Iron-Catalyzed Decarboxylative Olefination of Cycloketone Oxime Esters with α,β-Unsaturated Carboxylic Acids via C–C Bond Cleavage

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.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Copper Catalyst-Promoted Regioselective Multicomponent Cascade Cyclization of 3-Aza-1,5-enynes with Sulfur Dioxide and Cycloketone Oxime Esters to Access Cyanoalkylsulfonylated 1,2-Dihydropyridines

Radical cascade cyclization via the cracking of alkenyl C-H has emerged as an attractive and remarkable tool for the rapid construction of ring frameworks with endocyclic double bonds. We developed a cascade reaction of 3-aza-1,5-enynes with sulfur dioxide and cycloketone oxime esters to access cyanoalkylsulfonylated 1,2-dihydropyridines, which can be easily converted to pyridine derivatives. This protocol involves radical addition to the C≡C bond and 6-endo cyclization and features high regioselectivity and a broad substrate scope.

Diboron-promoted iron-catalyzed denitrative vinylation of β-nitrostyrenes with cycloketoximes

We have developed a new iron-catalyzed ring opening alkenylation of cycloketoximes with β-nitrostyrenes for the facile synthesis of flexible distal cyanoalkyl alkenes. Both non-active strained cycloketoximes and less-strained cycloketoximes could be coupled with β-nitrostyrenes with excellent regio- and stereoselectivity under base-free redox-neutral conditions. The diboron reagent played an irreplaceable role in this transformation. A reasonable radical-mediated process could be involved in this catalytic cycle.

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.

Photochemical and Single Electron Transfer Generation of 2'-Deoxycytidin-N4-yl Radical from Oxime Esters

A 2'-deoxycytidin-N4-yl radical (dC·), a strong oxidant that also abstracts hydrogen atoms from carbon-hydrogen bonds, is produced in a variety of DNA damaging processes. We describe here the independent generation of dC· from oxime esters under UV-irradiation or single electron transfer conditions. Support for this σ-type iminyl radical generation is provided by product studies carried out under aerobic and anaerobic conditions, as well as electron spin resonance (ESR) characterization of dC· in a homogeneous glassy solution at low temperature. Density functional theory (DFT) calculations also support fragmentation of the corresponding radical anions of oxime esters 2d and 2e to dC· and subsequent hydrogen atom abstraction from organic solvents. The corresponding 2'-deoxynucleotide triphosphate (dNTP) of isopropyl oxime ester 2c (5) is incorporated opposite 2'-deoxyadenosine and 2'-deoxyguanosine by a DNA polymerase with approximately equal efficiency. Photolysis experiments of DNA containing 2c support dC· generation and indicate that the radical produces tandem lesions when flanked on the 5'-side by 5'-d(GGT). These experiments suggest that oxime esters are reliable sources of nitrogen radicals in nucleic acids that will be useful mechanistic tools and possibly radiosensitizing agents when incorporated in DNA.

Divergent Synthesis of 2-Cyanoaryl Carbamate and 2-Cyanoaryl Urea Derivatives via Hypervalent Iodine-Induced C-C Bond Cleavage

Divergent synthetic methods for transforming isatins to 2-cyanoaryl carbamate and 2-cyanoaryl urea derivatives were developed using ammonium carbamate as the nitrogen source and iodobenzene diacetate as the oxidant. This reaction features mild conditions, broad substrate scope, and moreover, the use of toxic cyano-containing compounds is avoided.

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.

Fe-catalyzed denitrative cyanoalkylation of nitroalkenes with cycloketone oxime esters via reductive C–C bond formation

An iron catalyzed reductive denitrative cyanoalkylation of nitroalkenes with cycloketone oxime esters using Zn as the reductant has been successfully established in which the NO2 of nitroalkenes eventually acts as a leaving group.

Recent Advance in Iminyl Radical Triggered C–H and C–C Bond Functionalization of Oxime Esters via 1,5-HAT and β-Carbon Scission

The direct functionalization of C(sp3)–H and C(sp3)–C(sp3) bonds is considered as one of the most valuable synthetic strategies because of its high efficiency and step-economy for the rapid assembly of complex molecules. However, the relatively high bond disassociation energies (BDEs) and similar chemical environment lead to large obstacles in terms of low reactivity and selectivity. Using a radical-based strategy has proved to be an efficient approach to overcome these difficulties via a hydrogen atom transfer (HAT) process for selective C(sp3)–H functionalization and β-carbon scission for C(sp3)–C(sp3) bond derivatization. Oxime esters have emerged as outstanding precursors of iminyl radicals for versatile chemical transformations. This short review summaries the recent advances in site-specific C(sp3)–H functionalization and C(sp3)–C(sp3) bond cleavage starting from oxime esters by our group and pioneering work by others, mainly focusing on the reaction design as well as the reaction mechanism.

1 Introduction

2 C(sp3)–H Bond Functionalization via 1,5-HAT of Acyclic Oxime Esters­

2.1 1,5-HAT/Cyclization

2.2 1,5-HAT/C–C or C–Heteroatom Bond Formation

3 C(sp3)–C(sp3) Bond Functionalization via β-Carbon Scission of Cyclic Oxime Esters

3.1 β-Carbon Scission/C–C or C–Heteroatom Bond Formation

3.2 β-Carbon Scission/Cyclization

4 Conclusion and Outlook

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.

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.

Dual Nickel/Ruthenium Strategy for Photoinduced Decarboxylative Cross-Coupling of α,β-Unsaturated Carboxylic Acids with Cycloketone Oxime Esters

Herein, a dual nickel/ruthenium strategy is developed for photoinduced decarboxylative cross-coupling between α,β-unsaturated carboxylic acids and cycloketone oxime esters. The reaction mechanism is distinct from previous photoinduced decarboxylation of α,β-unsaturated carboxylic acids. This reaction might proceed through a nickelacyclopropane intermediate. The C(sp²)-C(sp³) bond constructed by the aforementioned reaction provides an efficient approach to obtaining various cyanoalkyl alkenes, which are synthetically valuable organic skeletons in organic and medicinal chemistry, under mild reaction conditions. The protocol tolerates many critical functional groups and provides a route for the modification of complex organic molecules.

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.

Generation of Sulfonylated Tetrazoles through an Iron-Catalyzed Multicomponent Reaction Involving Sulfur Dioxide

As a privileged motif, tetrazoles can be widely found in pharmaceuticals and materials science. Herein, a five-component reaction of cycloketone oxime esters, alkynes, DABCO·(SO₂)₂, and two molecules of trimethylsilyl azide under iron catalysis is developed, giving rise to a range of cyano-containing sulfonylated tetrazoles in moderate to good yields. This multicomponent reaction exhibits excellent selectivity and enables the formation of multiple new chemical bonds in one pot. A possible mechanism involving azidosulfonylation of alkynes, C-C bond cleavage of both cycloketone oxime esters and alkynes, and [3 + 2] cycloaddition of trimethylsilyl azide and the nitrilium cation intermediate is proposed. Additionally, the potential of terminal alkynes acting as powerful synthons for the synthesis of tetrazoles in a radical initiated process is demonstrated for the first time.

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High-value tetrazole motifs were synthesized via a five-component reaction

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Fixing sulfur dioxide into tetrazole molecules under mild conditions

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Low-cost iron catalyst initiated the transformation

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Excellent selectivity with the formation of multiple new chemical bonds

Visible-light-driven palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade

A visible-light-induced palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade is described. A range of six to nine-membered β-alkenylated cyclic ketones possessing a quaternary carbon center were accessed under mild conditions. Besides styrenes, the electron-rich alkenes such as silyl enol ethers and enamides were also compatible, providing the desired β-alkylated cyclic ketones in moderate to good yields.

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.

Copper-catalyzed oxidative decarboxylative alkylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines

We have developed a new oxidative decarboxylation of cinnamic acids with 4-alkyl-1,4-dihydropyridines to construct C(sp3)-C(sp2) bonds in the presence of copper catalyst and dicumyl peroxide (DCP). A variety of internal alkenes have been obtained with mild conditions, broad substrate scope and excellent functional group tolerance. This method has significant potential for application by using inexpensive and stable cinnamic acids instead of alkenyl halides and nitro-olefins.

Recent Progress in Radical Decarboxylative Functionalizations Enabled by Transition-Metal (Ni, Cu, Fe, Co or Cr) Catalysis

Aliphatic carboxylic acids are abundant in natural and synthetic sources and are widely used as connection points in many chemical transformations. Radical decarboxylative functionalization promoted by transition-metal catalysis has achieved great success, enabling carboxylic acids to be easily transformed into a wide variety of products. Herein, we highlight the recent advances made on transition-metal (Ni, Cu, Fe, Co or Cr) catalyzed C–X (X = C, N, H, O, B, or Si) bond formation as well as syntheses of ketones, amino acids, alcohols, ethers and difluoromethyl derivatives via radical decarboxylation of carboxylic acids or their derivatives, including, among others, redox-active esters (RAEs), anhydrides, and diacyl peroxides.

1 Introduction

2 Ni-Catalyzed Decarboxylative Functionalizations

3 Cu-Catalyzed Decarboxylative Functionalizations

4 Fe-Catalyzed Decarboxylative Functionalizations

5 Co- and Cr-Catalyzed Decarboxylative Functionalizations

6 Conclusions

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.

Photocatalytic dual decarboxylative alkenylation mediated by triphenylphosphine and sodium iodide

An efficient photocatalytic dual decarboxylative alkenylation of α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide (NHP) esters mediated by triphenylphosphine and sodium iodide has been developed. This protocol proceeds under 456-nanometer irradiation by visible blue light in the absence of transition metals or organic dye based photoredox catalysts. The reaction is successfully applied to a wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) and α-amino acids, enabling transformations of diverse α,β-unsaturated carboxylic acids to α,β-alkylated styrenes with high efficiency and excellent selectivity under mild conditions.

O-Perfluoropyridin-4-yl Oximes: Iminyl Radical Precursors for Photo- or Thermal-Induced N-O Cleavage in C(sp2)-C(sp3) Bond Formation

O-Perfluoropyridin-4-yl group was first installed onto cycloketone oximes as a new electrophore, which was proven to be efficient iminyl radical precursors under photocatalytic and thermal conditions. A range of O-perfluoropyridin-4-yl oximes were successfully utilized in C(sp²)-C(sp³) bond formations of quinoxalin-2(1H)-ones and alkenes, providing facile accesses to a range of functionalized alkylnitriles.

Copper-catalyzed alkylarylation of vinylarenes with cycloalkylsilyl peroxides and boronic acids

A mild and efficient copper-catalyzed alkylarylation of vinylarenes with cycloalkylsilyl peroxides and boronic acids is described.

Iron catalyzed ketoalkylation and ketoalkylation/etherification of styrenes initiated by selective C–C bond cleavage

A mild and efficient iron-catalyzed ketoalkyl-Heck-type coupling initiated by radical C–C bond cleavage is described. Furthermore, this concise catalytic system was also applicable for the three-component ketoalkylation/etherification of styrenes.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.