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

Photoredox nickel-catalyzed radical cyclization of N-arylacrylamides with alkyl bromides

3,3-Disubstituted oxindoles constitute a significant class of biologically active molecules, often found in a wide range of bioactive compounds. In this work, we present a photoredox nickel-catalyzed intermolecular cyclization between N-arylacrylamides and readily accessible alkyl bromides, which affords a diverse range of 3,3-disubstituted oxindoles in moderate to high yields. Our mechanistic studies reveal that the reduction of alkyl bromides via single-electron transfer from a reactive Ni(I) species is a critical step in driving this radical cascade transformation. This approach offers several advantages, including mild reaction conditions, broad functional group tolerance, and a simple workup procedure.

Photoinduced radical cyclization reaction of isocyanides with α-carbonyl bromides to access 11-alkyl-substituted 1,4-dibenzodiazepines

1,4-Dibenzodiazepines, an important component of nitrogen-containing heterocycles, are widely present in drugs. Herein, we developed a photochemical radical cascade cyclization reaction of isocyanides with α-carbonyl bromides under mild conditions. A sequence of 11-alkyl-substituted 1,4-dibenzodiazepines were produced in 53%-85% yields, demonstrating excellent tolerance towards various functional groups. Primary, secondary, and tertiary α-carbonyl bromides were effectively employed as alkyl reagents.

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.

Alcohols as Alkyl Electrophiles for Deoxygenative Heck Reaction Enabled by Excited State Pd Catalysis

Here, we present a general method for the photoinduced Pd-catalyzed deoxygenative Heck reaction of vinyl arenes with ortho-iodophenyl-thionocarbonate derived from alcohols. Mechanistic studies reveal that the deoxygenation involves a 5-endo-trig cyclization and fragmentation process, with radical addition identified as the rate-determining step in this transformation. This one-pot procedure demonstrates excellent selectivity for less hindered hydroxyl groups in diols, facilitating late-stage functionalization of complex molecules and scalability to gram-scale synthesis. The protocol highlights significant synthetic potential and can be extended to the cascade 1,1-difunctionalization of isocyanides and the intermolecular radical cascade cyclization of N-arylacrylamides.

Photoinduced Palladium-Catalyzed 1,2-Aminoalkylation of Aromatic Alkenes with Hydroxyl as the Directing Group

The hybrid nature of Pd(I)-alkyl radical species has enabled a wide array of radical-based transformations. However, in this transformation, the secondary Pd(I)-alkyl radical species are prone to recombining into Pd(II)-alkyl species to give Heck-type products via β-H loss. Herein, we report a visible-light-induced, three-component Pd-catalyzed 1,2-aminoalkylation of alkenes with readily available alkyl halides and amines to construct C-C and C-N bonds simultaneously. Mechanistic investigation shows that the intermediate of o-quinone methide produced is the key factor in the transformation.

Recent Advances in Visible Light Induced Palladium Catalysis

Visible light-induced Pd catalysis has emerged as a promising subfield of photocatalysis. The hybrid nature of Pd radical species has enabled a wide array of radical-based transformations otherwise challenging or unknown via conventional Pd chemistry. In parallel to the ongoing pursuit of alternative, readily available radical precursors, notable discoveries have demonstrated that photoexcitation can alter not only oxidative addition but also other elementary steps. This Minireview highlights the recent progress in this area.

Photochemical Radical Bicyclization of 1,5-Enynes: Divergent Synthesis of Fluorenes and Azepinones

A dual nickel- and iridium-photocatalyzed radical cascade bicyclization reaction for the synthesis of highly complex molecular structures in an atom- and step-economic manner has been described. A series of radical precursors are utilized for the divergent synthesis of diversely substituted fluorenes and indenoazepinones bearing quaternary carbons by using cascade cyclization reactions of 1,5-enynes. This reaction is characterized by its mild conditions, broad substrate scope, excellent selectivity, and satisfactory yield including facile scale-up synthesis.

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.

Unified Approach to Diverse Heterocyclic Synthesis: Organo-Photocatalyzed Carboacylation of Alkenes and Alkynes from Feedstock Aldehydes and Alcohols

We report an organo-photocatalyzed carboacylation reaction that offers a springboard to create chemical complexity in a diversity-driven approach. The modular one-pot method uses feedstock aldehydes and alcohols as acyl surrogates and commercially available Eosin Y as the photoredox catalyst, making it simple and affordable to introduce structural diversity. Several biologically relevant skeletons have been easily synthesized under mild conditions in the presence of visible light irradiation by fostering a radical acylation/cyclization cascade. The proposed reaction mechanism was further illuminated by a number of spectroscopic studies. Furthermore, we applied this protocol for the late-stage functionalization of pharmaceuticals and blockbuster drugs.

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.

Photocatalytic Alkylation/Arylative Cyclization of N-Acrylamides of N-Heteroarenes and Arylamines with Dihydroquinazolinones from Unactivated Ketones

We describe a visible-light photoredox-catalyzed alkylation/arylative cyclization of N-acrylamides─from 2-arylindoles, 2-arylbenzimidazoles, or N-substituted anilines─with ketone-derived dihydroquinazolinones, accessing indolo- and benzimidazolo[2,1-a]isoquinolines or 2-oxindoles. The consecutive incorporation of alkyl- and aryl-carbogenic motifs across a C=C bond via formal cleavage of ketone α-C-C and arene C-H bonds leads to the formation of five- and six-membered rings, with an all-carbon quaternary stereocenter. This dicarbofunctionalization elaborates aromatization-driven radical C-C functionalization of unactivated aliphatic ketones to construct diverse cyclic structures with functionality tolerance.

Visible-Light-Mediated Photoredox Carbon Radical Formation from Aqueous Sulfoxonium Ylides

The visible-light-induced photoredox carbon radical formation from aqueous sulfoxonium ylides has been demonstrated for the first time. While direct reduction of sulfoxonium ylides by H₂O efficiently generates the corresponding hydrocarbon compounds, the use of additional alkenes as radical acceptors alters the chemical reactivity to achieve alkene carboarylation of N-arylacrylamides. Mechanistic studies reveal two different reaction pathways involved in the carbon radical formation from aqueous sulfoxonium ylides resulting in reduction to release dimethyl sulfone and carboarylation to form DMSO.

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.

Highly efficient synthesis of C3-heteroaryl 3-fluorooxindoles via a one-pot stepwise Ce(iii)/photoassisted cross-dehydrogenative coupling/fluorooxidation process

A one-pot stepwise strategy has been developed to afford C3-heteroaryl 3-fluorooxindoles via a Ce(iii)/photoassisted cross-dehydrogenative coupling/fluorooxidation process in moderate-to-good yields with excellent functional group compatibility.

Visible-Light-Driven Palladium-Catalyzed Radical Tandem Dearomatization of Indoles with Unactivated Alkenes

A mild visible-light-driven palladium-catalyzed radical tandem dearomatization of indoles with unactivated alkenes is described with moderate to good yields and good to excellent diastereoselectivities. Under visible-light irradiation, the photoexcited state of the palladium complex was formed, which could transfer a single electron to N-(2-bromobenzoyl)indoles, leading to a hybrid palladium radical chemistry. This provides efficient and atom-economical access to diverse 2,3-disubstituted indoline derivatives.

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.

Discovery of an Oxidative System for Radical Generation from Csp3-H Bonds: A Synthesis of Functionalized Oxindoles

A facile and versatile method for generating radicals from Csp³-H bonds under metal-free and organic-peroxide-free conditions was developed. By combining safe persulfate and low-toxic quaternary ammonium salt, a wide variety of Csp³-H compounds including ethers, (hetero)aromatic/aliphatic ketones, alkylbenzenes, alkylheterocycles, cycloalkanes, and haloalkanes were selectively activated to generate the corresponding C-centered radicals, which could be further captured by N-arylacrylamides to deliver the valuable functionalized oxindoles. Good functional group tolerance was demonstrated. The useful polycarbonyl compound and esters were also modified with the strategy. Moreover, the combination can also be applied to the practical coupling between simple haloalkanes and N-hydroxyphthalimide (NHPI).

Unactivated Alkyl Chloride Reactivity in Excited-State Palladium Catalysis

Excited-state palladium catalysis is an efficient process for the alkylation of diverse organic compounds via the generation of alkyl radicals from alkyl bromides and iodides. However, the generation of alkyl radicals from more stable alkyl chlorides remains challenging. Herein, we demonstrate the excited-state palladium-catalyzed synthesis of oxindoles and isoquinolinediones via alkylation/annulation reaction by overcoming inherent limitations associated with unactivated C(sp³)-Cl bond activation at room temperature.