Aromatization-driven deconstructive functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis

Aromatization-driven deconstruction and functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis is developed. The aromatization effect was introduced to synergistically drive unstrained cyclic C-C bond cleavage, with the aim of overcoming the ring-size limitation of nitrogen-centered radical induced deconstruction of carbocycles. Herein, we demonstrate the synergistic photoredox/nickel catalyzed deconstructive cross-coupling of spiro dihydroquinazolinones with organic halides. Remarkably, structurally diverse organic halides including aryl, alkenyl, alkynyl, and alkyl bromides were compatible for the coupling. In addition, this protocol is also characterized by its mild and redox-neutral conditions, excellent functional group compatibility, high atom economy, and easy scalability. A telescoped procedure involving condensation and ring-opening/coupling was found to be accessible. This work provides a complementary strategy to the existing radical-mediated C-C bond cleavage of unstrained carbocycles.

Photoinduced Palladium-Catalyzed Radical Heck-Type Coupling of Cyclobutanone Oxime Esters with Vinyl Arenes

A palladium-catalyzed radical Heck-type coupling reaction of cyclobutanone oxime esters with olefins under visible-light irradiation has been developed. The cyanoalkyl/Pd(I) hybrid species generated by selected ring-opening C-C bond cleavage of imino/Pd(I) species reacted smoothly with vinyl arenes, delivering the cyanoalkylation olefins under mild conditions. This elegant strategy has a broad scope and functional group tolerance. Subsequently, late-stage functionalization of bioactive molecules and synthetic transformations of the product further confirm the practicality.

Construction of C-P Bonds from Free Cyclobutanone Oximes and Chlorophosphines via Radical-Radical Coupling

Herein, we report a catalyst-free reaction of cyclobutanone oximes with chlorophosphines (R2PCl), which forms a fragile C═N-O-PR2 species that undergoes N-O homolysis, fragmentation, and radical-radical coupling, leading to the formation of cyano-containing phosphine oxides in good yields. The reaction features an in situ activation of cyclobutanone oximes for radical generation, in which R2PCl plays a dual role as both an activator and a reactant.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

N-Iminopyridinium Compounds in Giese Reaction: Photoinduced Homolytic N-N and C-C Bond Cleavage for Cyanoalkyl Radical Generation

We present an innovative photoinduced cyanoalkyl radical addition methodology using N-iminopyridinium reagents derived from cyclic ketones. Mechanistic investigations reveal the association of the excited Hantzsch ester and iminopyridinium with pyridyl radical generation. The ensuing cascade involving homolytic N-N bond and C-C bond cleavage of the pyridyl radical ultimately leads to the formation of cyanoalkyl radical species, leading to diverse Giese-type products. The method showcases versatility and synthetic utility in late-stage functionalization.

Photoredox-Catalyzed Three-Component 1,2-Cyanoalkylpyridylation of Styrenes with Nonredox-Active Cyclic Oximes

Three-component alkene 1,2-difunctionalizations have emerged as a powerful strategy for rapid buildup of diverse and complex alkylpyridines, but the distal functionalized alkyl radicals for the alkene 1,2-alkylpyridylations were still rare. Herein, we report an example of regioselective three-component 1,2-cyanoalkylpyridylation of feedstock styrenes with accessible nonredox-active cyclic oximes through visible-light photoredox catalysis, providing a series of structurally diverse β-cyanoalkylated alkylpyridines. This protocol proceeds through a radical relay pathway including the generation of iminyl radicals enabled by phosphoranyl radical-mediated β-scission, radical transposition through C-C bond cleavage, highly selective radical addition, and precise radical-radical cross-coupling sequence, thus facilitating the regioselective formation of two distinct C-C single bonds in a single-pot operation. This synthetic strategy features mild conditions, broad compatibility of functional groups and substrate scope, diverse product derivatization, and late-stage modification.

Synthesis of aliphatic nitriles from cyclobutanone oxime mediated by sulfuryl fluoride (SO2F2)

A SO₂F₂-mediated ring-opening cross-coupling of cyclobutanone oxime derivatives with alkenes was developed for the construction of a range of δ-olefin-containing aliphatic nitriles with (E)-configuration selectivity. This new method features wide substrate scope, mild conditions, and direct N-O activation.

Dicarbofunctionalization of unactivated alkenes via organo-photoredox catalysis in water: access to cyanoalkylated fused quinazolinones

A visible light-induced C-C bond cleavage/addition/cyclization cascade of oxime esters and unactivated alkenes has been developed using water as the solvent. This green protocol offers an easy access to medicinally valuable cyanoalkylated quinazolinones. Mild reaction conditions, functional group tolerance and late-stage functionalization of complex molecules are the important features of this transformation.

Synthesis and Application Dichalcogenides as Radical Reagents with Photochemical Technology

Dichalcogenides (disulfides and diselenides), as reactants for organic transformations, are important and widely used because of their potential to react with nucleophiles, electrophilic reagents, and radical precursors. In recent years, in combination with photochemical technology, the application of dichalcogenides as stable radical reagents has opened up a new route to the synthesis of various sulfur- and selenium-containing compounds. In this paper, synthetic strategies for disulfides and diselenides and their applications with photochemical technology are reviewed: (i) Cyclization of dichalcogenides with alkenes and alkynes; (ii) direct selenylation/sulfuration of C-H/C-C/C-N bonds; (iii) visible-light-enabled seleno- and sulfur-bifunctionalization of alkenes/alkynes; and (iv) Direct construction of the C(sp)-S bond. In addition, the scopes, limitations, and mechanisms of some reactions are also described.

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.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

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.

Photoinduced radical cascade cyclization of acetylenic acid esters with oxime esters to access cyanalkylated coumarins

A photoinduced radical cascade cyclization of acetylenic acid esters with oxime esters is described, providing cyanalkylated coumarins in superior yields under mild conditions. Radical capture and luminescence quenching experiments showed that this transformation was accomplished via a radical addition/5-exo spirocyclization/1,2-ester migration process.

Organophotoredox-catalyzed cyanoalkylation of 1,4-quinones

A visible-light-induced metal-free cyanoalkylation of 1,4-quinones under mild and redox-neutral conditions is described. This reaction proceeds at room temperature without the need of extra base or additive and is suitable for a variety of 1,4-quinones and differently substituted cyclobutanone oxime esters. Further transformation of cyano functionality to tetrazole and amine has also been demonstrated to showcase the advantage of this method to prepare drug-like molecules.

Photoinduced remote regioselective radical alkynylation of unactivated C-H bonds

A method for the remote regioselective alkynylation of unactivated C(sp³)-H bonds in diverse aliphatic amides by photogenerated amidyl radicals has been developed. The site-selectivity is dominated via a 1,5-hydrogen atom transfer (HAT) process of the amide. Mild reaction conditions and high regioselectivity are demonstrated in this methodology.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Divergent cyanoalkylation/cyanoalkylsulfonylation of enamides under organophotoredox catalytic conditions

An organophotoredox catalyzed divergent cyanoalkylation/cyanoalkylsulfonylation of enamides is described.

Visible-Light-Driven Photoredox-Catalyzed C(sp3)-C(sp3) Cross-Coupling of N-arylamines with Cycloketone Oxime Esters

A novel photoredox-catalyzed C(sp3)-C(sp3) cross-coupling between N-arylamines and cycloketone oxime esters under mild conditions has been accomplished. The redox-neutral reaction proceeds good functional group tolerance and excellent regioselectivity without any...

Oxime ester-enabled anti-Markovnikov hydrosilylation of alkenes

An oxime ester-enabled anti-Markovnikov hydrosilylation of alkenes is developed.

Cyanomethylative cyclization of unactivated alkenes with nitriles for the synthesis of cyano-containing ring-fused quinazolin-4(3H)-ones

The synthesis of cyano-containing pyrrolo- and piperidino-quinazolinones was developed using alkyl nitriles through radical cascade addition/cyclization under metal-free conditions.

Visible-Light Photoredox-Catalyzed Dicarbofunctionalization of Styrenes with Oxime Esters and CO2: Multicomponent Reactions toward Cyanocarboxylic Acids and γ-Keto Acids

A photoredox-catalyzed dicarbofunctionalization of styrenes with oxime esters and CO₂ has been achieved. Notably, a series of four-, five-, or six-membered cyclic ketone oximes worked well to furnish a wide range of ε-, ζ-, and η-cyanocarboxylic acids in good yields. Furthermore, a series of γ-keto acids also could be obtained by employing acyclic ketone oxime esters as the carbonyl radical precursor. It provides convergent access to diverse biologically important cyanocarboxylic and γ-keto acids.

Transition-Metal- and Light-Free Generation of an Iminyl Radical: Facile Approach to Oxindoles and Isoquinolinediones with a Quaternary Carbon Center via Cyanoalkylarylation

We have developed an efficient and non-toxic method for the environmental-friendly generation of an iminyl radical from cyclobutanone oxime ester via direct thermolysis in the absence of light, transition metals, "tin", and other activators. This redox-neutral cyanoalkylarylation protocol enjoys a wide substrate scope and a good functional group tolerance, providing facile access to oxindoles and isoquinolinediones with a quaternary carbon center that are difficult to prepare by traditional methods.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Iron-catalyzed radical cascade cyclization of oxime esters with isocyanides: synthesis of 1-cyanoalkyl isoquinolines and 6-cyanoalkyl phenanthridines

An iron-catalyzed radical cascade cyclization of oxime esters with isocyanides for the synthesis of 1-cyanoalkyl isoquinolines and 6-cyanoalkyl phenanthridines has been developed. This demonstrates excellent functional group tolerance and broad substrate scope. A diverse range of potentially valuable 1-cyanoalkyl isoquinolines and 6-cyanoalkyl phenanthridines were obtained in moderate to good yields.

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.

Photocatalytic cross-couplings via the cleavage of N-O bonds

N-(Acyloxy)phthalimide and oxime derivatives containing N-O bonds are important chemicals and synthetic intermediates, and visible light photoredox reductions of the N-O bonds provide carbon- or nitrogen-centered radicals for N-(acyloxy)phthalimide derivatives and iminyl radicals for oxime derivatives. This feature article summarises the recent progress in the visible light photoredox organic reactions, including decarboxylative addition reactions, alkylation, allylation, alkenylation, alkynylation, arylation, heteroarylation and cascade annulation of N-(acyloxy)phthalimide derivatives through the formation of carbon-carbon bonds, decarboxylative borylation, amination, oxygenation, sulfuration, selenylation, fluorination and iodination of N-(acyloxy)phthalimide derivatives through the formation of carbon-heteroatom bonds, and additions to arenes and alkenes, hydrogen atom transfer and the cleavage of α-carbon-carbon bonds via the iminyl radical intermediates for oxime derivatives.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Visible-Light-Promoted Selenocyanation of Cyclobutanone Oxime Esters Using Potassium Selenocyanate

We report the visible-light-promoted selenocyanation of cyclobutanone oxime esters using potassium selenocyanate in the presence of a fac-Ir(ppy)₃ catalyst for the first time. Because of the mild conditions employed and use of readily accessible potassium selenocyanate, this method is an effective and green strategy for the synthesis of cyano and selenocyano bifunctional substituted alkanes.

Photoinduced 1,2-dicarbofunctionalization of alkenes with organotrifluoroborate nucleophiles via radical/polar crossover

Alkene 1,2-dicarbofunctionalizations are highly sought-after transformations as they enable a rapid increase of molecular complexity in one synthetic step. Traditionally, these conjunctive couplings proceed through the intermediacy of alkylmetal species susceptible to deleterious pathways including β-hydride elimination and protodemetalation. Herein, an intermolecular 1,2-dicarbofunctionalization using alkyl N-(acyloxy)phthalimide redox-active esters as radical progenitors and organotrifluoroborates as carbon-centered nucleophiles is reported. This redox-neutral, multicomponent reaction is postulated to proceed through photochemical radical/polar crossover to afford a key carbocation species that undergoes subsequent trapping with organoboron nucleophiles to accomplish the carboallylation, carboalkenylation, carboalkynylation, and carboarylation of alkenes with regio- and chemoselective control. The mechanistic intricacies of this difunctionalization were elucidated through Stern-Volmer quenching studies, photochemical quantum yield measurements, and trapping experiments of radical and ionic intermediates.

Deacylative Carbon-Carbon Bond Cleavage of Ketone Equivalents: Applications to Radical Carbon-Carbon Bond Formation Reactions

This article describes the synthetic application of ketone-derived oxaziridines as alkyl radical precursors in copper-catalyzed Carbon-Carbon bond formation reactions. Experimental and computational studies indicate a free radical mechanism, where alkyl radicals are efficiently generated via cleavage of a Carbon-Carbon bond of oxaziridines. Acyclic and unstrained cyclic oxaziridines are applicable to the present radical process, allowing for the generation of various alkyl radicals with good functional group compatibility.

Organophotoredox-Catalyzed Three-Component Coupling of Heteroatom Nucleophiles, Alkenes, and Aliphatic Redox Active Esters

This manuscript describes a visible-light-mediated organophotoredox catalytic process for vicinal difunctionalization of alkenes using heteroatom nucleophiles and aliphatic redox active esters. A wide range of heteroatom nucleophiles including alcohols, water, carboxylic acids, amides, and halogens can be used for this reaction. This radical relay type reaction allows forging of C(sp³)-C(sp³) with a carbon-centered radical and C(sp³)-heteroatom bonds with a benzyl cation on the vinylarenes with complete regioselectivity in a single step.

Generation of (E)-β-sulfonyl enamines from sulfur dioxide via a radical process

An iron(ii)-catalyzed three-component reaction of O-acyl oximes, sulfur dioxide, and N-vinylacetamides is accomplished. Diverse (E)-β-sulfonyl enamines are obtained in moderate to good yields by using this protocol with excellent stereoselectivity and regioselectivity.

Cyanation: a photochemical approach and applications in organic synthesis

This review summarises the photocatalytic cyanation strategies to construct C(sp²)–CN, C(sp³)–CN and X–CN (X = N, S) bonds.

Visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates leading to coumarins via SO2 insertion

A visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates with cycloketone oxime compounds for the preparation of 3-cyanoalkylsulfonylcoumarins via SO₂ insertion is reported.

Organocatalytic Three-Component 1,2-Cyanoalkylacylation of Alkenes via Radical Relay

Here, we report an unprecedented regioselective, intermolecular 1,2-cyanoalkylacylation of feedstock alkenes with readily available oxime esters and aldehydes by N-heterocyclic carbene (NHC) organocatalysis. The crux of this success is the exquisite control over the radical relay process by an NHC organocatalyst. This protocol offers a general platform for diversity-oriented synthesis of valuable ketonitriles under mild, transition-metal-free, and redox-neutral conditions and highlights its potential in the late-stage functionalization of pharmaceutical architectures and natural products.

Visible-Light Photoredox-Catalyzed Ring-Opening Carboxylation of Cyclic Oxime Esters with CO2

The carboxylation of cyclic oxime esters with carbon dioxide via visible-light photoredox catalysis is demonstrated for the first time. A variety of cyclic oxime esters undergo ring-opening C-C bond cleavage and carboxylation to give cyanoalkyl-containing carboxylic acids in moderate to good yields. Moreover, this methodology features mild reaction conditions (room temperature, 1 atm), wide substrate scope, good functional group tolerance as well as facile derivations of products. Mechanistic studies indicate that the benzylic radicals and anions might be the key intermediates.

Synthesis of trisubstituted alkenes by Ni-catalyzed hydroalkylation of internal alkynes with cycloketone oxime esters

A method for Ni-catalyzed hydroalkylation of internal alkynes with cycloketone oxime esters was developed. The reaction has a broad substrate scope. This hydroalkylation shows excellent regio- and stereo-selectivity. This method enables readily available starting materials to be used to access a range of cyano-substituted single-configuration trisubstituted alkenes. These are valuable feedstock chemicals and are widely used in synthetic and medicinal chemistry.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.