Photocatalytic Hydroacylation of Alkenes by Directly Using Acyl Oximes

Synthesis of Acylation Polycyclic Derivatives via Regioselective Acylation/Cyclization of 1,7-Dienes with Acyl Oxime Esters

A visible-light-induced radical cascade regioselective acylation/cyclization of 1,7-dienes with acyl oxime esters for the preparation of acylation polycyclic compounds via NCR-mediated C-C σ-bond cleavage is established. The transformation involves the cleavage of the C-C σ-bond in acyl oxime esters and selective addition of the electron neutral C═C bonds in 1,7-dienes for the synthesis of acyl polycyclic quinolinone derivatives, not the traditional seven-membered ring products. The strategy offers several advantages, including broad substrate tolerance, no need for bases, hyperstoichiometric radical initiators, and other auxiliaries.

Metal- and photocatalyst-free approach to visible-light-induced acylation of quinoxalinones

A transition-metal- and photocatalyst-free photochemical reaction was successfully developed for the direct acylation of quinoxalin-2(1H)-ones, which was enabled by the formation of electron donor-acceptor (EDA) complexes. The use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the electron donor allows efficient and operationally simple access to a series of C3-aroylated and acylated quinoxalin-2(1H)-ones with moderate to good yields.

Light-Driven Photocatalyst-Free Synthesis of β, δ-Functionalized Ketones from Aldehydes

The synthesis of ketones has been a long focus of chemistry research, on account of its unique reactivity. Herein, we report a simple light-driven photocatalyst-free synthesis of β, δ-functionalized ketones from aldehydes, using inexpensive and commercially abundant feedstock chemicals. This reaction is enabled by the direct acyl radical generation via hydrogen atom transfer and the subsequent radical addition process, avoiding the need for prefunctionalized substrates and organometallic reagent.

Photocatalytic Phosphine-Mediated Thioesterification of Carboxylic Acids with Disulfides

Herein, a practical and effective synthesis of thioesters from readily available carboxylic acids and odorless disulfides was developed under photocatalytic conditions. This approach involves phosphoranyl radical-mediated fragmentation to generate acyl radicals and allows for incorporation of both S atoms of the disulfides into the desired products. In addition to batch reactions, a continuous-flow reactor was employed, enabling rapid thioester synthesis on a gram scale. Preliminary experimental mechanistic studies and the rapid synthesis of dalcetrapib are also demonstrated.

Recent Advances in Molecule Synthesis Involving C-C Bond Cleavage of Ketoxime Esters

The synthetic strategies of oxime derivatives participating in radical-type reactions have been rapidly developed in the last few decades. Among them, the N–O bond cleavage of oxime esters leading to formation of nitrogen-centered radicals triggers adjacent C–C bond cleavage to produce carbon-centered free radicals, which has been virtually used in organic synthesis in recent years. Herein, we summarized the radical reactions involving oxime N–O bond and C–C bond cleavage through this special reaction form, including those from acyl oxime ester derivatives and cyclic ketoxime ester derivatives. These contents were systematically classified according to different reaction types. In this review, the free radical reactions involving acyl oxime esters and cyclic ketoxime esters after 2021 were included, with emphasis on the substrate scope and reaction mechanism.

Visible-Light-Driven Hydroacylation of Unactivated Alkenes Using Readily Available Acyl Donors

Herein, we report visible-light-driven hydroacylation of unactivated alkenes. We employed benzimidazolines as new acyl donors and achieved perfect regioselectivity, high functional-group tolerance, and excellent substrate generality. We also performed mechanistic experiments to elucidate the detailed reaction mechanism. This is the first example of (1) hydroacylation of unactivated alkenes using (2) easily prepared acyl donors under (3) visible-light irradiation. Our findings offer a new strategy to synthesize a wide variety of ketones under mild conditions.

Photoredox Catalytic Phosphine-Mediated Deoxygenation of Hydroxylamines Enables the Construction of N-Acyliminophosphoranes

The chemistry of phosphoranyl radicals has received increasing attention in recent years. Here, we report the generation of amidyl radicals through photocatalytic deoxygenation of hydroxylamines with triphenylphosphine. This methodology offers a novel and convenient route to a diverse range of N-acyliminophosphoranes in moderate to good yields under visible-light photoredox conditions. Fluorescence quenching experiments suggest that the excited-state of the organic photocatalyst (4CzIPN) was oxidatively quenched by a Cu(II) salt.

Visible-light-induced acylation/cyclization of active alkenes: facile access to acylated isoquinolinones

Nitrogen heterocycles, especially polycyclic compounds, are significant skeletons in valuable molecules. Herein, we developed an efficient and practical visible-light-induced acylation/cyclization of active alkenes with acyl oxime derivatives for constructing acylated indolo/benzimidazo-[2,1,a]isoquinolin-6(5H) ones. This reaction was compatible with various functional groups and a series of fused indole/imidazole skeletons were prepared in up to 95% yield at room temperature.

Visible-Light-Induced Dual Acylation of Alkenes for the Construction of 3-Substituted Chroman-4-ones

Heterocyclic compounds, especially oxygen-containing heterocyclic compounds, are crucial moieties in bioactive compounds and drug leads. Substituted chroman-4-ones are a kind of the most significant structural skeletons. Herein, we report a visible-light-induced dual acylation of alkenes for constructing 3-substituted chroman-4-ones, which undergoes a radical tandem cyclization reaction through carbon-carbon bond cleavage of oxime esters by a nitrogen-centered radical strategy. A series of 3-substituted chroman-4-ones were prepared with up to 86% yield.

Phosphine/Photoredox Catalyzed Anti-Markovnikov Hydroamination of Olefins with Primary Sulfonamides via α-Scission from Phosphoranyl Radicals

New strategies to access radicals from common feedstock chemicals hold the potential to broadly impact synthetic chemistry. We report a dual phosphine and photoredox catalytic system that enables direct formation of sulfonamidyl radicals from primary sulfonamides. Mechanistic investigations support that the N-centered radical is generated via α-scission of the P-N bond of a phosphoranyl radical intermediate, formed by sulfonamide nucleophilic addition to a phosphine radical cation. As compared to the recently well-explored β-scission chemistry of phosphoranyl radicals, this strategy is applicable to activation of N-based nucleophiles and is catalytic in phosphine. We highlight application of this activation strategy to an intermolecular anti-Markovnikov hydroamination of unactivated olefins with primary sulfonamides. A range of structurally diverse secondary sulfonamides can be prepared in good to excellent yields under mild conditions.

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.

Alternative and Uncommon Acylating Agents - An Alive and Kicking Methodology

Functionalizing and derivatising organic molecules is a centerpiece in organic synthesis. Succinctly manipulating and installing acyl moieties in organic molecules spurred the interest of chemists owing to its occurrence in natural products, bioactive molecules, pharmaceuticals, and advanced materials. Traditionally, access to acylation reaction was achieved by Friedel-Crafts reaction, Schotten-Baumann, and Vilsmeier-Haack acylation, however, these protocols own pitfalls. Further to make the acylation process attractive and environmentally friendly, toluene, aldehydes, alcohols, α-keto acids, amines, amides, esters, ethers, nitriles, alkynes, alkenes, ketenes, N-acylbenzotriazoles, ketones, thioacids, oximes, thiazolium carbinols, PIDA, diacyl disulfides and acyl salts were used as an acyl surrogates/reagents. Amusingly, these acylating reagents are considered uncommon and alternative to carboxylic acids, acid chlorides and acetic anhydrides. This short review aims to encompass the usage of acylating agents in transition-metal, metal-free, light-driven and other demanding conditions, and thus reveals their practicality.

Visible-Light-Induced Transition-Metal-Free Nitrogen-Centered Radical Strategy for the Synthesis of 2-Acylated 9H-Pyrrolo[1,2-a]indoles

A convenient and efficient visible-light-induced tandem acylation/cyclization of N-propargylindoles with aryl- or alkyl-substituted acyl oxime esters for the synthesis of 2-acyl-substituted 9H-pyrrolo[1,2-a]indoles under transition-metal-free conditions, which proceeds via nitrogen-centered radical-mediated cleavage of the C-C σ-bond in acyl oxime esters, is established. The aryl or alkyl acyl radicals, which come from acyl oxime esters, attack the C-C triple bonds in N-propargylindoles and then go through intramolecular cyclization/isomerization.

Visible-light-facilitated P-center radical addition to C[double bond, length as m-dash]X (X = C, N) bonds results in cyclizations

Visible-light-facilitated phosphorus radical reactions have been developed as a powerful and sustainable tool for the synthesis of various organophosphorus compounds. In general, these reactions require stoichiometric amounts of oxidants, and reductants, bases, and radical initiators, leading to uneconomical and complicated processes. Progress has been made over the past few years toward using reactions that proceed under eco-benign and mild reaction conditions. Furthermore, these reactions have broad functional group tolerance, with some facile and economical pathways. Herein, we summarize the discoveries and achievements pertaining to C-P bond formation through a visible light photocatalysis procedure with high atom economy, made by our group and other research groups. It was established that greener and more environmentally friendly approaches do not require an additional oxidant or base. Moreover, we have designed and synthesized a new type of P-radical precursor, which can take part in reactions without the requirement for any additional bases, oxidants, and additives. This breakthrough, pertaining to novel visible-light-induced transformations, will be discussed and a plausible mechanism is proposed, based on corresponding experiments and the literature.

Synthesis of γ-Oxo-α-amino Acids via Radical Acylation with Carboxylic Acids

Herein we present a highly efficient, light-mediated, deoxygenative protocol to access γ-oxo-α-amino acid derivatives. This radical methodology employs photoredox catalysis, in combination with triphenylphosphine, to generate acyl radicals from readily available (hetero)aromatic and vinylic carboxylic acids. This approach allows for the straightforward synthesis of γ-oxo-α-amino acids bearing a wide range of functional groups (e.g., Cl, CN, furan, thiophene, Bpin) in synthetically useful yields (∼60% average yield). To further highlight the utility of the methodology, several deprotection and derivatization reactions were carried out.

O-Perhalopyridin-4-yl Hydroxylamines: Amidyl-Radical Generation Scaffolds in Photoinduced Direct Amination of Heterocycles

Reported herein is the design and synthesis of new O-perhalopyridin-4-yl hydroxylamines as shelf-stable and versatile amidyl-radical precursors. The novel amination reagents can be easily prepared via a single synthetic step from inexpensive commercially available starting materials using monoprotected HONH₂ as amino source. The synthetic potency of the developed reagents was well demonstrated by direct amination of a series of quinoxalin-2(1H)-ones and their analogues under photocatalytic conditions, even without any additive and photocatalysts.

Visible-light-induced 4CzIPN/LiBr system: a tireless electron shuttle to enable reductive deoxygenation of N-heteroaryl carbonyls

A mild visible-light-induced photoredox system was found to be a tireless electron shuttle to enable reductive deoxygenation of N-heteroaryl carbonyls.