Aerobic Radical-Cascade Alkylation/Cyclization of α,β-Unsaturated Amides: an Efficient Approach to Quaternary Oxindoles

Synthesis and antifungal activity of hexahydropyrrolidoindole alkaloids

Twenty-one hexahydropyrrolidoindole alkaloids were designed and synthesised via acylation reaction at the 3-N position from the commercially available indole-3-acetonitrile as the starting material in excellent yields. The effects of all target compounds against Verticillium dahlia, Fusarium oxysperium sp., Cytospora juglandis, Aspergillu sflavu, Aspergillus niger and Fusarium oxysporum were determined. The results of bioassays indicated that the majority of tested compounds displayed comparable or better in vitro bioactivity than the positive control. Notably, compounds 8 and 17 revealed potent activity against C. juglandis and A. sflavu, both with the same minimum inhibitory concentration value of 1.9 µg mL-1, which has fungicidal activity far exceeded that of amphotericin B and chlorothalonil.

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.

Pd-Catalyzed Photochemical Alkylative Functionalization of C═C and C═N Bonds

The development of excited-state palladium-catalyzed alkylative cyclization of acrylamides and the alkylation of quinoxalinones is described. The application of a variety of primary, secondary, and tertiary unactivated alkyl halides as alkyl radical precursors and the use of a simple catalyst system are the highlights of this reactivity manifold. The reactions exhibit wide scope, occur under mild conditions, and furnish the products in excellent yields.

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.

Copper-Catalyzed 1,2-Dicarbonylative Cyclization of Alkenes with Alkyl Bromides via Radical Cascade Process

Herein, we developed a new procedure on 1,2-dicarbonylative cyclization of 4-aryl-1-butenes with alkyl bromides. Using simple copper catalyst, two molecules of carbon monoxide were introduced into the double bond with the formation of four new C-C bonds and a new ring. Various α-tetralones and 2,3-dihydroquinolin-4-ones were formed in moderate to good yields.

Synthesis of 3-substituted 2-oxindoles from secondary α-bromo-propionanilides via palladium-catalyzed intramolecular cyclization

In contrast to aromatic halides, coupling reactions involving oxidative addition of alkyl halides, especially secondary or tertiary halides, to transition metals tend to be more challenging. Herein a palladium-catalyzed intramolecular cyclization of α-bromo-propionanilides has been developed, delivering a series of 3-substituted 2-oxindoles in high yields. The method features easy to prepare starting materials, broad substrate scope and excellent functional group tolerance. A detailed mechanistic investigation has been performed.

Nickel-Catalyzed Reductive Cascade Arylalkylation of Alkenes with Alkylpyridinium Salts

Herein, we describe a nickel-catalyzed reductive deaminative arylalkylation of tethered alkenes with pyridinium salts as C(sp³) electrophiles. This two-component dicarbofunctionalization reaction enables the efficient synthesis of various benzene-fused cyclic compounds bearing all-carbon quaternary centers. The approach presented in this paper proceeds under mild conditions, tolerating a wide variety of functional groups and heterocycles. It has been used to functionalize complicated molecules at a late stage.

Cu-Catalyzed alkylation-cyanation type difunctionalization of styrenes with aliphatic aldehydes and TMSCN via decarbonylation

A copper-catalyzed decarbonylative alkylation-cyanation of styrene derivatives with aliphatic aldehydes and trimethylsilyl cyanide to provide chain elongated nitriles is reported. Using TBHP as an oxidant and free radical initiator, the reaction can smoothly convert abundant α-di-substituted, α-mono-substituted and linear aliphatic aldehydes into the corresponding 3°, 2° and 1° alkyl radicals to initiate the subsequent radical-type difunctionalization of various styrenes.

Iron-catalyzed decarboxylative and oxidative decarbonylative cross-coupling: a new strategy for the synthesis of monofluoroalkenes

Herein, an iron(ii)-catalyzed decarboxylative and oxidative decarbonylative cross-coupling of α-fluoro cinnamic acids with aliphatic aldehydes is presented.

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.

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.

Photoinduced efficient synthesis of cyanoalkylsulfonylated oxindoles via sulfur dioxide insertion

A visible-light-promoted radical cascade reaction of N-arylacrylamide and cyclobutanone oxime esters with sulfur dioxide insertion is established. Mainly through the exploration of the visible light wavelength, it is found that the light source has a certain influence on the formation of cyanoalkylsulfonylated oxindoles, furnishing a range of sulfones in good to excellent yields. This protocol presents good functional group compatibility and does not require transition metals, photosensitizers, external bases, or oxidants.

Donor-Acceptor Complex Enables Cascade Radical Cyclization of N-Arylacrylamides with Katritzky Salts

Cascade radical cyclization of N-arylacrylamides is an attractive method to prepare 3,3-disubstituted oxindoles. As the reported methods often require additives and/or photocatalysts, we herein report an additive- and photocatalyst-free deaminative strategy for their synthesis under mild conditions, enabled by photoactivation of an electron donor-acceptor (EDA) complex. DFT studies indicated that the involvement of an explicit xylene solvent molecule can greatly enhance the photoactivity of the EDA complex between N-arylacrylamides and Katritzky salts.

Photoinduced Palladium-Catalyzed Intermolecular Radical Cascade Cyclization of N-Arylacrylamides with Unactivated Alkyl Bromides

A mild visible-light-induced Pd-catalyzed intermolecular radical cascade reaction of N-arylacrylamides with unactivated alkyl bromides is disclosed. Photoexcited Pd complexes transfer a single electron in this protocol, and hybrid alkyl Pd-radical species are involved as the key reaction intermediates. Sophisticated bioactive oxindole derivatives bearing various substituents and substitution patterns can be efficiently afforded through this approach.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Reagent-free aerobic oxidative synthesis of amides from aldehydes and isothiocyanates

A reagent-free autoxidative reaction of aldehydes with isothiocyanates is developed to readily access amides, involving capture of carboxylic acids in situ generated from aldehydes by isothiocyanates as both coupling mediators and amine surrogates.

Biobased Spiroimides from Itaconic Acid and Formamides: Molecular Targets for a Novel Synthetic Application of Renewable Chemicals

Spiroimides exhibit a wide range of biological activities, such as anticonvulsant, antiarrhythmic, and antihyperglycemic activities. Herein, a novel synthetic application of renewable chemicals, itaconic acid and formamides, is described. Proper exploitation of the reactivity of itaconic acid and formamide allows for the development of an efficient synthetic approach for the production of several new biobased spiroimides, spiro[dihydroquinolin-2-one-succinimides] and spiro[indolin-2-one-glutarimides], in excellent overall yields (up to 98%).

Iron-Catalyzed Oxidative Decarbonylative α-Alkylation of Acyl-Substituted Furans with Aliphatic Aldehydes as the Alkylating Agents

A protocol for FeCl₂-catalyzed oxidative decarbonylative α-alkylation of acyl furans using alkyl aldehydes as the alkylating agents has been developed. This protocol affords α-alkyl-α-acylfurans in moderate to good yields in a practical and sustainable fashion. Mechanistic studies suggest that the reaction proceeds via generation of an alkyl radical from the alkyl aldehyde, addition of the radical to the furan ring, and subsequent rearomatization.

Catalytic Asymmetric Allylic Amination with Isatins, Sulfonamides, Imides, Amines, and N-Heterocycles

A generally useful palladium-catalyzed method for the asymmetric allylic amination with a large variety of isatins, sulfonamides, imides, amines, and N-heterocycles is introduced. A single protocol with a readily available catalyst accomplishes this reaction at room temperature with high yields and enantioselectivities often exceeding 90%, which is demonstrated with 31 examples.

Palladium-Catalyzed Decarboxylative ortho-C(sp2 )-H Aroylation of N-Sulfoximine Benzamides at Room Temperature

A palladium-catalyzed method for the decarboxylative ortho C-H acylation of N-sulfoximine benzamides is developed at room temperature. The catalytic method enables easy access to various functionalized 2-aroylaromatic carboxylic acid derivatives in good isolated yields. Based on our mechanistic studies, a Pd(II)/Pd(IV) catalytic cycle that involves aroyl radical intermediate is proposed for the reaction.

Fe-catalyzed Decarbonylative Alkylative Spirocyclization of N-Arylcinnamamides: Access to Alkylated 1-Azaspirocyclohexadienones

For the convenient introduction of simple linear/branched alkyl groups into biologically important azaspirocyclohexadienones, a practical Fe-catalyzed decarbonylative cascade spiro-cyclization of N-aryl cinnamamides with aliphatic aldehydes to provide alkylated 1-azaspiro-cyclohexadienones was developed. Aliphatic aldehydes were oxidative decarbonylated into primary, secondary and tertiary alkyl radicals conveniently and allows for the subsequent cascade construction of dual C(sp3)-C(sp3) and C=O bonds via radical addition, spirocyclization and oxidation sequence.

Fe-Catalyzed decarbonylative cascade reaction of N-aryl cinnamamides with aliphatic aldehydes to construct 3,4-dihydroquinolin-2(1H)-ones

A practical Fe-catalyzed decarbonylative cascade reaction of N-aryl cinnamamides with aliphatic aldehydes to provide C3 alkylated 3,4-dihydroquinolin-2(1H)-ones is developed. Aliphatic aldehydes were oxidatively decarbonylated into 1°, 2° and 3° alkyl radicals conveniently, allowing for the subsequent cascade construction of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds via radical addition and HAS-type cyclization. The importance of the amide linkage and the selectivity of the 6-endo-trig over 5-exo-trig cyclization pathway were elucidated by experimental results and DFT calculations.

Copper-Catalyzed Alkylarylation of Unactivated Alkenes: Synthesis of 3-Alkyl Indolines from N-Allyl Anilines and Alkanes

A rare example of C(sp³ )-H functionalization of simple alkanes with unactivated alkenes is presented. In the presence of a copper salt and di-tert-butyl peroxide (DTBP), N-allyl anilines underwent exo-selective alkylation/cyclization cascade with unactivated alkenic bonds as radical acceptors and simple alkanes as radical precursors, providing a direct access to 3-alkyl indolines. The present protocol features simple operation, broad substrate scope and great exo selectivity.

Catalytic Enantioselective Ynamide Additions to Isatins: Concise Access to Oxindole Alkaloids

The highly enantioselective addition of terminal ynamides to a variety of isatins, catalyzed by a bisoxazolidine copper complex under mild, base-free reaction conditions, is described. The reaction is broad in scope, scalable, applicable to unprotected isatins, and provides efficient access to 3-hydroxyoxindoles carrying a tetrasubstituted chiral center with excellent yields and enantioselectivities. Synthetically versatile, multifunctional 3-hydroxyindolinones are obtained by hydration, partial hydrogenation, or hydroxyacyloxylation of the ynamide moiety at room temperature and exhaustive hydrogenation followed by reductive detosylation and spontaneous cyclization affords cinchonamidine alkaloids.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.

Regio- and stereo-selective decarbonylative alkylative arylation of terminal alkynes with aliphatic aldehydes and arenes via dual C–H bond functionalization

Readily available linear/branched aliphatic aldehydes including sugar derivatives were oxidatively decarbonylated into 1˙, 2˙ and 3˙ alkyl radicals for the alkylative arylation of terminal alkynes via dual C–H bond functionalization.

Co-Catalyzed decarbonylative alkylative esterification of styrenes with aliphatic aldehydes and hypervalent iodine(iii) reagents

Decarbonylation of aliphatic aldehydes into 1°, 2° and 3° alkyl radicals to construct C(sp³)–C(sp³) bond via radical addition and C(sp³)–O bond via the interconversion of Co^II–Co^III–Co^I.

Molecular Oxygen-Promoted General and Site-Specific Alkylation with Organoboronic Acid

A general alkylating method using organoboronic acid under 1 atm of oxygen is developed. It allows a facile access to a wide range of functionalized molecules with privileged scaffolds in drugs and natural products such as oxindoles, quinolinones, chromones, naphthoquinones, coumarins, and quinolones. In contrast to previous alkylation approaches that generally requiring transition-metal catalysis and a stoichiometric chemical oxidant, the present strategy features metal-free, molecular oxygen as the terminal oxidant and site specificity.

Decarbonylative Formation of Homoallyl Radical Capable of Annulation with N-Arylpropiolamides via Aldehyde Auto-oxidation

A new metal-free aldehyde auto-oxidation strategy that allows the decarbonylative formation of homoallyl radical capable of cascade annulations with alkynes is described. By using various N-arylpropiolamides, the oxidative radical [3 + 2]/[5 + 2] cascade annulation reaction was achieved to produce benzo[ b]cyclopenta[ e]azepin-4(1 H)-ones, which represent a powerful new platform for the intermolecular cycloadditions of alkyne with broad substrate scope and high selectivity.