Recent advances in 4CzIPN-mediated functionalizations with acyl precursors: single and dual photocatalytic systems

4CzIPN (1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene) has emerged as a key metal-free photocatalyst for sustainable organic synthesis. Due to its unique design enabling high photoluminescence quantum yield, thermally activated delayed fluorescence (TADF) and long excited state lifetime, 4CzIPN facilitates diverse reactions, such as C-C and C-X bond formation reactions, under mild reaction conditions. This review highlights its application in decarboxylation, acylation and cyclisation reactions involving α-keto acids, carboxylic acids and aldehydes in a single catalytic system, as well as the combination of a dual catalytic system with transition metals to enhance selectivity and scope. 4CzIPN contributes to the advancement of sustainable chemistry by enabling green, efficient and scalable reactions and this review covers studies published between 2020 and 2024.

NHPI-Mediated FeTiO3-Photocatalyzed Semiheterogeneous Decarboxylative Acylarylation of Acrylamides with α-Oxocarboxylic Acids under Nitrogen

FeTiO3 has emerged as an interesting semiconductor photocatalyst in organic synthesis. We herein describe a visible-light-induced semiheterogeneous strategy for the synthesis of 3-(2-oxoethyl)indolin-2-ones with moderate to good yields and good functional group compatibility using recyclable FeTiO3 as a photocatalyst and NHPI as a redox catalyst.

General Defluoroalkylation of Trifluoromethylarenes with Both Electron-Donating and -Withdrawing Alkenes

The incorporation of gem-difluoromethylene units into organic molecules remains a formidable challenge. Conventional methodologies for constructing aryldifluoromethyl derivatives relied on the use of high-functional fluorinating regents under harsh conditions. Herein, we report general and efficient photoredox catalytic systems for defluoroalkylation of readily available trifluoromethylarenes through selective C-F cleavage to deliver gem-difluoromethyl radicals which proceed through reductive addition to both electron-donating and withdrawing alkenes under transition-metal free conditions. Mechanistic studies reveal that thiol serves as both photocatalyst and HAT reagent under visible light irradiation. This synergistic photocatalysis and HAT catalysis protocol exhibits ample and salient features such as high chemo- and regioselectivity, broad substrate scope, amenable gram-scale synthesis and late-stage modification of bioactive molecules.

Organocatalyzed Photoelectrochemistry for the Generation of Acyl and Phosphoryl Radicals through Hydrogen Atom-Transfer Process

An organocatalyzed photoelectrochemical method for the generation of acyl and phosphoryl radicals from formamides, aldehydes, and phosphine oxides has been developed. This protocol utilizes 9,10-phenanthrenequinone (PQ) as both a molecular catalyst and a hydrogen atom-transfer (HAT) reagent, eliminating the requirement for external metal-based reagents, HAT reagents, and oxidants. The generated acyl radicals can be applied to a range of radical-mediated transformation reactions, including C-H carbamoylation of heteroarenes, intermolecular tandem radical cyclization of CF3-substituted N-arylacrylamides, as well as intramolecular cyclization reactions. The use of acyl radicals in these transformations offers an efficient and sustainable approach to accessing structurally diverse carbonyl compounds.

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.

Organoelectrophotocatalytic Generation of Acyl Radicals from Formamides and Aldehydes: Access to Acylated 3-CF3-2-Oxindoles

Organoelectrophotocatalytic generation of acyl radicals from formamides and aldehydes to synthesize acylated 3-CF3-2-oxindoles has been developed. This protocol features a monocatalytic system using 9,10-phenanthrenequinone (PQ) both as a catalyst and as a hydrogen atom transfer (HAT) reagent, which avoids the use of an external HAT reagent, metal reagent, and oxidant. A variety of acylated 3-CF3-2-oxindoles have been obtained in satisfactory yields from CF3-substituted N-arylacrylamides via a tandem radical cyclization.

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-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

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.

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.

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.

TBHP-mediated photochemical coupling/cyclization of N-arylacrylamides with thiols

The effective photoredox-mediated oxidative thiolation and cyclization of N-arylacrylamides with thiols leads to biologically interesting 3-thionated oxindoles through C-S and C-C bond formation. This process represents a straightforward reaction that starts from non-prefunctionalized thiolating reagents. Mechanistic studies demonstrated that the TBHP serves as a key radical initiator with visible-light catalysis.

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.

Organophotoredox and Hydrogen Atom Transfer Cocatalyzed C-H Alkylation of Quinoxalin-2(1H)-ones with Aldehydes, Amides, Alcohols, Ethers, or Cycloalkanes

Described is a mild method that merges organophotoredox catalysis with hydrogen atom transfer to enable C-H alkylation of quinoxalin-2(1H)-ones with feedstock aldehydes, amides, alcohols, ethers, or cycloalkanes. This reaction occurred under environmentally benign and external oxidant-free reaction conditions, providing a general and sustainable access to various C3-alkylated quinoxalinone derivatives with broad substituent diversity and good functional group compatibility.