Nickel-Catalyzed Reductive Electrophilic Ring Opening of Cycloketone Oxime Esters with Aroyl Chlorides

Nickel-catalysed highly regioselective synthesis of β-acyl naphthalenes under reductive conditions

Over the past decade, significant progress has been made in the direct C-H acylation of naphthalenes, occurring at the α or β-positions to yield valuable ketones through Friedel-Crafts acylation or transition-metal-catalysed carbonylative coupling reactions. Nevertheless, highly regioselective acylation of naphthalenes remains a formidable challenge. Herein, we developed a nickel-catalysed reductive ring-opening reaction of 7-oxabenzonorbornadienes with acyl chlorides as the electrophilic coupling partner, providing a new method for the exclusive preparation of β-acyl naphthalenes.

I2 Catalyzed and TBHP/Ammonium-Promoted Conversion of Arylethanone to Nitriles via β-Scission of Iminyl Radicals

Herein, we report a general I2-catalyzed and TBHP/ammonium-promoted conversion of arylethanone to aromatic nitriles under air. This procedure proceeded with the β-scission of iminyl radical, which was facilitated via quenching the released alkyl radical by tert-butyl peroxyl radical leading to peroxide followed with Kornblum-DeLaMare rearrangement. A series of aryl methyl ketone and alkyl aryl ketone worked well with good functional group tolerance in high yields. As such, this metal-free procedure represents a facile, safe, green, and practical procedure in conversion of arylethanone to aromatic nitriles.

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

Photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access 2,4-disubstituted quinolines

We herein report an efficient photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access a wide range of 2,4-disubstituted quinolines. Preliminary mechanism experiment results suggested that this reaction was initiated by an acyl radical generated from acyl chlorides through a single-electron-transfer (SET) process. This transformation showed good substrate suitability and functional group compatibility at room temperature.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Stereoselective Synthesis Axially Chiral Arylnitriles through Base-Induced Chirality-Relay β-Carbon Elimination of α-Hydroxyl Ketoxime Esters

We report herein a point-to-axial chirality transfer reaction of α-hydroxyl oxime esters for the synthesis of axially chiral arylnitriles. The reaction proceeds smoothly through a base-promoted retro-benzoin condensation reaction of α-hydroxyl oxime esters, where the axial chirality is created via the C-C bond cleavage based on a proper distorted conformation of the biaryl structure induced by its stereogenic carbon center.

Nickel-Catalyzed Diastereoselective Cross-Electrophile Ring Opening of 7-Oxabenzonorbornadienes with Aromatic Aldehydes

In this protocol, we developed a nickel-catalyzed diastereoselective cross-electrophile ring opening reaction of 7-oxabenzonorbornadienes with aromatic aldehydes as the electrophilic coupling partner utilizing Zn as the stoichiometric reductant. In this reaction, a challenging stereoselective bond formation between two disubstituted sp³-hybridized carbon centers has been achieved, furnishing a variety of 1,2-dihydronaphthalenes with full diastereocontrol of three consecutive stereogenic centers.

Nickel-Catalyzed Decarbonylative Reductive Alkylation of Aroyl Fluorides with Alkyl Bromides

This paper describes the nickel-catalyzed reductive alkylation of aroyl fluorides with alkyl bromides in a decarbonylative manner. In this reaction, various functional groups are well tolerated and the C(sp²)-C(sp³) bond can be constructed directly without the use of organometallic reagents. The present reaction is a cross-electrophile coupling via the radical pathway, affording the corresponding alkylarenes in moderate to good yields.

Synthesis of Multisubstituted Allylic Alcohols via a Nickel-Catalyzed Cross-Electrophile Ring-Opening Reaction

Herein we report a nickel-catalyzed cross-electrophile ring-opening reaction of vinyl epoxides wherein aryl iodides, alkyl iodides, and benzyl chlorides can all serve as the electrophilic coupling partners, providing a new approach to preparing multisubstituted allylic alcohols. This new method features broad substrate scope (76 examples), good step-economy, and high L/B- and E/Z selectivity as well as mild reaction conditions.

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.

Insights into the Mechanism of Metal-Catalyzed Transformation of Oxime Esters: Metal-Bound Radical Pathway vs Free Radical Pathway

Controlling of radical reactivity by binding a radical to the metal center is an elegant strategy to overcome the challenge that radical intermediates are "too reactive to be selective". Yet, its application has seemingly been limited to a few strained-ring substrates, azide compounds, and diazo compounds. Meanwhile, first-row transition-metal-catalyzed (mainly, Fe, Ni, Cu) transformations of oxime esters have been reported recently in which the activation processes are assumed to follow free-radical mechanisms. In this work, we show by means of density functional theory calculations that the activation of oxime esters catalyzed by Fe(II) and Cu(I) catalysts more likely affords a metal-bound iminyl radical, rather than the presumed free iminyl radical, and the whole process follows a metal-bound radical mechanism. The as-formed metal-bound radical intermediates are an Fe(III)-iminyl radical (S_total = 2, S_Fe = 5/2, and S_iminyl = -1/2) and a Cu(II)-iminyl radical (S_total = 0, S_Cu = 1/2, and S_iminyl = -1/2). The discovery of such novel substrates affording metal-bound radical intermediates may facilitate the experimental design of metal-catalyzed asymmetric synthesis using oxime esters to achieve the desired enantioselectivity.

Ni-Catalyzed Remote Radical/Cross-Electrophile Coupling Cascade for Selective C(sp3)-H Arylation

An innovative 1,5-HAT cascade strategy has been advanced for the nickel-catalyzed distal arylation via cross-electrophile coupling. Through specific migration, the remote C(sp³)-H bond is regioselectively activated, and Ar-I as the available electrophile is used for the construction of the C(sp³)-C(sp²) bond. This method also has broad applicability for benzylic and aliphatic N-fluorocarboxamides with yields up to 80%. Furthermore, a series of control experiments demonstrated that this reaction is probably initiated by a radical process.

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.

Oxime ester-enabled anti-Markovnikov hydrosilylation of alkenes

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

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.

DFT Insights into the Hydrodenitrogenation and Ring-Opening of Indole on an M (M = Ni, Pt, Ni–Pt) Slab Model

Density functional theory (DFT) calculation has been used to study the hydrodenitrogenation (HDN) and ring-opening of indole on an M (M = Ni, Pt, Ni–Pt) slab surface. The possible reaction pathway of indole hydrogenation has been investigated in order to reveal the bimetallic synergistic effects of an M slab surface. Compared to the mechanism of indole hydrogenation on an M slab surface, it was found that a PtNi(111) surface favors indole hydrogenation. According to the results of DFT calculation, it suggests that the bimetallic effect of the M surface plays an important role in indole hydrogenation.

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.

C-C Bond Acylation of Oxime Ethers via NHC Catalysis

An N-heterocyclic carbene (NHC)-catalyzed strategy has been developed to address the issue of using toxic transitional metals in the field of C-C bond activation. The novel reaction mode enables an efficient docking between the cyanoalkyl from the cycloketone oxime derivative and the acyl group from the aldehyde, affording ketonitrile in moderate to good yields, which is one kind of useful building block for synthesizing nitrogen-containing pharmacophores.

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.

Nickel-Catalyzed Directed Cross-Electrophile Coupling of Phenolic Esters with Alkyl Bromides

Herein, we demonstrate the successful use of robust phenolic esters as an electrophilic acyl source in the reaction with diverse primary and secondary unactivated alkyl bromides. The cleavage of the relatively inert C-O bond is facilitated by the neighboring coordinating hydroxyl or sulfonamide moiety. By circumventing the use of pregenerated organometallics, this method allows efficient preparation of a variety of o-hydroxyl and tosyl-protected o-amino aryl ketones with high compatibility with a wide range of functionalities.

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.

Nickel-Catalyzed Transformation of Alkene-Tethered Oxime Ethers to Nitriles by a Traceless Directing Group Strategy

Nickel-catalyzed transformation of alkene-tethered oxime ethers to nitriles using a traceless directing group strategy has been developed. A series of alkene-tethered oxime ethers derived from benzaldehyde and cinnamyl aldehyde derivatives were converted into the corresponding benzonitriles and cinnamonitriles in 46-98% yields using the nickel catalyst system. Control experiments showed that the alkene group tethered to an oxygen atom on the oximes via one methylene unit plays a key role as a traceless directing group during the catalysis.

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.

Metal-free iminyl radical-mediated C–C single bond cleavage/functionalization of redox-active oxime esters

By photoactivating Hantzsch ester (HE) to act as an electron donor, we have achieved single-electron reduction of cycloketone oxime esters, thus providing a novel catalyst-free strategy for alkylation, allylation, vinylation and alkynylation.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

A dual photoredox-nickel strategy for remote functionalization via iminyl radicals: radical ring-opening-arylation, -vinylation and -alkylation cascades

A divergent strategy for the remote arylation, vinylation and alkylation of nitriles is described. These processes proceed through the photoredox generation of a cyclic iminyl radical and its following ring-opening reaction. The distal nitrile radical is then engaged in nickel-based catalytic cycles to form C-C bonds with aryl bromides, alkynes and alkyl bromides.

Iminyl radical-promoted imino sulfonylation, imino cyanogenation and imino thiocyanation of γ,δ-unsaturated oxime esters: synthesis of versatile functionalized pyrrolines

We reported an iminyl radical-promoted imino-functionalization of γ,δ-unsaturated oxime esters to synthesize versatile functionalized pyrrolines.

Copper-catalyzed ring-opening C(sp3)–N coupling of cycloketone oxime esters: access to 1°, 2° and 3° alkyl amines

Access to 1°, 2° and 3° alkyl amines through copper-catalyzed C(sp³)–N coupling of cycloketone oxime esters was realized.