Visible light photoredox-catalyzed arylative cyclization to access benzimidazo[2,1-a]isoquinolin-6(5H)-ones

Photo-induced decarboxylative radical cascade cyclization of unactivated alkenes: access to CF- and CF2-substituted ring-fused imidazoles

A mild and effective visible-light-induced decarboxylative radical cascade reaction of olefin-containing imidazoles with α-fluorinated carboxylic acids as building blocks containing CF or ArCF2 moieties, has been developed to afford a series of monofluoromethylated or aryldifluoromethylated polycyclic imidazoles in medium to excellent yields with features of simple operation, available raw materials, and wide substrate scopes. In addition, the mechanistic experiments indicated that the methodology involved a radical pathway.

Photoinduced Vicinal Difunctionalization of Diaryliodonium Salts To Access Bis(tetraphenylphosphonium) Salts

Vicinal bis(tetraarylphosphonium) salts have scarcely been reported in the literature. In this study, we demonstrate that visible-light-induced difunctionalization of ortho-trifluoromethylsulfonylated diaryliodonium salts conveniently furnishes bis(phosphonium) salts without additional catalysts or photoinitiators. The methodology establishes a practical platform for the preparation of bis(phosphonium) salts using readily available tertiary phosphines. The bis(tetraarylphosphonium) salts are anticipated to garner a great deal of interest in catalytic and medicinal chemistry.

Visible-Light-Mediated Cascade 1,4-Hydrogen Atom Transfer versus Dearomative Spirocyclization of N-Benzylacrylamides: Divergent Access to Functionalized γ-Ketoamides and 2-Azaspiro[4.5]decanes

Visible-light-driven metal- and photocatalyst-free cascade 1,4-HAT and dearomative spirocyclization of N-benzylacrylamides are described for sustainable synthesis of a variety of pharmaceutically important γ-ketoamides and 2-Azaspiro[4.5]decanes in one pot in good to excellent yields. Readily accessible and nontoxic materials, expensive Ir or Ru photocatalyst-free mild conditions, excellent functional group tolerance, operational simplicity, and scalability enhance the practical value of this protocol. Mechanistic studies reveal that acyl radicals generated from α-oxocarboxylic acids trigger the rare 1,4-HAT and dearomative spirocyclization. The synthetic potential of this environmentally benign method is further showcased by late-stage functionalization of drug molecules, amino acid, and peptides.

Photoredox-catalysed radical difluoromethylation/cyclization of N-acryloyl-2-arylbenzimidazole to access CF2H-substituted benzimidazo[2,1-a]isoquinolin-6(5H)-ones

An efficient visible-light-promoted cascade difluoromethylation/cyclization reaction to access various CF2H-substituted benzimidazo[2,1-a]isoquinolin-6(5H)-ones was developed using difluoromethyltriphenylphosphonium bromide salt as the precursor of the -CF2H group under mild conditions. This protocol utilized an easily accessible and inexpensive organophotocatalyst, offering the benefits of a broad substrate scope, good functional group tolerance, and good to excellent yields, in addition to a simple operational procedure. Furthermore, the reaction mechanism was subjected to investigation, and it was demonstrated that a radical pathway constitutes a single electron transfer (SET) procedure.

Photoredox-catalyzed arylative and aryl sulfonylative radical cascades involving diaryliodonium reagents: synthesis of functionalized pyrazolones

We disclose a photoredox-catalyzed arylative radical cascade between N'-arylidene-N-acryloylhydrazides and diaryliodonium reagents to obtain the corresponding benzylated pyrazolones in good yields. The protocol was extended to three-component coupling involving the 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) (DABSO) adduct as a sulfur dioxide surrogate for the synthesis of arylsulfonylated pyrazolones. Both reactions exhibit broad scope, scalability, and high functional group tolerance.

Overriding Cage Effect in Electron Donor-Acceptor Photoactivation of Diaryliodonium Reagents: Synthesis of Chalcogenides

In recent times, diaryliodonium reagents (DAIRs) have witnessed a resurgence as arylating reagents, especially under photoinduced conditions. However, reactions proceeding through electron donor-acceptor (EDA) complex formation with DAIRs are restricted to electron-rich reacting partners serving as donors due to the well-known cage effect. We discovered a practical and high-yielding visible-light-induced EDA platform to generate aryl radicals from the corresponding DAIRs and use them to synthesize key chalcogenides. In this process, an array of DAIRs and dichalcogenides react in the presence of 1,4 diazabicyclo[2.2.2]octane (DABCO) as a cheap and readily available donor, furnishing a variety of di(hetero)aryl and aryl/alkyl chalcogenides in good yields. The method is scalable, features a broad scope with good yields, and operates under open-to-air conditions. The photoinduced chalcogenation technology is suitable for late-stage functionalizations and disulfide bioconjugations and facilitates access to biologically relevant thioesters, dithiocarbamates, sulfoximines, and sulfones. Moreover, the method applies to synthesizing diverse pharmaceuticals, such as vortioxetine, promazine, mequitazine, and dapsone, under amenable conditions.

Visible Light-Induced Regioselective E to Z Isomerization of Polarized 1,3-Dienes: Experimental and Theoretical Insights

The stereocontrol of E → Z isomerization on a (1E,3E)-diene, instead of a simple alkene, can be more challenging due to the increased number of isomerization possibilities. Herein, we report visible light-mediated regioselective (1E,3E → 1E,3Z) isomerization of (1E,3E)-diene. The reaction conditions are mild and easy to apply and can be applied to a wide range of substances, with an excellent yield and selectivity (90:10). It is evident from the crystal structures that the cause of regioselectivity for the isomerization of 1,3-diene may not be limited to 1,3-allylic strains; CH-π interaction may also play a vital role. Computational studies suggest that this regioselective photoisomerization is a thermodynamically feasible process and requires the accumulation of spin density on the modified double bond for the transformation to occur.

Non-directed oxidative annulation of 2-arylindazoles with electron deficient olefins via visible light photocatalysis

A new visible-light-mediated non-directed oxidative annulation between 2-arylindazoles and electron-deficient olefins using commercially available piperidine-1-sulfonyl chloride as the radical precursor to afford fused 5,6-dihydroindazolo[2,3-a]quinolines has been developed under mild reaction conditions. This transformation occurs via two consecutive C-H bond functionalizations. The mechanistic investigation results indicate that the reaction progresses through a radical pathway forming a 2-(2-aryl-2H-indazol-3-yl)-3-piperidin-1-ylsulfonyl derivative as an intermediate.

Visible light-mediated difluoromethylation/cyclization in batch and flow: scalable synthesis of CHF2-containing benzimidazo- and indolo[2,1-a]isoquinolin-6(5H)-ones

We report here a practical and cost-effective method for the synthesis of CHF2-containing benzimidazo- and indolo[2,1,a]-isoquinolin-6(5H)-ones through a visible light-mediated difluoromethylation/cyclization cascade. The method, which affords functionalized multifused N-heterocyclic scaffolds in moderate to high yields under mild reaction conditions, is also easily scalable using low-cost 3D printed photoflow reactors.

Visible Light-Induced Radical Cascade Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Polycyclic Imidazoles

An efficient and mild protocol for the visible light-induced radical cascade difluoromethylation/cyclization of imidazoles with unactivated alkenes using easily accessible and bench-stable difluoromethyltriphenylphosphonium bromide as the precursor of the -CF2H group has been developed to afford CF2H-substituted polycyclic imidazoles in moderate to good yields. This strategy, along with the construction of Csp3-CF2H/C-C bonds, is distinguished by mild conditions, no requirement of additives, simple operation, and wide substrate scope. In addition, the mechanistic experiments have indicated that the difluoromethyl radical pathway is essential for the methodology.

Exogenous photocatalyst-free aryl radical generation from diaryliodonium salts and use in metal-catalyzed C-H arylation

We demonstrate (1) detectable halogen bonding is not critical for enabling light-driven radical generation from diaryliodonium salts and (2) radicals generated by this route can be captured by transition-metals for C-H arylation reactions. These results are the first step toward developing new metal-catalyzed aryl radical couplings without exogenous photocatalysts.

Visible Light-Induced Imide Alkylation of Azauracils with Aryl Diazoesters

A visible light-induced green and sustainable N-H functionalization of (aza)uracils with α-diazo esters leading to imide alkylation is described. The reaction does not require any catalyst or additive and proceeds under mild conditions. Moreover, an intriguing three component coupling was observed when (aza)uracils were allowed to react with α-diazo esters in cyclic ethers (e. g. 1,4-dioxane, THF) as a solvent. Both the insertion and three-component coupling features broad scope with good to excellent yields and appreciable functional group tolerance. Notably, the divergent method enables modification of natural products and pharmaceuticals, thereby facilitates access to potentially biologically active compounds.

A General Electron Donor-Acceptor Photoactivation Platform of Diaryliodonium Reagents: Arylation of Heterocycles

We report a photoredox system comprising sodium iodide, triphenyl phosphine, and N,N,N',N'-tetramethylethylenediamine (TMEDA) that can form a self-assembled tetrameric electron donor-acceptor (EDA) complex with diaryliodonium reagents (DAIRs) and furnish aryl radicals upon visible light irradiation. This practical mode of activation of DAIRs enables arylation of an array of heterocycles under mild conditions to provide the respective heteroaryl-(hetero)aryl assembly in moderate to excellent yields. Detailed mechanistic investigations comprising photophysical and DFT studies provided insight into the reaction mechanism.

Organophotoredox-Catalyzed Arylation and Aryl Sulfonylation of Morita-Baylis-Hillman Acetates with Diaryliodonium Reagents

We report an organophotoredox-catalyzed stereoselective allylic arylation of MBH acetates with a palette of diaryliodonium triflates (DAIRs) to provide the corresponding trisubstituted alkenes in moderate to good yields. The method could be extended to three-component coupling involving 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) adduct (DABSO) as a sulfur dioxide surrogate for the synthesis of biologically relevant allylic sulfones. Both of these reactions were carried out under mild conditions featuring broad scope, robustness, and appreciable functional group tolerance.

Visible-Light-Induced Metal- and Photocatalyst-Free Radical Cascade Cyclization of Cinnamamides for Synthesis of Functionalized Dihydroquinolinones

Visible-light-promoted metal- and photocatalyst-free radical cascade cyclization of cinnamamides with α-oxocarboxylic acids is described for sustainable synthesis of diverse pharmaceutically important dihydroquinolinone scaffolds in one pot under mild conditions. The decarboxylative cascade cyclization proceeded efficiently at room temperature without the need for expensive photocatalysts such as Ir or Ru complexes, which indicates the practicability and environmentally benign nature of this protocol. Preliminary mechanistic studies reveal that the blue LED irradiation efficiently cleaves the I-O bond of the hypervalent iodine reagent PhI(O₂CCOAr)₂ formed through ligand exchange between iodobenzene diacetate and arylglyoxylic acid to initiate the cascade reaction. The synthetic value of this operationally simple and energy-efficient method is further demonstrated by late-stage functionalization of drug molecules in excellent yields.