Aerobic Transition-Metal-Free Visible-Light Photoredox Indole C-3 Formylation Reaction

Modulating confinement space in metal-organic frameworks enables highly selective indole C3-formylation

Here, Selective C3-formylation of indole was achieved under mild conditions using a metal-organic framework (MOF) catalyst. The confined reaction space within the MOF pores effectively suppressed undesired side reactions and promoted the formation of the targeted product by controlling the reaction pathway. Density functional theory (DFT) calculations corroborated the experimental observations.

TBHP/Et3N-Promoted Chemoselective Formylation and Peroxidation of Pyrrolo[2,1-a]isoquinolines

An efficient formylation of pyrrolo[2,1-a]isoquinoline derivatives has been reached by the use of TBHP (tBuOOH) and Et3N as the mediator. In this strategy, CHO and CDO can be readily incorporated into heteroarenes by the utilization of CHCl3 and CDCl3 as the carbonyl sources. Interestingly, a solvent-controlled chemoselectivity was observed. The use of PhCl as a solvent resulted in dearomatization and peroxidation of pyrrolo[2,1-a]isoquinolines, delivering functionalized peroxides in 53-64% yields.

Metal-free visible light mediated direct C-H amination of benzoxazole with secondary amines

An efficient visible light mediated, eosin Y catalyzed direct C-H oxidative amination of benzoxazoles with secondary amines has been developed, which providing a straightforward, green, and environmentally benign access to a wide variety of substituted benzoxazole-2-amines under mild reaction conditions. The biological studies such as drug-likeness and molecular docking are also carried out on the molecule.

HAT-Mediated Electrochemical C(sp2)-H Acylation of Quinolines with Alcohols

Herein, an electrochemical hydrogen atom transfer (HAT) strategy for C(sp2)-H formylation of electron-deficient quinolines and isoquinolines is described. The cheap methanol acts as a formyl source with a catalytic amount of N-hydroxyphthalimide (NHPI) as the hydrogen atom transfer (HAT) catalyst. The advantages of this reaction are transition-metal-catalyst- and chemical-oxidant-free conditions, and the protocol could also be applied to the direct C(sp2)-H acetylation or propionylation of quinolines.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Visible Light Induced C-H/N-H and C-X Bonds Reactions

Herein, we report efficient visible light-induced photoredox reactions of C–H/N–H and C–X Bonds. These methods have provided access to varied portfolio of synthetically important γ-ketoesters, azaspirocyclic cyclohexadienones spirocyclohexadienones, multisubstituted benzimidazole derivatives, substituted N,2-diarylacetamide, 2-arylpyridines and 2-arylquinolines in good yields and under mild conditions. Moreover, we have successfully discussed the construction through visible light-induction by an intermolecular radical addition, dearomative cyclization, aryl migration and desulfonylation. Similarly, we also spotlight the visible light-catalyzed aerobic C–N bond activation from well-known building blocks through cyclization, elimination and aromatization. The potential use of a wide portfolio of simple ketones and available primary amines has made this transformation very attractive.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Synthesis of C-1 Deuterated 3-Formylindoles by Organophotoredox Catalyzed Direct Formylation of Indoles with Deuterated Glyoxylic Acid

Direct formylation of feedstock indoles with newly developed, cost-effective deuterated glyoxylic acid as formylation agent under visible light and air (O₂) as terminal oxidant has been developed. An isatin byproduct produced from the corresponding indole reactant serves as a facilitator for the formylation process. The simple, mild, metal- and oxidant-free protocol enables the synthesis of structurally diverse C1-deuterated 3-formylindoles with broad functional group tolerance and late-stage functionalization at a high level of D-incorporation (95-99%).

Visible Light Promoted Indole C-3 Formylation Using Eosin Y as a Photoredox Catalyst

A visible-light-mediated C-3 formylation of indole catalyzed by eosin Y has been developed using tetramethylethylenediamine as a carbon source and air as an oxidant. This protocol shows high tolerance to a large quantity of functional groups under mild conditions, and providing 3-formylated indoles with good yields. This method is a highly attractive alternative to the approach of traditional formylation.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Synthesis of methylene-bridged α,β-unsaturated ketones: α-Csp3-H methylenation of aromatic ketones using Selectfluor as a mild oxidant

A three starting material four component reaction (3SM-4CR) is developed for the synthesis of α,β-unsaturated ketones and β-amino ketones in good yields. The reaction employs tetramethylethylenediamine (TMEDA) as a methylene and terminal olefin source, and Selectfluor as a mild oxidant. TMEDA worked as a dual synthon to provide two carbons in this metal-free transformation process. The scope and versatility of the methods have been demonstrated with 23 examples. A Selectfluor-promoted oxidative reaction mechanism is proposed based on the results of the experimental studies.

Formylation and Bromination of Pyrrolo[2,1-a]isoquinoline Derivatives with Bromoisobutyrate and Dimethyl Sulfoxide

We have developed an efficient formylation of pyrroloisoquinolines using bromoisobutyrate and dimethyl sulfoxide as carbonyl reagent. Various formylated pyrroloisoquinolines could be prepared in good yields (up to 94%). This formylation process can be easily scaled up to gram scale with good yield. In most cases of pyrroloisoquinolines without methoxy groups, the combination of bromoisobutyrate and dimethyl sulfoxide could act as a bromination reagent, delivering brominated pyrroloisoquinolines in acceptable to good yields (up to 82%).

N,N,N',N'-Tetramethylethylenediamine-Enabled Photoredox-Catalyzed C-H Methylation of N-Heteroarenes

Aiming at the valuable methylation process, readily available and inexpensive N,N,N',N'-tetramethylethylenediamine (TMEDA) was first identified as a new methyl source in photoredox-catalyzed transformation in this work. By virtue of this simple methylating reagent, a facile and practical protocol for the direct C-H methylation of N-heteroarenes was developed, featuring mild reaction conditions, broad substrate scope, and scalability. Mechanistic studies disclosed that a sequential photoredox, base-assisted proton shift, fragmentation, and tautomerization process was essentially involved.

Molecular iodine mediated oxidative cleavage of the C–N bond of aryl and heteroaryl (dimethylamino)methyl groups into aldehydes

An efficient iodine mediated conversion of aryl or heteroaryl (dimethylamino)methyl compunds to aryl or heteroaryl aldehydes is achieved via cleavage of C-N bond.

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.

The aerobic oxidative hydroxysulfurization of gem-difluoroalkenes to produce α,α-difluoro-β-hydroxysulfides

The efficient aerobic oxidative hydroxysulfurization of gem-difluoroalkenes with aryl or even alkyl thiols to produce α,α-difluoro-β-hydroxysulfides has been developed.

A General and Scalable Synthesis of Polysubstituted Indoles

A consecutive 2-step synthesis of N-unprotected polysubstituted indoles bearing an electron-withdrawing group at the C-3 position from readily available nitroarenes is reported. The protocol is based on the [3,3]-sigmatropic rearrangement of N-oxyenamines generated by the DABCO-catalyzed reaction of N-arylhydroxylamines and conjugated terminal alkynes, and delivers indoles endowed with a wide array of substitution patterns and topologies.

Synthesis of Pyrrolo[2,1,5-cd]indolizine Rings via Visible-Light-Induced Intermolecular [3+2] Cycloaddition of Indolizines and Alkynes

A range of indolizine smoothly underwent visible-light-induced intermolecular [3+2] annulations with internal alkynes to afford pyrrolo[2,1,5-cd]indolizine in good to excellent yields with high regioselectivity. Through this cascade reaction, a series of fluoroactive fused indolizines with a large π-system were conveniently synthesized. The usage of visible light as energy source with air as a stoichiometric oxidant under simple conditions makes this process attractive and practical.

Photo-induced 1,2-carbohalofunctionalization of C–C multiple bonds via ATRA pathway

Carbohalofunctionalization of C–C multiple bonds via atom transfer radical processes constitutes an efficient method for the construction of halogenated building blocks with complete atom economy. This review summarizes the recent advancements.

Recent applications of Rose Bengal catalysis in N-heterocycles: a short review

The visible light harnessing ability of Rose Bengal, an organic dye, has been extensively employed in organic chemistry over the last few years. In visible light mediated reactions, this photoredox catalyst operates through multiple pathways and has the ability to provide distinctly different and valuable results. The most significant of these results are bond creation, bond functionalization, particularly for C–H and C–heteroatom bonds, and cross couplings. It is crucial to study these cases whenever these bond formations and couplings lead to the formation of heterocyclic compounds or their functionalization. The diverse biological activity and medicinal applications of heterocyclic compounds is an extensively explored area. This review primarily attempts to demonstrate the synthetic potential of Rose Bengal for synthesis and site selective functionalization of nitrogen containing heterocycles.

The recent applications of Rose Bengal as a photocatalyst for the synthesis and functionalization of N-heterocycles have been discussed.

Aerobic C(sp3)–H oxidation and oxygenation of quaternarized quinolines and pyridines by visible-light-induced photocatalysis

One stone two birds: two kinds of oxygenation products (>60 new products) including carboxylate inner salts and alcohols have been accessed under mild aerobic photocatalysis.

Electrochemically Enabled C3-Formylation and -Acylation of Indoles with Aldehydes

Reported herein is an effective strategy for oxidative cross-coupling of indoles with various aldehydes. The strategy is based on a two-step transformation via a well-known Mannich-type reaction and a C-N bond cleavage for carbonyl introduction. The key step-the C-N bond cleavage of the Mannich product-was enabled by electrochemistry. This strategy (with over 40 examples) ensures excellent functional-group tolerance as well as late-stage functionalization of pharmaceutical molecules.

Oxidative Deprotection of p-Methoxybenzyl Ethers via Metal-Free Photoredox Catalysis

An efficient and greener deprotection method for p-methoxybenzyl (PMB) ethers using a metal-free visible light photoredox catalyst and air and ammonium persulfate as the terminal oxidants is presented. Various functional groups and protecting groups were tolerated in the developed method to achieve good to excellent yields in short reaction times. Significantly, the developed method was compatible with PMB ethers derived from primary, secondary, and tertiary alcohols and a gram-scale reaction. Mechanistic studies support a proposed reaction mechanism that involves single electron oxidation of the PMB ether.

Recent advances in photocatalytic manipulations of Rose Bengal in organic synthesis

This review highlights the recent advances in photocatalytic manipulations of Rose Bengal in organic synthesis.

Visible-light-induced oxidative ring expansion of indoles with amidines

An efficient and mild visible-light-enabled reaction involving the oxidative ring expansion of indoles with amidines in the aqueous phase at room temperature is developed.

Visible light-mediated chemistry of indoles and related heterocycles

The impact of visible light-promoted chemistry on the functionalization of indoles and related heterocycles is reviewed.

Synthesis of 2-(Trifluoromethyl)indoles via Domino Trifluoromethylation/Cyclization of 2-Alkynylanilines

A new method for the synthesis of 2-(trifluoromethyl)indoles using easily accessible 2-alkynylanilines and a well-established fluoroform-derived CuCF₃ reagent is described. This method utilizes a domino trifluoromethylation/cyclization strategy to construct the indole cores with no ambiguity of the CF₃ position. The intriguing 3-formyl-2-(trifluoromethyl)indoles can also be synthesized by this protocol, which are useful intermediates for the preparation of trifluoromethylated drug analogues. The ultimate CF₃ source is the inexpensive industrial byproduct fluoroform.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.