Visible-Light-Induced Regioselective Alkylation of Coumarins via Decarboxylative Coupling with N-Hydroxyphthalimide Esters

Halogen-Bond-Promoted Direct Cross-Coupling of Trifluoromethylated Alkyl Bromides with Coumarins/Quinolinones: Unraveling Trifluoromethyl Effects

This study introduces a novel approach involving XB-mediated cross-coupling of α-trifluoromethylated alkyl bromides with coumarins and quinolinones under visible light irradiation. Both density functional theory (DFT) calculations and experimental studies converge to suggest that the noncovalent interaction between alkyl bromides and DMAP, intensified by the α-trifluoromethyl group, plays a pivotal role in facilitating this chemoselective reaction.

Photo-Induced C-H Methylation Reactions

Direct C-H methylation is a highly valuable approach for introducing methyl groups into organic molecules, particularly in pharmaceutical chemistry. Among the various methodologies available, photo-induced methylation stands out as an exceptional choice due to its mild reaction conditions, energy efficiency, and compatibility with functional groups. This article offers a comprehensive review of photochemical strategies employed for the direct and selective methylation of C(sp3 )-H, C(sp2 )-H, and C(sp)-H bonds in various organic molecules. The discussed methodologies encompass transition-metal-based photocatalysis, organophotocatalysis, as well as other metal-free approaches, including electron donor-acceptor (EDA)-enabled transformations. Importantly, a wide range of easily accessible agents such as tert-butyl peroxide, methanol, DMSO, methyl tert-butyl ether, TsOMe, N-(acetoxy)phthalimide, acetic acid, methyl halides, and even methane can serve as effective methylating reagents for modifying diverse targets. These advancements in photochemical C-H methylation are anticipated to drive further progress in the fields of organic synthesis, photocatalysis, and pharmaceutical development, opening up exciting avenues for creating novel organic molecules and discovering new drug compounds.

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.

Photosynthesis of 3-Alkylated Coumarins from Carboxylic Acids Catalyzed by a Na2S-Based Electron Donor-Acceptor Complex

A simple and practical electron donor-acceptor (EDA) strategy to synthesize various 3-alkylated coumarins from easily available coumarins and naturally abundant carboxylic acids under photocatalyst-, oxidant-, and additive-free and mild conditions is reported. Using Na₂S as the catalytic electron donor, a series of primary, secondary, and tertiary carbon radicals can be efficiently generated, and the EDA complex can be regenerated without an alkaline additive.

Visible-Light-Mediated Vicinal Difunctionalization of Activated Alkynes with Boronic Acids: Substrate-Controlled Rapid Access to 3-Alkylated Coumarins and Unsaturated Spirocycles

A visible-light-mediated difunctionalization of activated alkynes with boronic acid is unveiled to synthesize 3-alkylated coumarins and unsaturated spiro-lactones. The substituent at the para-position of the aryl ring of aryl alkynoate plays a pivotal role in the selective formation of chain-alkylated coumarins or spirocyclic compounds under mild conditions. The reaction employs a hypervalent iodine reagent and ruthenium photocatalyst. The spiro-lactones thus obtained were subjected to another novel mode of visible-light-mediated radical addition cascade cyclization (RACC) to access various new fused spirocyclic compounds.

Photoredox-Catalyzed Tandem Radical Cyclization/Hydroxylation for the Synthesis of 4-Hydroxyalkyl-3,3-difluoro-γ-lactams

The photoredox-catalyzed radical difluoroalkylation/cyclization/hydroxylation cascade reaction of various 2-bromo-2,2-difluoro-N-arylacetamides containing unactivated alkene moieties has been developed, providing green and efficient access to various 4-hydroxyalkyl-3,3-difluoro-γ-lactams. Control experiments confirmed a radical process, and inexpensive air acted as the sole hydroxy resource. In addition, the highlights of this protocol include good tolerance for a variety functional groups, lower photocatalyst loading, and ease of operation.

Recent developments in the synthesis of the isoquinoline-1,3(2H,4H)-dione by radical cascade reaction

In recent years, isoquinoline-1,3(2H,4H)-dione compounds have attracted extensive attention from synthetic chemists, with the aim of finding simple, mild, green and efficient synthetic methods. In this review, we summarize the diverse range of synthetic methods employing acryloyl benzamides as key substrates to furnish isoquinoline-1,3-diones using different radical precursors, such as those containing carbon, sulphur, phosphorus, nitrogen, silicon and bromine. This will stimulate the interest of readers to engage in research in this field.

Cobalt-catalyzed synthesis of aryl ketones and aldehydes from redox-active esters

A cobalt-catalyzed decarboxylative oxidation of benzylic redox-active esters is described. This protocol efficiently converts secondary or primary aliphatic carboxylic acids into aromatic ketones or aldehydes. A wide range of substrates selectively reacted in good to excellent yields with broad functional group tolerance. Notably, various biologically active molecules could also work well, which indicated the synthetic application of such a methodology.

Plant Growth Suppressive Activity of (R)-3-(7'-Aryl-9'-hydroxyprop-8'-yl)coumarin, Structural Isomer of Z-2-Hydroxybenzylidene-γ-butyrolactone-type Lignan

3-(7'-Aryl-9'-hydroxyprop-8'-yl)coumarin, which is a structural isomer of a Z-2-hydroxybenzylidene-γ-butyrolactone-type lignan, was stereoselectively synthesized and subjected to plant growth regulation examination. (R)-4'-Methoxyphenyl derivative 3 showed stereospecific plant growth suppressive activity. The significance of the presence of hydroxy group at the 9'-position for the activity was clarified. The effect of the substituent at the 7'-aryl group was also shown. The 3'-methoxy, 4'-methoxy, and 4'-trifluoromethyl derivatives 10, 3, and 22 led to the most significant growth suppression of Italian ryegrass roots. The 2'-methoxy derivative 9 and 4'-methoxy derivative 3 provided the most growth suppressive activity against lettuce shoots and roots, respectively.

Selective Mono- and Diamination of Ketones in a Combined Copper-Organocatalyst System

Herein, we report a simple and mild protocol for the chemoselective mono- and diamination of ketone using pyrazole as the amine source in a combined copper-organocatalyst system. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient solution for the synthesis of α-pyrazole and α,α-dipyrazole ketone derivatives. The control experiment demonstrates that in situ generated hydrazone is a key intermediate in the transformation.

An Earth-abundant cobalt based photocatalyst: visible light induced direct (het)arene C-H arylation and CO2 capture

In this work, we have reported a noble metal free heterogeneous photocatalyst to carry out direct (het)arene C-H arylation and solvent-free CO₂ capture via single-electron transfer processes at room temperature and under pressure. The catalytic system comprises a cobalt(III) complex grafted over the silica coated magnetic support for the efficient recovery of the photocatalytic moiety without hampering its light-harvesting capability. The novel Earth-abundant cobalt(III) based photocatalyst possesses various fascinating properties such as high surface area to volume ratios, large pore volume, crystalline behaviour, high metal loading, excellent stability and reusability. The general efficacy of the highly abundant and low-cost cobalt based heterogeneous nanocatalyst was checked for the selective conversion of aryldiazonium salts into synthetically and pharmaceutically significant biaryl motifs under ambient conditions upon irradiation with visible light. The highly efficient photocatalytic conversion of carbon dioxide (CO₂) to a value-added chemical was accomplished under mild reaction conditions with high selectivity, showing the added benefit of operational simplicity.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Electron donor–acceptor complex-catalyzed photoredox reactions mediated by DIPEA and inorganic carbonates

A DIPEA–NHPI ester–inorganic carbonate catalytic EDA complex is reported as an efficient and sustainable radical generation platform for developing photocatalytic reactions.

A visible-light-induced photocatalyst-free approach for C-3 dicarbonyl coumarin production

A mild and efficient visible-light-induced synthesis of C-3 dicarbonyl coumarins from 3-arylacetylene coumarins without a photocatalyst was reported. This iodide-mediated method exhibited broad substrate scope and good functional group tolerance, and a series of C-3 dicarbonyl coumarins were obtained in moderate to excellent yields. Based on the control experimental results, it was found that the visible-light-induced oxidation might be via both radical and ionic processes. Moreover, some synthesized compounds displayed high sensitivity to hydrogen peroxide (H₂O₂) with a low detection limit (DL, down to 0.149 μM).

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Visible-Light-Induced C(sp2)-C(sp3) Coupling Reaction for the Regioselective Synthesis of 3-Functionalized Coumarins

A photocatalysis strategy for the regioselective synthesis of 3-functionalized coumarins is reported. With visible light irradiation, a direct and regioselective C(sp²)-C(sp³) coupling reaction of 3-(2-hydroxyphenyl)acrylates with ethers or thioethers occurs by using Ru(bpy)₃Cl₂·6H₂O as a photocatalyst and TBHP as an oxidant. The cascade process involves alkenylation of the C(sp³)-H bond of ethers and lactonization, furnishing 3-alkylated coumarins as the final products. This approach is characterized by a broad substrate scope, mild reaction conditions, and simplified operation. The synthesis of 3-alkylated coumarins could be realized by a one-pot procedure, starting from commercially available salicylaldehyde.

Photoinduced homolytic decarboxylative acylation/cyclization of unactivated alkenes with α-keto acid under external oxidant and photocatalyst free conditions: access to quinazolinone derivatives

A novel and green strategy for the synthesis of acylated quinazolinone derivatives via photo-induced decarboxylative cascade radical acylation/cyclization of quinazolinone bearing unactivated alkenes has been developed. The protocol provides a novel route to access acyl radicals from α-keto acids through a self-catalyzed energy transfer process. Most importantly, the reaction proceeded smoothly without any external photocatalyst, additive or oxidant, and could be easily scaled-up in flow conditions with sunlight irradiation.

Metal-Free Direct C-H Functionalization of Quinoxalin-2(1H)-Ones to Produce 3-Vinylated Quinoxalin-2(1H)-Ones in the Presence of Alkenes

A novel and efficient C 3-H vinylation reaction with quinoxalin-2(1H)-one as the substrate, in the presence of alkenes, under metal-free conditions, is reported herein. The reaction leads to the formation of new carbon-carbon bonds that exhibit moderate to good reactivities. The vinylation of quinoxalin-2(1H)-ones, in the presence of alkenes, is an attractive process that can be potentially utilized to produce biologically active 3-vinylated quinoxalin-2(1H)-ones.

Ir-Porphyrin-Based Metal-Organic Framework as a Dual Metallo- and Photocatalyst for Inert Alkyl C(sp3)H Bond Activation and Direct Functionalization

The activation and transformation of inert alkyl C(sp³)-H bonds to obtain high-value fine chemicals by sustainable solar energy are of great significance. Herein, by incorporating Ir^III-porphyrin into metal-organic frameworks (MOFs) to stabilize the highly active carbene, we reported a new approach to combining metallo- and photocatalysis to efficiently accelerate carbene migratory insertion and C-H bond activation via the radical coupling pathway for inert alkane functionalization. The in situ-formed carbene was restricted into the pores of MOFs to produce Ir^III-carbene, allowing the first-time isolation and structural characterization of the Ir^III-carbene intermediate which are not stabilized by a heteroatom. The product of the reaction, especially the cyclic ethers as substrates, suggested that the functionalization of the α position of the alkoxy group was favored. Additionally, the new approach could be extended to stabilize the metal carbene intermediates to realize C(sp³)-H bond alkylation and arylation.

Photocatalytic transition-metal-free direct 3-alkylation of 2-aryl-2H-indazoles in dimethyl carbonate

A general transition-metal-free photocatalytic decarboxylative 3-alkylation reaction of 2-aryl-2H-indazoles was developed under visible-light irradiation under mild conditions.

An ultrastable olefin-linked covalent organic framework for photocatalytic decarboxylative alkylations under highly acidic conditions

An ultrastable olefin-linked covalent organic framework 2D-COF-2 offers an alternative heterogeneous photocatalyst for photocatalytic decarboxylative alkylations, exhibiting impressive effciency, sustainabilty and promising industrial potential.

Photocatalytic intramolecular radical cyclization involved synergistic SET and HAT: synthesis of 3,3-difluoro-γ-lactams

A mild and metal-free protocol for visible-light induced intramolecular radical cyclization of N-allyl(propargyl)-2-bromo-2,2-difluoro-N-arylacetamide has been developed.

Fe(III)-Catalyzed Decarboxylative C3-Difluoroarylmethylation of Coumarins with α,α-Difluoroarylacetic Acids

A facile Fe(III)-catalyzed oxidative decarboxylative radical coupling reaction of α,α-difluoroarylacetic acids with coumarins has been developed. This transformation, which provides a series of C-3 difluoroarylmethylated coumarins containing various functional groups in moderate-to-good yields, features easily accessible starting materials and operational simplicity.

Metal-free regioselective C-H chalcogenylation of coumarins/(hetero)arenes at ambient temperature

A novel, practical and metal-free approach for the regioselective selenation of coumarins employing (bis(trifluoroacetoxy)iodo)benzene (PIFA) at room temperature is presented. The developed method is suitable for a wide substrate scope and affords 3-selenyl coumarins in good to excellent yields with high selectivity. A radical mechanism is proposed for this new transformation. Furthermore, the application of sulfenylation with coumarines and selenation with other (hetero)arenes in this transformation is successful.