Acid-Catalyzed Oxidative Radical Addition of Ketones to Olefins

Advances in radical peroxidation with hydroperoxides

Organic peroxides have become sought-after functionalities, particularly following the multi-tone consumption in polymer production and success in medicinal chemistry. The selective introduction of a peroxide fragment at different positions on the target molecule is a priority in the modern reaction design. The pioneering Kharasch-Sosnovsky peroxidation became the basic universal platform for the development of peroxidation methods, with its great potential for rapid generation of complexity due to the ability to couple the resulting free radicals with a wide range of partners. This review discusses the recent advances in the radical Kharasch-type functionalization of organic molecules with OOR fragment including free-component radical couplings. The discussion has been structured by the type of the substrate of radical peroxidation: C(sp 3 )-H substrates; aromatic systems; compounds with unsaturated C-C or C-Het bonds.

Photoinduced fluoroalkylation-peroxidation of alkenes enabled by ligand-to-iron charge transfer mediated decarboxylation

We report here a photoinduced iron-catalyzed fluoroalkylation-peroxidation of activated and/or unactivated alkenes with fluoroalkyl carboxylic acids and hydroperoxide. The ligand-to-iron charge transfer strategy effectively overcomes the high redox potential of the fluoroalkyl carboxylic acids, facilitating the difunctionalization reaction to occur smoothly under mild reaction conditions. The late-stage functionalization of drug and natural product derivatives was also demonstrated.

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.

Generation and Radical-Radical Cross-Coupling of Alkenyloxy Radical

Alkene-attached oxygen radicals are rarely used, as highly reactive oxygen radicals are incompatible with the alkene moiety. The direct radical-radical cross-coupling of O radicals is also challenging (limited to N-O bond formation) because of the lack of suitable persistent radical species. This study demonstrated the feasibility of using Breslow intermediate radical (BIR) as a persistent radical to capture unstable π-conjugated O radicals and allow the C-O radical-radical cross-coupling.

Multicomponent Reactions between Heteroatom Compounds and Unsaturated Compounds in Radical Reactions

In this mini-review, we present our concepts for designing multicomponent reactions with reference to a series of sequential radical reactions that we have developed. Radical reactions are well suited for the design of multicomponent reactions due to their high functional group tolerance and low solvent sensitivity. We have focused on the photolysis of interelement compounds with a heteroatom-heteroatom single bond, which readily generates heteroatom-centered radicals, and have studied the photoinduced radical addition of interelement compounds to unsaturated compounds. First, the background of multicomponent radical reactions is described, and basic concepts and methodology for the construction of multicomponent reactions are explained. Next, examples of multicomponent reactions involving two interelement compounds and one unsaturated compound are presented, as well as examples of multicomponent reactions involving one interelement compound and two unsaturated compounds. Furthermore, multicomponent reactions involving intramolecular cyclization processes are described.

TBAI-Catalyzed Regioselective Hydroxyperoxidation of 1-Aryl/Alkyl-1,3-dienes

An efficient, mild, and economical approach for regioselective synthesis of 4-aryl/alkyl-1-peroxy-but-3-en-2-ols from 1-substituted-1,3-butadienes using hydroperoxides and catalyzed by TBAI has been developed. This method can be executed in a simple operation with no dry conditions required and having tolerance to a wide range of substrates to access corresponding hydroxyperoxidates in good yields. Thus, an excellent regioselective orthogonal dioxygenation in a diene system has been achieved.

An Ionic-Radical Approach to Vicinally Functionalized Cyclopentanones and Cyclohexanones

A versatile strategy to access vicinally functionalized cyclopentanones and cyclohexanones is described. It takes advantage of new reagents, α-xanthyl enones, which can be prepared from the reaction of xanthate salts with the corresponding epoxyketones under mild conditions. β-Functionalization of these compounds can be performed by conjugate additions without affecting the xanthate moiety. This significantly expands the pool of xanthate substrates, allowing the synthesis of open chain and fused bicyclic building blocks useful in the synthesis of natural products.

Azodioxy compounds as precursors for C-radicals and their application in thermal styrene difunctionalization

An atom-economic thermal α,β-difunctionalization of various styrenes with readily prepared azodioxy compounds is reported. Mechanistic studies reveal that the starting azodioxy compounds can thermally be cleaved to the corresponding C-nitroso compounds, which under these thermal conditions further homolyze to generate reactive C-radicals along with the persistent NO radical. In the presence of a styrene, C-radical addition with subsequent nitrosylation followed by tautomerization is occurring, resulting in an overall styrene β-alkylation-α-oximation reaction.

Regioselective Approach to β-Peroxyl Alcohols and Ethers from Alkenes

A different regioselective three-component reaction of alkenes, oxygen sources, and hydroperoxides mediated by ammonium iodine to α-oxyperoxidates has been developed. Mechanistic studies demonstrated that regioselective radical addition and subsequent S_N2 nucleophilic substitution were possible for the formation of products. In addition to the traditional pathway of S_N2 reaction, that is, where nucleophiles attack the α-C atoms at the back side, an additional unusual transition configuration with the H₂O molecule attacking the α-C atom at the front side was obtained.

Visible-Light-Driven Organophotocatalyzed Multicomponent Approach for Tandem C(sp3)-H Activation and Alkylation Followed by Trifluoromethylthiolation

A visible-light-driven organophotocatalyzed multicomponent approach has been developed for tandem direct C(sp³)-H activation and alkylation followed by trifluoromethylthiolation in a one-pot operation. We report a completely metal-free, tandem, three-component approach for the difunctionalization of activated alkenes via the photoinduced radical pathway. This protocol allows the formation of two new C(sp³)-C(sp³) and C(sp³)-SCF₃ bonds using a bench-stable, easy-to-handle trifluoromethylthiolating reagent under mild reaction conditions. The generosity of this reaction is shown with a library of C(sp³)-H donors and alkenes derivatives. The reaction conditions can tolerate a wide variety of functional groups. Gram-scale synthesis using environmentally benign and straightforward conditions highlights the synthetic advancement of the methodology. Further functionalization of the final product is also successfully demonstrated.

Access to 3,3'-disubstituted peroxyoxindole derivatives and α-peroxyamides via azaoxyallyl cation-guided addition of hydroperoxides

Herein, a transition metal-free approach for access to 3,3'-disubstituted peroxyoxindole is disclosed, which harnesses a transient azaoxyallyl cation. This strategy is also applicable to the synthesis of structurally diverse α-peroxycarboxylic acid surrogates. The method exhibits good functional group tolerance and is suitable for generating a library of peroxy-containing compounds.

Oxidant-Switched Palladium-Catalyzed Regioselective Mono- versus Bis-ortho-Aroylation of 1-Aryl-1H-indazoles with Aldehydes via C-H Bond Activation

A highly efficient oxidant-switched palladium-catalyzed regioselective C_(sp²)-H/C_(sp²)-H cross-dehydrogenative coupling (CDC) for direct mono/bis-ortho-aroylation of substituted 1-phenyl-1H-indazoles 1a-j with various substituted aldehydes 3a-t via C_(sp²)-H bond activation has been developed. In this study, Pd-catalyzed chelation-assisted mono- or bis-aroylation of substituted 1-phenyl-1H-indazoles depends on the type of oxidant being used for the CDC reaction. While mono-ortho-aroylation of substituted 1-phenyl-1H-indazole was obtained using dicumylperoxide (DCP) as the oxidant, the bis-ortho-aroylation product has been afforded by the use of tert-butyl hydroperoxide (TBHP). Regardless of the greater activity at the C-3 position of 1H-indazoles, the greater coordinating capacity of the N atom directed the aroylating group to the ortho position, leaving behind the nondirected metalation pathway. The Pd-catalyzed operationally simplified methodology proceeded in the presence of oxidants with either DCP or TBHP in dichloroethane as the solvent at 110 °C for 16 h, which generated a miscellaneous variety of monosubstituted o-benzoyl/acyl-1-aryl-1H-indazoles 4a-t/5a-i and bis-substituted o-benzoyl-1-aryl-1H-indazoles 6a-j in ≤88% yields. The probable mechanistic pathway involves a free radical chelation-assisted approach that could be accomplished by the addition of an in situ-generated oxidant-promoted benzoyl/acyl radical to the ortho position of 1-phenyl-1H-indazoles. A wide range of substrates, a high functional group tolerance, gram-scale synthesis, control/competitive experiments, and a variety of synthetic applications further exemplify the versatility of the developed methodology.

Concomitant Functionalization of Two Different Ketones by Merging Brønsted Acid Catalysis and Radical Relay Coupling

In previous literature, incorporation of functional groups at the α-position of unactivated carbonyl compounds was mainly restricted to one kind of corresponding precursors. Herein, we report the concomitant functionalization of...

Iron-Catalyzed Ring-Opening Reactions of Cyclopropanols with Alkenes and TBHP: Synthesis of 5-Oxo Peroxides

The ring opening of cyclopropanols is rarely used in multicomponent reactions. Herein we report the three-component reaction of cyclopropanols with alkenes and tert-butyl hydroperoxide (TBHP) catalyzed by an iron catalyst. This protocol enables the incorporation of both the β-carbonyl fragment and a peroxy unit across the C═C double bond regioselectively, thus allowing an efficient, facile access to 5-oxo peroxides. Modification of the biologically active molecules and various downstream derivatizations of the peroxides are also demonstrated.

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

Annulative π-Extension (APEX) of Indoles to Pyrido[1,2-a]indoles Using 4-Oxo Peroxides as C4 Units

Annulative π-extension (APEX) of 3-substituted indoles to pyrido[1,2-a]indoles is developed by using 4-oxo peroxides as π-extending reagents, which are employed as versatile C4 building blocks. This transformation is initiated by Brønsted acid-mediated Hock rearrangement of the peroxyl group. Notably, the pyrido[1,2-a]indole products are obtained by elimination of the indole moiety from the corresponding dihydropyrido[1,2-a]indoles, which could be selectively formed at room temperature.

A Mn-catalyzed remote C(sp3)-H bond peroxidation triggered by radical trifluoromethylation of unactivated alkenes

A manganese-catalyzed radical relay strategy for the remote trifluoromethylation-peroxidation of unactivated alkenes is disclosed. The electrophilic CF3 group was added to the C[double bond, length as m-dash]C double bonds to afford remote C-centered radicals upon 1,5-HAT, which could be efficiently trapped by Mnn+1OOBu-t species to deliver 1,6-difunctionalized products selectively under mild conditions. t-BuOOH serves as both the oxidant and the peroxy precursor in this transformation.

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

Directed Cobalt-Catalyzed C-H Activation to Form C-C and C-O Bonds in One Pot via Three-Component Coupling

Herein, we disclose an efficient cobalt-catalyzed three-component coupling of benzamides, diazo compounds, and tert-butyl hydroperoxide, which provides an efficient approach to construct C(sp²)-C(sp³) and C-O bonds in one-pot accompanied with C-H activation. This protocol features low catalyst loading (4 mol %), the avoidance of additives, and excellent functional group compatibility, providing three-component coupling adducts with high yields under mild conditions (up to 88%). Mechanism studies show that the reaction may involve a radical process.

Cascade Radical Annulation of 2-Alkynylthio(seleno)anisoles with Acetone or Acetonitrile: Synthesis of 3-Acetomethyl- or Cyanomethyl-Substituted Benzothio(seleno)phenes

An efficient method for the direct preparation of 3-aceto(cyano)methyl-substituted benzothio(seleno)phenes has been achieved through C(sp³)-H bond activation of easily available acetone or acetonitrile and cascade radical cyclization reaction. In this cascade radical cyclization reaction, C(sp²)-C(sp³) and C(sp²)-S bonds, as well as benzenethio(seleno)phene skeletons, can be built along with the cleavage of the C(sp³)-S bond, demonstrating the high step-economics and efficiency of this approach.

Iron-catalyzed oxidative cyclization of olefinic 1,3-dicarbonyls with ketone C(sp3)–H bonds: facile access to 2,3-dihydrofurans

The reaction involves the addition of an α-carbonyl radical to the CC bond of olefinic 1,3-dicarbonyls followed by intramolecular 5-endo-trig cyclization.

Meta-Dehydrogenative Alkylation of Arenes with Ethers, Ketones, and Esters Catalyzed by Ruthenium

A meta-dehydrogenative alkylation of arenes with cyclic ethers, ketones, and esters catalyzed by ruthenium is achieved in the presence of a di-tert-butyl peroxide (DTBP) oxidant. Interestingly, when quinoline and isoquinoline are employed as the directing group, or a chain ether as alkylation reagent, the system produces Minisci reaction products. Mechanistic study indicates that meta-dehydrogenative alkylation is a radical process initiated by DTBP with the assistance of a C_Ar-Ru bond ortho/para-directing effect.

Neighboring Thioether Participation in Bioinspired Radical Oxidative C(sp3)-H α-Oxyamination of Pyruvate Derivatives

A bioinspired radical oxidative α-oxyamination of pyruvate with an oxoammonium salt through multiple-site concerted proton-electron transfer process has been developed, which was facilitated by anchoring the mercapoto chains as a "hopping" site at the γ-position of α-keto esters.

Three component hydroxyletherification and hydroxylazidation of (trifluoromethyl)alkenes: access to α-trifluoromethyl β-heteroatom substituted tertiary alcohols

The three component hydroxyletherification and hydroxylazidation reactions of (trifluoromethyl)alkenes are reported, providing various useful α-trifluoromethyl β-heteroatom substituted tertiary alcohols in high yields.

Copper-catalyzed trifluoromethylthiolation-peroxidation of alkenes and allenes

Cu-catalyzed trifluoromethylthiolation-peroxidation of alkenes and allenes using AgSCF₃ and tert-butyl hydroperoxide has been developed. The method provides a variety of β-SCF₃ and β-vinyl-SCF₃ peroxides with excellent regio- and chemo-selectivities.

Acid promoted radical-chain difunctionalization of styrenes with stabilized radicals and (N,O)-nucleophiles

A radical chain difunctionalization of styrenes does not require a catalyst, but activation of benzoyl peroxide by a strong acid.

Facile access to evodiakine enabled by aerobic copper-catalyzed oxidative rearrangement

Oxidation as a fundamentally important method for the synthesis of complex structures is difficult to achieve in a selective manner. Evodiakine, a complex natural product possessing an unprecedented ring system (6/5/5/7/6), has a high oxidation state without a practical solution. Herein, we report the first synthesis of evodiakine via aerobic copper-catalyzed late-stage functionalization of evodiamine.

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.

Free Radical Addition of Nitrile, Ketone, and Ester to Alkyne and the Selectivity Discussion

A general and practical atom transfer radical addition (ATRA) of simple nitriles, ketones, and esters to alkynes was developed. It can allow an efficient access to a wide range of β,γ-unsaturated nitriles, ketones, and esters. The unique chemoselectivity in this system is also discussed.

Bioinspired radical cyclization of tryptamines: synthesis of peroxypyrroloindolenines as potential anti-cancer agents

Inspired by the heme iron-catalyzed radical insertion of dioxygen to the tryptophan indole ring, herein we utilize alkylperoxy radical species as a coupling partner to trigger a peroxycyclization of readily accessible tryptophan derivatives and enable the first synthesis of peroxypyrroloindolenines.

Oxidative radical addition–chlorination of alkenes to access 1,1-dichloroalkanes from simple reagents

This protocol provides a facile regioselective method for the synthesis of 1,1-dichloroalkanes from terminal alkenes using simple chloro reagents.

Four-component radical-dual-difunctionalization (RDD) and decarbonylative alkylative peroxidation of two different alkenes with aliphatic aldehydes and TBHP

From difunctionalization of a single alkene to radical-dual-difunctionalization of two different alkenes! A alkylative peroxidation and cross-coupling of two alkenes was established with the aliphatic aldehydes as the alkyl radical source.

Radical addition of ketones and cyanide to olefinsviaacid catalyzed formation of intermediate alkenyl peroxides

γ-Cyanoketones are formed by double radical addition from olefins, ketones and sulfonyl cyanidesviareactive alkenyl peroxide intermediates.