Photocatalyst-Free, Visible-Light-Mediated C(sp3)–H Arylation of Amides via a Solvent-Caged EDA Complex

EDA Complex-Enabled Annulation to Access CF2-Containing Tetralones and Quinazolinones Using Persulfates as Electron Donors

A photocatalyst-free and EDA complex-enabled radical cascade cyclization reaction of inactive alkenes with bromodifluoroacetamides was reported for the divergent synthesis of fluorine-containing tetralones and quinazolinones. In this transformation, persulfates as electron donors and difluoro bromamide as electron acceptors generate the EDA complex. This is a promising photochemical method with advantages such as mild reaction conditions, simple operation, being metal-free, and excellent functional group tolerance.

Visible-Light-Induced Domino Perfluoroalkylation/Cyclization to Access Perfluoroalkylated Quinazolinones by an EDA Complex

The electron donor-acceptor (EDA) complexes have been extensively studied, which formed an electronically excited state, obviating the need for an exogenous photocatalyst. Herein, we report a mild and efficient strategy for photoinduced radical domino perfluoroalkylation/cyclization using N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) as an electron donor. This protocol could be well expanded to access various polycyclic quinazolinones containing perfluoroalkyl groups, exhibiting photocatalyst-free, good functional group tolerance, and environmentally friendly features.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

Visible light as a sole requirement for alkylation of α-C(sp3)-H of N-aryltetrahydroisoquinolines with alkylboronic acids

An alkylation of α-C(sp3)-H at N-aryltetrahydroisoquinolines with alkylboronic acids was developed under visible-light irradiation in the absence of additional photocatalyst. The reaction proceeded well, tolerating a variety of functional groups, and featured low-cost and mild reaction conditions. A preliminary mechanistic study indicated that an electron donor-acceptor (EDA) complex between an electron-rich N-aryltetrahydroisoquinoline and an electron-poor alkylboronic acid was involved in the reaction.

Direct decarboxylative Giese amidations: photocatalytic vs. metal- and light-free

A direct intermolecular decarboxylative Giese amidation reaction from bench stable, non-toxic and environmentally benign oxamic acids has been developed, which allows for easy access to 1,4-difunctionalised compounds which are not otherwise readily accessible. Crucially, a more general acceptor substrate scope is now possible, which renders the Giese amidation applicable to more complex substrates such as natural products and chiral building blocks. Two different photocatalytic methods (one via oxidative and the other via reductive quenching cycles) and one metal- and light-free method were developed and the flexibility provided by different conditions proved to be crucial for enabling a more general substrate scope.

Visible-Light-Mediated Synthesis of α-Aryl Ester Derivatives via an EDA Complex

We report an efficient radical-based and photocatalyst-free method for the C(sp2)-C(sp3) cross-coupling reaction to synthesize α-aryl ester derivatives. The process starts from a β-keto ester and an electron-deficient halogenated aryl halide under alkaline conditions to form an electron donor-acceptor complex and is driven by visible light. From the synthetic point of view, this newly established method represents a simple way to access arylpropionic acids from commercially available and cheap starting materials.

Phototriggered Self-Catalyzed Three-Component Minisci Reaction: A Route to β-C(sp3) Heteroarylated Alcohols/Ethers

Herein, a visible-light-triggered photocatalyst-free radical cascade Minisci reaction of heteroarenes, alkenes, and water/alcohols to obtain diverse β-C(sp³) heteroarylated alcohols/ethers has been developed. Achieved under mild and simple conditions, this protocol is scalable and features broad substrate scope and functional group tolerance. Mechanistic studies demonstrate that the heteroarene can be served as a photocatalyst to engage single-electron transfer with persulfate.

Benzoates as photosensitization catalysts and auxiliaries in efficient, practical, light-powered direct C(sp3)-H fluorinations

Of the methods for direct fluorination of unactivated C(sp3)-H bonds, photosensitization of SelectFluor is a promising approach. Although many substrates can be activated with photosensitizing catalysts, issues remain that hamper fluorination of complex molecules. Alcohol- or amine-containing functional groups are not tolerated, fluorination regioselectivity follows factors endogenous to the substrate and cannot be influenced by the catalyst, and reactions are highly air-sensitive. We report that benzoyl groups serve as highly efficient photosensitizers which, in combination with SelectFluor, enable visible light-powered direct fluorination of unactivated C(sp3)-H bonds. Compared to previous photosensitizer architectures, the benzoyls have versatility to function both (i) as a photosensitizing catalyst for simple substrate fluorinations and (ii) as photosensitizing auxiliaries for complex molecule fluorinations that are easily installed and removed without compromising yield. Our auxiliary approach (i) substantially decreases the reaction's induction period, (ii) enables C(sp3)-H fluorination of many substrates that fail under catalytic conditions, (iii) increases kinetic reproducibility, and (iv) promotes reactions to higher yields, in shorter times, on multigram scales, and even under air. Observations and mechanistic studies suggest an intimate 'assembly' of auxiliary and SelectFluor prior/after photoexcitation. The auxiliary allows other EnT photochemistry under air. Examples show how auxiliary placement proximally directs regioselectivity, where previous methods are substrate-directed.

Electrochemical Friedel–Crafts-type amidomethylation of arenes by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborate

Electrochemical Friedel–Crafts-type amidomethylation was successfully carried out by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborates, such as iPr₂NHEtBF₄. Constant current electrolysis of 1,3,5-trimethoxybenzene or indoles in DMA containing 0.1 M iPr₂NHEtBF₄ using an undivided cell equipped with a Pt plate cathode and a Pt wire anode (a quasi-divided cell) resulted in selective formation of N-acyliminium ions of DMA at the anode, which reacted with arenes to give the corresponding amidomethylated products in good to high yields.

Ritter-type amination of C(sp3)-H bonds enabled by electrochemistry with SO42

By merging electricity with sulfate, the Ritter-type amination of C(sp³)-H bonds is developed in an undivided cell under room temperature. This method features broad substrate generality (71 examples, up to 93% yields), high functional-group compatibility, facile scalability, excellent site-selectivity and mild conditions. Common alkanes and electron-deficient alkylbenzenes are viable substrates. It also provides a straightforward protocol for incorporating C-deuterated acetylamino group into C(sp³)-H sites. Application in the synthesis or modification of pharmaceuticals or their derivatives and gram-scale synthesis demonstrate the practicability of this method. Mechanistic experiments show that sulfate radical anion, formed by electrolysis of sulfate, served as hydrogen atom transfer agent to provide alkyl radical intermediate. This method paves a convenient and flexible pathway for realizing various synthetically useful transformations of C(sp³)-H bonds mediated by sulfate radical anion generated via electrochemistry.

The amination of C(sp3)–H bonds is an appealing and challenging task in organic synthesis. Here, by using an electrogenerated sulfate radical an HAT agent, the authors report a practical Ritter-type amination of C(sp3)–H bonds.

Visible-Light-Promoted Cross-Coupling of O-Aryl Oximes and Nitrostyrenes to Access Cyanoalkylated Alkenes

A photoinduced, photocatalyst-free cyanoalkylation of nitrostyenes was explored, affording a series of cyanoalkylated alkenes in moderate to good yields. Mechanistic studies reveal that an electron donor-acceptor complex formed between O-aryl oximes and DIPEA is presumably involved in this process. The excellent functional group compatibility of this newly designed synthetic protocol allows for cyanoalkylation of structurally varied substrates, which offers an eco-friendly pathway for the assembly of cyanoalkylated alkenes.

Synthesis of Unnatural α-Amino Acid Derivatives via Photoredox Activation of Inert C(sp3)-H Bonds

The synthesis of unnatural, tertiary amino acids is a challenging task. While decarboxylation-radical addition has been an important strategy for their formation, the use of alkyl radicals from C(sp³)-H bonds has not been fully explored. Herein, we report a photocatalytic protocol for the synthesis of unnatural α-amino esters employing abundant alkanes and imines retaining full atom economy. When this method is applied, several amino acid derivatives are synthesized in moderate to good yields.

Site-Selective C(sp3)–H Functionalizations Mediated by Hydrogen Atom Transfer Reactions via α-Amino/α-Amido Radicals

Amines and amides, as N-containing compounds, are ubiquitous in pharmaceutically-active scaffolds, natural products, agrochemicals, and peptides. Amides in nature bear a key responsibility for imparting three-dimensional structure, such as in proteins. Structural modifications to amines and amides, especially at their positions α to N, bring about profound changes in biological activity oftentimes leading to more desirable pharmacological profiles of small drug molecules. A number of recent developments in synthetic methodology for the functionalizations of amines and amides omit the need of their directing groups or pre-functionalizations, achieving direct activation of the otherwise relatively benign C(sp3)–H bonds α to N. Among these, hydrogen atom transfer (HAT) has proven a very powerful platform for the selective activation of amines and amides to their α-amino and α-amido radicals, which can then be employed to furnish C–C and C–X (X = heteroatom) bonds. The abilities to both form these radicals and control their reactivity in a site-selective manner is of utmost importance for such chemistries to witness applications in late-stage functionalization. Therefore, this review captures contemporary HAT strategies to realize chemo- and regioselective amine and amide α-C(sp3)–H functionalization, based on bond strengths, bond polarities, reversible HAT equilibria, traceless electrostatic-directing auxiliaries, and steric effects of in situ-generated HAT agents.

1 Introduction

2 Functionalizations of Amines

3 Functionalizations of Carbamates

4 Functionalizations of Amides

5 Conclusion

Organic Sulfinic Acids and Salts in Visible Light-Induced Reactions

Sulfinic acids and their salts are a useful source of sulfur-containing structures. Photocatalysis of these compounds with visible light enables to achieve various transformations under mild conditions. This review summarizes visible-light-induced reactions of sulfinic acids and their salts. It is organized by reaction type and brief discussions on plausible reaction mechanisms for typical transformations are presented.

1 Introduction

2 Sulfonylation Reactions

2.1 Sulfonylation of Alkenes

2.2 Sulfonylation of Alkynes

2.3 Sulfonylation of Arenes

2.4 sp3 C–H Functionalization

3 Desulfonylation Reactions

4 Sulfenylation Reactions

4.1 Sulfenylation of Heteroarenes

4.2 Sulfenylation of Carbonyl Chlorides

5 Conclusions

Electrophilic amidomethylation of arenes with DMSO/MeCN reagents

An efficient electrophilic amidomethylation of aromatics was described with DMSO as the CH2 source and nitrile as the nitrogen source.

An organophotocatalytic late-stage N–CH3 oxidation of trialkylamines to N-formamides with O2 in continuous flow

We report an organophotocatalytic, N–CH₃-selective oxidation of trialkylamines in continuous flow. Based on the 9,10-dicyanoanthracene (DCA) core, a new catalyst (DCAS) was designed with solubilizing groups for flow processing. This allowed O₂ to be harnessed as a sustainable oxidant for late-stage photocatalytic N–CH₃ oxidations of complex natural products and active pharmaceutical ingredients bearing functional groups not tolerated by previous methods. The organophotocatalytic gas–liquid flow process affords cleaner reactions than in batch mode, in short residence times of 13.5 min and productivities of up to 0.65 g per day. Spectroscopic and computational mechanistic studies showed that catalyst derivatization not only enhanced solubility of the new catalyst compared to poorly-soluble DCA, but profoundly diverted the photocatalytic mechanism from singlet electron transfer (SET) reductive quenching with amines toward energy transfer (E_nT) with O₂.

An N–CH₃-selective trialkylamine oxidation to N-formamides is reported in continuous flow using gaseous O₂. A novel, enhanced-solubility dicyanoanthracene organophotocatalyst switched the photochemical mechanism from electron to energy transfer.

Photocatalytic C-H Azolation of Arenes Using Heterogeneous Carbon Nitride in Batch and Flow

The functionalization of aryl C(sp² )-H bonds is a useful strategy for the late-stage modification of biologically active molecules, especially for the regioselective introduction of azole heterocycles to prepare medicinally-relevant compounds. Herein, we describe a practical photocatalytic transformation using a mesoporous carbon nitride (mpg-CN_x ) photocatalyst, which enables the efficient azolation of various arenes through direct oxidation. The method exhibits a broad substrate scope and is amenable to the late-stage functionalization of several pharmaceuticals. Due to the heterogeneous nature and high photocatalytic stability of mpg-CN_x , the catalyst can be easily recovered and reused leading to greener and more sustainable routes, using either batch or flow processing, to prepare these important compounds of interest in pharmaceutical and agrochemical research.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

Visible-Light-Driven Sulfonation of α-Trifluoromethylstyrenes: Access to Densely Functionalized CF3-Substituted Tertiary Alcohol

Reported herein is a visible-light-induced sulfonation of α-trifluoromethylstyrenes with sodium sulfinates, which provides a series of α-trifluoromethyl-β-sulfonyl tertiary alcohols. This new synthetic protocol is enabled by a charge-transfer complex between oxygen and sulfinates, featuring broad substrate scope and scalability. Excellent functional group compatibility and chemoselectivity render this method suitable for sulfonation of pharmaceutically relevant molecules. In the presence of D₂O, deuteriotrifluorinated products were also obtained, further demonstrating the flexibility and synthetic potentials of this strategy.

Donor-Acceptor Complex Enables Cascade Radical Cyclization of N-Arylacrylamides with Katritzky Salts

Cascade radical cyclization of N-arylacrylamides is an attractive method to prepare 3,3-disubstituted oxindoles. As the reported methods often require additives and/or photocatalysts, we herein report an additive- and photocatalyst-free deaminative strategy for their synthesis under mild conditions, enabled by photoactivation of an electron donor-acceptor (EDA) complex. DFT studies indicated that the involvement of an explicit xylene solvent molecule can greatly enhance the photoactivity of the EDA complex between N-arylacrylamides and Katritzky salts.

Hole-mediated photoredox catalysis: tris(p-substituted)biarylaminium radical cations as tunable, precomplexing and potent photooxidants

Triarylamines are demonstrated as novel, tunable electroactivated photocatalysts that use dispersion precomplexation to harness the full potential of the visible photon (>4.0 V vs. SCE) in anti-Kasha photo(electro)chemical super-oxidations of arenes.