor (EDA) complexes under visible light irradiation. This protocol is a promising photochemical method due to its advantages of mild conditions, simple operation, wide substrate scope and high yields. And the obtained products may have great potential in the field of medicine.

Direct synthesis of N-functionalized indoles through isomerization of azomethine ylides

An unexpected isomerization of azomethine ylides generated in situ from isatin with indoline-2-carboxylic acid has been disclosed, providing direct access to N-functionalized indole scaffolds. This protocol has good functional group tolerance and provides various 3-(1H-indol-1-yl)indolin-2-one derivatives in moderate to high yields simply by using alcohol as the solvent, with no additional additive being required.

Cp2Fe-Mediated Electrochemical Synthesis of Phosphorylated Oxindoles and Indolo[2,1-a]isoquinolin-6(5H)-ones

An efficient and environmentally friendly electrochemical synthesis of phosphorylated oxindoles and indolo[2,1-a]isoquinolin-6(5H)-ones mediated by readily available Cp2Fe has been developed, which illustrated a broad substrate scope and diverse functional group compatibility. This protocol featured an external oxidant-free process and was at room temperature, which was proposed to be driven by the anodic oxidation of Cp2Fe.

Catalyst- and additive-free cascade radical addition/cyclization of N-arylacrylamides with trifluoropyruvates

Herein, we introduce a photocatalyst- and additive-free method for the preparation of valuable 3,3-disubstituted oxindoles bearing trifluoromethyl alcohol moieties from readily available acrylamides and cheap trifluoropyruvates. The excited trifluoropyruvates under ultraviolet-light irradiation react efficiently with acrylamides delivering a variety of trifluoromethyl oxindoles with broad functional group tolerance and moderate to good yields. This protocol features mild reaction conditions, simple operation and ready scalability.

Electrochemical Fe-catalysed radical cyclization for the synthesis of oxindoles

We report an efficient and sustainable electrocatalytic approach for the synthesis of 3,3-disubstituted 2-oxindoles bearing ester groups from readily accessible N-arylacrylamides and carbazates. The reaction proceeds through an electrochemical iron-catalyzed radical addition/cyclization sequence with a commercially available iron catalyst and carbazates as alkoxycarbonyl radical precursors. This mild and operationally simple method transforms a wide range of structurally diverse N-arylacrylamides into oxindole derivatives in good yields and can be smoothly scaled up for the preparation of synthetically valuable oxindoles that are key intermediates for the synthesis of natural products.

Visible-Light-Mediated Generation of Acyl Radicals from Triazine Esters

Acyl radicals are significant synthetic active species in organic synthesis. However, their generation via green and compatible methods remains challenging. Herein, we report an unprecedented visible-light-mediated approach for generating aryl acyl radicals from readily available triazine esters. This protocol with mild and redox-neutral conditions affords a diverse array of oxindoles attached to alcohol groups in a single operation. The recycling of leaving groups and a range of visible-light-mediated reactions using triazine ester as an acyl radical precursor demonstrate the synthetic potential of this methodology.

Photoinduced Alkyl/Aryl Radical Cascade for the Synthesis of Quaternary CF3-Containing Oxindoles and Indoline Alkaloids

Metal- and additive-free, photoinduced decarboxylative radical alkylation-cyclization of CF₃-acrylamides with alkyl redox-active esters provided the corresponding quaternary CF₃-oxindole derivatives in good yields. Notably, diaryliodonium salts also efficiently participated in the arylation-cyclization of CF₃-acrylamides in environmentally benign H₂O as a solvent. The present approach has been extended for the concise synthesis of CF₃-attached indoline alkaloid analogues, i.e., CF₃-(±)-desoxyeseroline, CF₃-(±)-esermethole, and CF₃-(±) progesterone receptor antagonists. The preliminary mechanistic studies revealed that the reaction is likely to proceed through initial photoexcitation of redox-active ester/diaryliodonium salts followed by the SET process with acrylamide.

Practical, scalable, and transition metal-free visible light-induced heteroarylation route to substituted oxindoles

The synthetic application of (hetero)aryl radicals in organic synthesis has been known since the last century. However, their applicability has significantly suffered from ineffective generation protocols. Herein, we present a visible-light-induced transition metal-free (hetero)aryl radical generation from readily available (hetero)aryl halides for the synthesis of 3,3'-disubstituted oxindoles. This transformation is amenable to a wide range of (hetero)aryl halides as well as several easily accessible acrylamides, and it is also scalable to multigram synthesis. Finally, the versatility of the oxindole products is demonstrated through their conversion to a variety of useful intermediates applicable to target-directed synthesis.

NaI/PPh3-catalyzed visible-light-mediated decarboxylative radical cascade cyclization of N-arylacrylamides for the efficient synthesis of quaternary oxindoles

A practical NaI/PPh3-catalyzed decarboxylative radical cascade cyclization of N-arylacrylamides with redox-active esters is described, which is mediated by visible light irradiation. A wide range of substrates bearing different substituents and derived from ubiquitous carboxylic acids, including α-amino acids, were synthesized and examined under this very mild, efficient, and cost effective transition-metal-free synthetic method. These afforded various functionalized oxindoles featuring a C3 quaternary stereogenic center. Mechanistic experiments suggest a radical mechanism.

Photoredox aerobic oxidative cycliation of N-arylacrylamides with benzylalcohols

While the radical coupling/cyclization of N-arylacrylamides has been well established for the synthesis of functionalized oxindoles, in the present work, a visible-light-induced aerobic oxidative coupling/cyclization reaction of N-arylacrylamides with benzylalcohols has been developed. The combination of LiBr and benzophenone as the additive was found to be highly effective to enhance the catalytic efficacy. This protocol offers a mild alternative access to structurally valuable hydroxyalkyl oxindoles. Mechanistic studies reveal that the bromo radical in situ formed through the photoredox SET process under aerobic conditions enables efficient HAT to generate the key hydroxyalkyl radical intermediate.

Highly Diastereoselective Synthesis of Oxindoles Containing Vicinal Quaternary and Tertiary Stereocenters by a Domino Heck/Decarboxylative Alkynylation Sequence

A palladium-catalyzed domino Heck/decarboxylative alkynylation reaction of trisubstituted alkenes or enamines is reported. For two different types of substrates, the current domino reaction employing different solvents and bases led to 3,3-disubstituted oxindoles and hydropyrimidinyl spirooxindoles containing vicinal quaternary and tertiary stereocenters in moderate to good yields, respectively. The general applicability of this method was shown by gram-scale syntheses and diverse transformations of the reaction products. The enantioselective version for this domino process was also studied.

Alkylation/Ipso-cyclization of Active Alkynes Leading to 3-Alkylated Aza- and Oxa-spiro[4,5]-trienones

A method for the preparation of 3-alkylated spiro[4.5]trienones via alkylation/ipso-cyclization of activated alkynes with 4-alkyl-DHPs under transition-metal-free conditions is proposed. This alkylation successively undergoes the generation of alkyl radicals, addition of alkyl radicals to the alkynes, and intramolecular ipso-cyclization. The mechanism studies suggest that the alkylation/ipso-cyclization involves a radical process. This ipso-cyclization procedure shows a series of advantages, such as accessibility, mild conditions, high efficiency, greater safety, and an environmentally friendly method.

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.

Visible-light-mediated tandem phosphorylation/cyclization for the synthesis of phosphorylated oxindoles

The visible-light-mediated tandem phosphorylation/cyclization of N-arylacrylamides with H-phosphine oxides has been developed for the synthesis of phosphorylated oxindoles. This efficient and facile process was useful for the construction of a C-P bond and triggered the formation of a C-C bond with good compatibility with functional groups undermild reaction conditions.

Green and Sustainable Visible Light-Mediated Formation of Amide Bonds: An Emerging Niche in Organic Chemistry

Amide bond is one of the most fascinating functional groups in nature due to its stability, conformational diversity, high bond polarity, and abundance in numerous natural products and drug candidates,...

Visible-light-promoted radical amidoarylation of arylacrylamides towards amidated oxindoles

A visible-light-promoted intermolecular radical amidation/cyclization of arylacrylamides was realized by using N-aminopyridinium salts as the source of primary amidyl radicals. The reaction exhibits a broad scope and good functionality tolerance,...

Visible-light-induced radical cascade reaction to prepare oxindoles via alkyl radical addition to N-arylacryl amides

A photochemical approach towards oxindoles with C3 quaternary centers by the radical cascade reaction of α,β-unsaturated N-arylacryl amides with alkyl bromides or iodides upon visible light irradiation under mild reaction conditions was developed.

The N-heterocyclic carbene-catalyzed [3 + 2] annulation of isoindigos with enals: the enantioselective construction of three contiguous stereogenic centers

An NHC-catalyzed enantioselective [3 + 2] annulation of enals and isoindigo is introduced as an efficient strategy for the construction of dimeric spirocyclic bisindoline alkaloid derivatives with moderate yields and good enantioselectivities.

Visible light-mediated radical-cascade addition/cyclization of arylacrylamides with aldehydes to form quaternary oxindoles at room temperature

The visible light-induced oxidative radical cascade coupling of N-arylacrylamides with aldehydes using bromide as the hydrogen atom transfer agent to synthesize functional oxindoles is described.

Visible light mediated functionalization of allenes

This review summarizes the visible light mediated strategies for the functionalization of allenes.