Advances of N-Hydroxyphthalimide Esters in Photocatalytic Alkylation Reactions

Light-Mediated Radical Addition to Azomethine Compounds: Novel Reactivity and Activation Modes

Azomethines is a class of compounds, which have traditionally served as electrophilic substrates, but their reactions with radicals have long been limited. Photocatalysis provided ample opportunities for promoting these reactions, with wide variety of reagents serving as precursors of radicals. Besides regular addition mode at the azomethine fragment, the oxidative pathway, in which the C=N bond remains in the product, has become possible by proper selection of redox catalyst. This review summarizes new developments in this rapidly developing field over the past five years. New concepts on activation of the C=N bond towards radical attack are discussed.

Photocatalytic Sulfonylation: Innovations and Applications

Photosynthesis, converting sustainable solar energy into chemical energy, has emerged as a promising craft to achieve diverse organic transformations due to its mild reaction conditions, sustainability, and high efficiency. The synthesis of sulfonated compounds has drawn significant attention in the pharmaceuticals, agrochemicals, and materials industries due to the unique structure and electronic properties of the sulfonyl groups. Over the past decades, many photocatalytic sulfonylation reactions have been developed. In this review, the recent advances in photocatalyzed sulfonylation have been reviewed since 2020, with a primary focus on discussing reaction design and mechanism.

Zn-Mediated Fragmentation of N-Alkoxyphthalimides Enabling the Synthesis of gem-Difluoroalkenes

Zn-mediated generation of alkoxyl radicals from N-alkoxyphthalimides emerged as an efficient approach for forming diverse and valuable alkyl radicals through β-scission or a hydrogen atom transfer process. The alkyl radical species can be further trapped by α-trifluoromethyl alkenes to construct a series of gem-difluoroalkenes.

Dual Acridine/Decatungstate Photocatalysis for the Decarboxylative Radical Addition of Carboxylic Acids to Azomethines

A concept for the dual use of acridine and tetrabutylammonium decatungstate photocatalysts in the reactions of carboxylic acids is proposed. Imines generated in situ from aldehydes and p-methoxyaniline, as well as other azomethines, were used as radical acceptors. The role of the decatungstate is believed to facilitate the turnover of the acridine photocatalyst by means of hydrogen atom transfer.

Direct C-H Alkylation of Benzothiadiazoles via Organic Photoredox Catalysis

2,1,3-Benzothiadiazole is widely used as a privileged scaffold in pharmaceuticals and organic functional materials. Nonetheless, many current methods for the functionalization of 2,1,3-benzothiadiazole rely on preactivation, transition metal catalysts/promoters, or an elevated reaction temperature. Herein we disclose a transition-metal-free visible-light-induced photocatalytic method for the direct C-H alkylation of 2,1,3-benzothiadiazole using readily accessible carboxylic acid derivatives, i.e., N-hydroxyphthalimide esters (NHPEs), as alkylating reagents under room temperature. This mild and scalable method is highlighted by the late-stage installation of the benzothiadiazole scaffold in drugs and natural products.

Visible-light-induced decarboxylative cascade cyclization of acryloylbenzamides with N-hydroxyphthalimide esters via EDA complexes

A visible-light-induced decarboxylative cascade reaction of acryloylbenzamides with alkyl N-hydroxyphthalimide (NHP) esters for the synthesis of various 4-alkyl isoquinolinediones mediated by triphenylphosphine (PPh3) and sodium iodide (NaI) was developed. This operationally simple protocol proceeded via the photoactivation of electron donor-acceptor (EDA) complexes between N-hydroxyphthalimide esters and NaI/PPh3, resulting in multiple carbon-carbon bond formations without the use of precious metal complexes or synthetically elaborate organic dyes, which provided an alternative practical approach to synthesize diverse isoquinoline-1,3(2H,4H)-dione derivatives.

One-Pot Transformation of Aldehydes to Ketones via Minisci-Type Reaction of Imines

A method for the conversion of aldehydes to ketones via the preliminary formation of aldiminines is described. The imines are involved in acid promoted Minisci-type reaction with alkyl radicals generated from esters of N-hydroxylphthalimide under photoredox conditions. Aminyl radical cations formed after the addition of the iminium ions are believed to be key intermediates, which determine the reaction outcome.

Optoelectronic materials as emerging photocatalysts: opportunities in sustainable organic synthesis

To overcome the energy and environmental crisis, the development of efficient, sustainable photocatalysts to convert inexhaustible solar energy into chemical energy is of great significance. Due to their unique optoelectronic properties, organic electronic materials have been translated into the photocatalytic field. These emerging photocatalysts are attractive because of their metal-free nature, chemical stability, and structural diversity. However, as many small molecules fail to absorb visible light solely, incorporating them into crosslinked frameworks is found to be an effective strategy to extend the conjugation and enhance visible-light absorption. In addition, the photophysical properties of these heterogeneous materials can be adjusted through structural modification and linkage engineering. Finally, these insoluble photocatalysts exhibit good recyclability and reusability. As a representative illustration, this feature article describes recent examples of the use of two types of organic electronic materials including phenothiazine and truxene in heterogeneous photocatalytic organic transformations. The synthesis and key photophysical properties of both organic electronic material-based photocatalysts are discussed combined with specific synthetic applications. We anticipate this feature article will stimulate the implementation of more diverse organic electronic materials in the field of photocatalysis, which may lead to unprecedented synthetic applications.

Nickel-Catalyzed Asymmetric Decarboxyarylation with NHP Esters of α-Amino Acid to Chiral Benzylamines

A nickel-catalyzed asymmetric decarboxyarylation of NHP esters via reductive cross-coupling has been established. Utilizing the NHP of amino acid esters as radical precursors furnishes a new protocol in which structurally diverse chiral benzylamines could be accessible. This method has demonstrated excellent catalytic efficiency, high enantioselective control, mild conditions, and good functional group tolerance, thus enabling the late-stage modification of bioactive molecules and pharmaceuticals.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

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.

Silver-catalyzed cyclization of α-imino-oxy acids to fused tetralone derivatives

A silver-catalyzed intramolecular radical relay cyclization of α-imino-oxy acids under mild conditions has been described. This reaction offers facile access to a diverse range of fused tetralone derivatives with exquisite stereoselectivity in moderate to good yields (40-98%). Experimental studies show that the reaction undergoes a decarboxylation and acetone fragmentation/1,5-hydrogen atom transfer (HAT)/cyclization process.

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.

Diversified access to di- and trisubstituted allenes via nickel-catalysed reactions of 1,3-enynes with alkyl N-hydroxyphthalimide esters

Herein, a new nickel-catalysed protocol for the preparation of di- and trisubstituted allenes has been successfully developed via the reactions of 1,3-enynes with alkyl N-hydroxyphthalimide esters. The new method based on a reductive radical-polar crossover (RPC) process features broad substrate scope, wide functional group tolerance, and a simple catalyst system. The late-stage allenylation of drugs has also been illustrated.

Visible-light-initiated external photocatalyst-free synthesis of α,α-difluoro-β-ketoamides from 4-aminocoumarins

A concise and efficient ring-opening difluorination strategy was developed for the synthesis of highly functionalized hydroxy-containing α,α-difluoro-β-ketoamides from the one-pot multicomponent reaction of 4-aminocoumarins, NFSI, and water in dimethyl carbonate (DMC) as a green solvent. The reactions were smoothly achieved under visible light irradiation in air at room temperature without the addition of any other external photocatalysts. With this protocol, various α,α-difluoro-β-ketoamides were successfully synthesized under mild conditions (25 examples, 73-91% yields). This transition-metal-free synthetic procedure shows good functional group compatibility and attractive practical potential for large-scale synthesis.

Visible light-promoted transition metal-free direct C3-carbamoylation of 2H-Indazoles

We reported a general transition metal-free transformation to access C3-carbamoylated 2H-indazoles via visible light-induced oxidative decarboxylation coupling, in the presence of oxamic acids as the coupling sources, 4CzIPN as the photocatalyst, and Cs2CO3 as the base. The great application potential of this mild condition is highlighted by the late-stage modification of drugs, N-terminal modification of peptides, and the good antitumor activity of the novel desired product.

Visible-light-promoted decarboxylative radical cascade cyclization to acylated benzimidazo/indolo[2,1-a]isoquinolin-6(5H)-ones in water

A metal-free visible-light-induced decarboxylative radical addition/cyclization procedure at room temperature was described for the synthesis of acylated benzimidazo/indolo[2,1-a]isoquinolines. The procedure was prepared in water via a reaction of functionalized 2-arylbenzoimidazoles or 2,3-diarylindoles and α-oxocarboxylic acids in the presence of phenyliodine(iii) diacetate (PIDA) in one step under mild reaction conditions. In this procedure, traditional heating and metal reagents could be effectively avoided to access 1,4-dicarbonyl-containing benzimidazo/indolo[2,1-a]isoquinoline-6(5H)-ones in satisfactory yields.

tert-Butyl Nitrite as a Twofold Hydrogen Abstractor for Dehydrogenative Coupling of Aldehydes with N-Hydroxyimides

A synthetically practical transition metal/catalyst/halogen-free dehydrogenative coupling of aldehydes with N-hydroxyimides promoted solely by tert-butyl nitrite under mild conditions was developed. tert-Butyl nitrite generates two radicals (^tBuO and NO) and thus works as a twofold hydrogen abstractor. A diverse array of N-hydroxyimide esters were prepared from either aliphatic or aromatic aldehydes. Benzoyl-substituted aldehydes such as 2-oxo-2-phenylacetaldehyde are also suitable.

The visible-light-induced acylation/cyclization of alkynoates with acyl oximes for the construction of 3-acylcoumarins

A nitrogen-centered radical-mediated carbon-carbon bond cleavage strategy is described to synthesize functionalized 3-acylcoumarins. The strategy is enabled by the visible-light-induced acylation/cyclization of alkynoates with various acyl oxime compounds in acetonitrile. The difunctionalization of carbon-carbon triple bonds precedes the generation of iminyl radicals, which is followed by the formation of acyl radicals. The acyl radicals then attack the carbon-carbon triple bonds, followed by 5-exo-trig cyclization and 1,2-ester migration. This strategy has wide substrate adaptability and good substituent tolerance.

Photocatalytic Decarboxylative [3 + 2] and [4 + 2] Annulation of Enynals and γ,σ-Unsaturated N-(Acyloxy)phthalimides by NaI/PPh3 Catalysis

A practical and eco-friendly strategy for the radical-mediated decarboxylative [3 + 2] and [4 + 2] annulation of enynals and γ,σ-unsaturated N-(acyloxy)phthalimides through the photoactivation of an electron donor-acceptor (EDA) complex has been developed. A wide range of primary, secondary, and tertiary alkyl N-hydroxyphthalimide (NHP) esters can be used as suitable substrates for the synthesis of fused ketones without any transition-metal catalysts or oxidants. This protocol features a broad substrate scope, excellent selectivity, and clean reaction conditions.