Oxidative C–C Bond Cleavage of Aldehydes via Visible-Light Photoredox Catalysis

1O2 and Base Assisted Oxidative Conversion of β-Enaminoesters to α-Acyloxy-β-ketoesters under Visible Light Irradiation

Singlet oxygen (1O2) and base assisted conversion of β-enaminoesters to α-acyloxy-β-ketoesters is demonstrated under visible light irradiation. The reaction involves formation of an imine intermediate via ene-type pathway initiated by 1O2 followed by base promoted dimerization and hydrolysis steps. The method is mild, environmentally friendly, requires air as the oxidant, and gives the products in moderate to high yields.

Cobalt(II)-Catalyzed Synthesis of γ-Diketones from Aryl Alkenes and Its Utilization in the Synthesis of Various Heterocyclic Compounds

An earth-abundant Co(II) salt-catalyzed mild and affordable synthetic route has been developed for the synthesis of industrially relevant 1,4-dicarbonyl compounds (or γ-diketones) via oxidative coupling between aryl alkenes and ketones (both cyclic and acyclic) using TBHP and DBU as the oxidant and base, respectively. 1,4-Dicarbonyl compounds are known to be synthesized using expensive metal catalysts, dual catalysts, or low-cost metal complexes combined with an additive or ligand template, which further needs to be synthesized. Herein, we report the synthesis of 1,4-dicarbonyl compounds using cobalt(II) acetate as a catalyst without any expensive co-catalyst or ligand templates. This methodology has a broad substrate scope with significant yields and good functional group tolerance. Generation of unsymmetrical 1,4-dicarbonyls at room temperature and its versatile synthetic expansion to produce synthetically and biologically valuable heterocyclic compounds are salient features of this novel methodology. In addition, various controlled experiments such as primary kinetic isotope effect study, Hammett analysis with variation of the nature of the substituents on the styrene ring, and theoretical calculations (density functional theory) unravel the mechanistic intricacies involved in this new, simple, and atom-economic methodology.

Catalytic Light-Driven Strategy for Transforming Oximes to Carbonyls

Oxime and carbonyl functional groups serve as powerful chemical hubs for constructing complex synthetic targets and valuable molecular scaffolds. In furthering this value, we report a photopromoted catalytic deoximation protocol for converting oximes and their derivatives to carbonyl functional groups. This strategic approach benefits from the use of renewable light energy input and ambient air conditions, in addition to demonstrating good substrate scope, functional group tolerance, and product yields. In offering, insights into these reactivity mechanistic studies are communicated, and the value of this protocol is further shown through one-pot operations.

Oxidative Amination of Aldehydes with Amines into α-Amino Ketones

Oxidative amination for the installation of nitrogen functional molecules from nitrogen nucleophiles has always been a very challenging topic in organic synthesis. Here we report a novel conversion of different aldehydes with secondary amines for the synthesis of diversified α-amino ketones. This method can be achieved through oxidative rearrangement of an in situ-generated enamine intermediate promoted by commercially available sodium percarbonate. Furthermore, this one-pot process is also suitable for the functional modification of complex molecules.

Enantioconvergent Palladium-Catalyzed Alkylation of Tertiary Allylic C-H Bonds

Enantioconvergent catalysis enables the conversion of racemic molecules into a single enantiomer in perfect yield and is considered an ideal approach for asymmetric synthesis. Despite remarkable advances in this field, enantioconvergent transformations of inert tertiary C-H bonds remain largely unexplored due to the high bond dissociation energy and the surrounding steric repulsion that pose unparalleled constraints on bond cleavage and formation. Here, we report an enantioconvergent Pd-catalyzed alkylation of racemic tertiary allylic C-H bonds of α-alkenes, providing a unique approach to access a broad range of enantioenriched γ,δ-unsaturated carbonyl compounds featuring quaternary carbon stereocenters. Mechanistic studies reveal that a stereoablative event occurs through the rate-limiting cleavage of tertiary allylic C-H bonds to generate σ-allyl-Pd species, and the achieved E/Z-selectivity of σ-allyl-Pd species effectively regulates the diastereoselectivity via a nucleophile coordination-enabled SN 2'-allylation pathway.

Diversity-Oriented Metal-Free Synthesis of Nitrogen-Containing Heterocycles Using Atropaldehyde Acetals as a Dual C3/C2-Synthon

A highly efficient and elegant diversity-oriented reaction paradigm employing atropaldehyde acetals as new dual C2/C3 synthons was developed under metal-free conditions using glycine esters as the counterpart reagents, which allowed rapid synthesis of two important nitrogen-containing heterocycles, pyrrolo[1,2-a]quinolines and 3,5-diarylpyridines. The divergent products are subtly controlled by the manipulation of the substitutional groups of glycine esters. When a N-arylglycine ester was used, pyrrolo[1,2-a]quinolines can be formed through cascade oxidative C-C cleavage/multiple cyclization. Instead, N-benzylglycine ester as the counter-reagent led to the synthesis of 3,5-diarylpyridines via two key C-N cleavages. Mild conditions, broad substrate scope, scalability and environmentally acceptable organic solvents rendered this method practical and attractive.

Applications of Photoredox Catalysis for the Radical-Induced Cleavage of C–C Bonds

Selective cleavage of C–C bonds forms one of the greatest challenges in current organic chemistry, due to the relative strength of these bonds. However, such transformations are an invaluable instrument to break down and construct new carbon–carbon bonds. To achieve this, photochemistry can be used as a tool to generate radicals and induce the cleavage of these bonds due to their high reactivity. This review examines some of the most influential contributions in this field since 2010.

1 Introduction

2 C–C Bond Cleavage

2.1 Homogeneous Catalyst

2.1.1 N-Centered Radical

2.2.2 O-Centered Radical

2.2 Heterogeneous Catalyst

3 C=C Bond Cleavage

3.1 Homogeneous Catalyst

3.2 Heterogeneous Catalyst

4 C≡C Bond Cleavage

4.1 Homogeneous Catalyst

4.2 Heterogeneous Catalyst

5 Conclusion

Visible light-mediated synthesis of α-alkoxy/hydroxy diarylacetaldehydes from terminal alkynes

A visible light-mediated approach enabling the use of alcohols as nucleophiles in a one-step synthesis of α-alkoxy/hydroxy diarylacetaldehydes is reported.

Visible light-driven copper(ii) catalyzed aerobic oxidative cleavage of carbon–carbon bonds: a combined experimental and theoretical study

By switching on visible blue light, aerobic oxidation of various substrates, such as α-substituted, β-substituted and α-halo styrenes, was first realized with a copper(ii) catalyst.

Acid-catalyzed cleavage of C-C bonds enables atropaldehyde acetals as masked C2 electrophiles for organic synthesis

Acid-catalyzed tandem reactions of atropaldehyde acetals were established for the synthesis of three important molecules, 2,2-disubstituted indolin-3-ones, naphthofurans and stilbenes. The synthesis was realized using novel reaction cascades, which involved the same two initial steps: (i) S_N2' substitution, in which the atropaldehyde acted as an electrophile; and (ii) oxidative cleavage of the carbon-carbon bond of the generated phenylacetaldehyde-type products. Compared with literature methods, the present protocol not only avoided the use of expensive noble metal catalysts, but also enabled a simple operation.

Visible-Light-Mediated Photocatalytic Oxidative C–C Bond Cleavage of Geminal Diazides: An Approach to Oxamates

Photoredox catalysis has received great attention in both academia and industry and remarkable progress has been made over the past decade. Now, it has been shown that a visible-light-mediated oxidative C–C bond cleavage of geminal diazides can be induced by organic dye catalysis for the synthesis of oxamates. A mechanistic study, confirmed by control experiments, indicates that this proceeds through single-electron transfer (SET). This methodology can be applied to convert a wide array of geminal diazides into oxamates.

Cobalt-Catalyzed Aerobic Oxidative Cleavage of Alkyl Aldehydes: Synthesis of Ketones, Esters, Amides, and α-Ketoamides

A widely applicable approach was developed to synthesize ketones, esters, amides via the oxidative C-C bond cleavage of readily available alkyl aldehydes. Green and abundant molecular oxygen (O₂ ) was used as the oxidant, and base metals (cobalt and copper) were used as the catalysts. This strategy can be extended to the one-pot synthesis of ketones from primary alcohols and α-ketoamides from aldehydes.

Transition-metal-free decarbonylative alkylation towards N-aryl α-hydroxy amides via triple C–C bond cleavages and their selective deuteration

A transition-metal-free decarbonylative alkylation reaction for the synthesis of N-aryl α-hydroxy amides via precise cleavages and reorganizations of three C–C σ bonds has been developed.

Gold(I)-Catalyzed Synthesis of Highly Substituted 1,4-Dicarbonyl Derivatives via Sulfonium [3,3]-Sigmatropic Rearrangement

An efficient and straightforward gold-catalyzed protocol for the synthesis of 2-substituted 4-oxo-4-arylbutanal derivatives from commercially available or easily accessible alkynes and vinylsulfoxide substrates has been developed. Extension of the methodology to the use of 1-cycloalkenyl sulfoxides allowed the facile synthesis of five-, six-, and seven-membered-ring cycloalkyl-1-one backbone. Subsequently, the tetrahydrocycloalkyl[b]pyrrole derivatives, which are found in many active pharmaceutical ingredients, were isolated in good yields. Mechanistic investigation highlighted a [3,3]-sigmatropic rearrangement of a sulfonium intermediate in this process.

Selective valorization of lignin to phenol by direct transformation of Csp2-Csp3 and C-O bonds

Phenol is an important commodity chemical in the industry, which is currently produced using fossil feedstocks. Here, we report a strategy to produce phenol from lignin by directly deconstructing C_sp2-C_sp3 and C-O bonds under mild conditions. It was found that zeolite catalyst could efficiently catalyze both the direct C_sp2-C_sp3 bond breakage to remove propyl structure and aliphatic β carbon-oxygen (C_β-O) bond hydrolysis to form OH group on the aromatic ring. The yield of phenol could reach 10.9 weight % with a selectivity of 91.8%. In this unique route, water was the only reactant besides lignin. A scale-up experiment showed that 4.1 g of pure phenol could be obtained from 50.0 g of lignin. The reaction pathway was proposed by a combination of control experiments and density functional theory studies. This work opens the way for producing phenol from lignin by direct transformation of C_sp2-C_sp3 and C-O bonds in lignin.

Eosin Y-Catalyzed Visible-Light-Mediated Aerobic Transformation of Pyrazolidine-3-One Derivatives

By utilizing an underexplored reactivity of N1-substituted pyrazolidine-3-ones, we developed a visible-light-induced aerobic oxidation of N1-substituted pyrazolidine-3-one derivatives yielding the corresponding azomethine imines. The resulting azomethine imines can be further reacted with ynones in situ under copper catalyzed [3 + 2] cycloaddition reaction conditions yielding the corresponding pyrazolo[1,2-a]pyrazoles in good yields. The methodology can be extended to other 1-aryl-substituted pyrazolidinones which undergo endocyclic oxidation deriving the corresponding pyrazolones as single products.

Copper Promoted Aerobic Oxidative C(sp3)-C(sp3) Bond Cleavage of N-(2-(Pyridin-2-yl)-ethyl)anilines

A strategy of aerobic oxidative C(sp³)-C(sp³) bond cleavage of N-ethylaniline derivatives bearing azaarenes for the synthesis of N-aryl formamides has been developed. This approach was carried out smoothly with the CuI/TEMPO/air system to give N-aryl formamides in yields of 50-90%. With this methodology, a mutagenically active compound was constructed in 90% yield. Moreover, the reaction also provided a one-pot synthetic tool for accessing a promoter of hematopoietic stem cells by difunctionalization in 61% yield.

Visible light-mediated photocatalytic oxidative cleavage of activated alkynes via hydroamination: a direct approach to oxamates

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature. In this reaction, the single electron oxidation of an in situ formed enamine followed by radical coupling with an oxidant finally delivers the oxamate. The key features of this photocatalytic reaction are the mild reaction conditions, metal-free organic dye as a photocatalyst, and TBHP playing a dual role as “O” source and for the regeneration of the photocatalyst.

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature.

Spirocyclic Tetramates by Sequential Knoevenagel and [1,5]-Prototropic Shift

Highly functionalized spirocyclic tetramates were prepared via a sequential Knoevenagel reaction and [1,5]-prototropic shift (T-reaction) of bicyclic tetramates. While these compounds isomerize in solution, stable analogues can be prepared via an appropriate choice of substituents. Further modification of these compounds allows for the introduction of aromatic groups, making them suitable as skeletons for application in medicinal chemistry.

Catalytic Aerobic Oxidation of C(sp3 )-H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk-scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂ , the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

Visible-Light-Mediated Ring-Opening Strategy for the Regiospecific Allylation/Formylation of Cycloalkanols

Here we describe a straightforward and efficient approach for regiospecific introduction of an allyl group into cycloalkanol molecules employing a visible-light-mediated ring-opening strategy. A wide range of distally allylated or formylated ketones is furnished from 1-aryl cycloalkanol precursors of variable ring sizes, providing a concise and practical access for the modification of complex natural products. Preliminary mechanistic studies demonstrate that the key O-centered radicals mediate the sequential ring cleavage and allylation/formylation.

Asymmetric Total Synthesis of Cyclocitrinol

The first and asymmetric total synthesis of cyclocitrinol, an unusual C25 steroid, has been accomplished in a linear sequence of 18 steps from commercially available compound 11. The synthetically challenging bicyclo[4.4.1] A/B ring system with a strained bridgehead (anti-Bredt) double bond of cyclocitrinol was constructed efficiently and diastereoselectively via a type II intramolecular [5 + 2] cycloaddition.

Enantioselective isoquinuclidine synthesis via sequential Diels–Alder/visible-light photoredox C–C bond cleavage: a formal synthesis of the indole alkaloid catharanthine

An enantioselective formation of isoquinuclidines useful for alkaloid synthesis is achieved through an organocatalyzed Diels–Alder reaction of dihydropyridines with acrolein and a subsequent photoredox catalyzed oxidative deformylation reaction.

Metal- and O2-Free Oxidative C-C Bond Cleavage of Aromatic Aldehydes

An oxidative C-C cleavage of aldehydes requiring neither metals nor O₂ was discovered. Homobenzylic aldehydes and α-substituted homobenzylic aldehydes were cleaved to benzylic aldehydes and ketones, respectively, using nitrosobenzene as an oxidant. This reaction is chemoselective for aromatic aldehydes, as an aliphatic aldehyde was unreactive under these conditions, and other reactive functionality such as ketones and free alcohols are tolerated. A mechanism accounting for the fate of the lost carbon is proposed.

Recent advances in visible-light-driven organic reactions

In recent years, visible-light-driven organic reactions have been experiencing a significant renaissance in response to topical interest in environmentally friendly green chemical synthesis. The transformations using inexpensive, readily available visible-light sources have come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In this review, we focus on recent advances in the development of visible-light-driven organic reactions, including aerobic oxidation, hydrogen-evolution reactions, energy-transfer reactions and asymmetric reactions. These key research topics represent a promising strategy towards the development of practical, scalable industrial processes with great environmental benefits.

Synthesis of carbonylated heteroaromatic compounds via visible-light-driven intramolecular decarboxylative cyclization of o-alkynylated carboxylic acids

An efficient strategy for the easy access to carbonylated heteroaromatic compounds has been developed via a visible-light-promoted intramolecular decarboxylative cyclization reaction of o-alkynylated carboxylic acids.