Radicals: Reactive Intermediates with Translational Potential

Anodically Coupled Electrolysis for the Heterodifunctionalization of Alkenes

The emergence of new catalytic strategies that cleverly adopt concepts and techniques frequently used in areas such as photochemistry and electrochemistry has yielded a myriad of new organic reactions that would be challenging to achieve using orthodox methods. Herein, we discuss the strategic use of anodically coupled electrolysis, an electrochemical process that combines two parallel oxidative events, as a complementary approach to existing methods for redox organic transformations. Specifically, we demonstrate anodically coupled electrolysis in the regio- and chemoselective chlorotrifluoromethylation of alkenes.

Selective Radical Trifluoromethylation of Native Residues in Proteins

The incorporation of fluorine can not only significantly facilitate the study of proteins but also potentially modulate their function. Though some biosynthetic methods allow global residue-replacement, post-translational fluorine incorporation would constitute a fast and efficient alternative. Here, we reveal a mild method for direct protein radical trifluoromethylation at native residues as a strategy for symmetric-multifluorine incorporation on mg scales with high recoveries. High selectivity toward tryptophan residues enhanced the utility of this direct trifluoromethylation technique allowing ready study of fluorinated protein constructs using 19F-NMR.

Selective electrochemical generation of benzylic radicals enabled by ferrocene-based electron-transfer mediators

The generation and intermolecular functionalisation of carbon-centred radicals has broad potential synthetic utility. Herein, we show that benzylic radicals may be generated electrochemically from benzylboronate derivatives at low electrode potentials (ca. -0.3 V vs. Cp2Fe0/+) via single electron oxidation. Use of a catalytic quantity of a ferrocene-based electron-transfer mediator is crucial to achieve successful radical functionalisation and avoid undesirable side reactions arising from direct electrochemical oxidation or from the use of stoichiometric ferrocenium-based oxidants.

Tertiary-Alcohol-Directed Functionalization of Remote C(sp3 )-H Bonds by Sequential Hydrogen Atom and Heteroaryl Migrations

Reported for the first time is a tertiary-alcohol-guided heteroarylation of remote C(sp³ )-H bonds. The mild and direct generation of alkoxyl radicals from alcohols is enabled by visible-light photocatalysis. A remote hydrogen atom and heteroaryl migration sequence are involved in the reaction. Many sensitive groups remain intact in the reaction, thus illustrating wide functional-group compatibility. This protocol provides a practical strategy for the late-stage modification of alkyl ketones.

Cu2O/TiO2 nanoparticles as visible light photocatalysts concerning C(sp2)–P bond formation

A novel and efficient method has been developed for the construction of an aromatic-phosphorus (Ar-P) bond under visible light irradiation using Cu₂O/TiO₂ nanoparticles as inexpensive and available photocatalysts.

A silver catalyzed domino reaction of N-cyanamide alkenes and 1,3-dicarbonyls for the synthesis of quinazolinones

A silver catalyzed domino reaction of N-cyanamide alkenes and 1,3-dicarbonyls has been developed for the facile synthesis of quinazolinones.

Copper-catalyzed selective radical–radical cross-coupling for C–S bond formation: an access to α-alkylthionitriles

Oxidative radical–radical cross-coupling: a new protocol for C–S bond formation was developed by selective cross-coupling between a thiyl radical and an isobutyronitrile radical.

Nickel(ii)-catalyzed enantioselective α-alkylation of β-ketoamides with phenyliodonium ylideviaa radical process

Enantioselective α-alkylation of β-ketoamides with iodonium ylide was realizedviaa radical process. In the presence of a chiralN,N′-dioxide/Ni(OTf)₂complex, the corresponding products were obtained with good results. Control experiments and EPR studies supported a radical process.

Electrochemical synthesis of 7-membered carbocycles through cascade 5-exo-trig/7-endo-trig radical cyclization

An electrochemical synthesis of functionalized 7-membered carbocycles through a 5-exo-trig/7-endo-trig radical cyclization cascade has been developed.

Organocatalytic decarboxylative alkylation of N-hydroxy-phthalimide esters enabled by pyridine-boryl radicals

The decarboxylative alkylation of N-hydroxyphthalimide (NHPI) based reactive esters with olefins has been achieved via an organocatalytic strategy.

Striving to exploit alkyl electrophiles: challenge and choice in transition metal-catalyzed cross-coupling reactions of sulfones

Transition metal-catalyzed cross-coupling reactions of alkyl sulfones as crystalline and readily available alkyl electrophiles via a radical process are highlighted.

Metal-free deoxygenative sulfonylation of quinoline N-oxides with sodium sulfinates via a dual radical coupling process

The first example of a metal- and reductant-free deoxygenative sulfonylation of quinoline N-oxides with sodium sulfinates via a dual radical coupling process is reported.

Photoredox generation of the trifluoromethyl radical from borate complexes via single electron reduction

A trifluoromethyl radical is generated by single electron reduction of CF₃-substituted borate complexes.

Copper-Catalyzed Redox-Neutral Cyanoalkylarylation of Activated Alkenes with Cyclobutanone Oxime Esters

The copper-catalyzed cyclization of activated alkenes with cyclobutanone O-acyl oximes under redox-neutral conditions has been reported. This facile protocol provided an efficient approach to a variety of cyanoalkylated oxindoles and dihydroquinolin-2(1H)-ones with a broad substrate scope and excellent functional group tolerance. In this reaction, sequential C-C bond cleavage, radical addition, and cyclization processes were involved, wherein multiple bonds were constructed in a one-pot reaction. Mechanistic studies suggest that the reaction probably proceeded via a radical pathway.

Synthesis of the Tetracyclic Structure of Batrachotoxin Enabled by Bridgehead Radical Coupling and Pd/Ni-Promoted Ullmann Reaction

The steroidal ABCD-ring system of the potent neurotoxin batrachotoxin was efficiently assembled in a convergent fashion. Bridgehead radical coupling between the simple AB-ring and D-ring fragments (3 and 4) formed the sterically congested linkage at the C9-oxygen-attached tetrasubstituted carbon. The C-ring was then cyclized by the Pd/Ni-promoted Ullmann reaction of the vinyl triflate and vinyl bromide of 19, giving rise to tetracyclic structure 1.

Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General

Explosive growth in the use of open shell reactivity, including neutral radicals and radical ions, in the field of synthetic organic chemistry has been observed in the past decade, particularly since the advent of ruthenium complexes in 2008. These complexes generally induce single-electron transfer (SET) processes via visible-light absorption. Additionally, recent significant advancements in organic electrochemistry involving SET processes to provide open shell reactivity offer a complementary method to traditional polarity-driven reactions described by two-electron transfer processes. In this Review, we highlight the importance of intramolecular SET processes in the field of synthetic organic chemistry, which seem to be more elusive than the intermolecular versions, since they are net redox-neutral and thus cannot simply be regarded as oxidations or reductions. Such intramolecular SET processes can rationally be understood in combination with concomitant bond formations and/or cleavages, and are regulated by a structural motif that we call a "redox tag." In order to describe modern radical-driven reactions involving SET processes, we focus on a classical formalism in which electrons are treated as particles rather than waves, which offers a practical yet powerful approach to explain and/or predict synthetic outcomes.

Transition-Metal-Free Radical C(sp3)-C(sp2) and C(sp3)-C(sp3) Coupling Enabled by 2-Azaallyls as Super-Electron-Donors and Coupling-Partners

The past decade has witnessed the rapid development of radical generation strategies and their applications in C-C bond-forming reactions. Most of these processes require initiators, transition metal catalysts, or organometallic reagents. Herein, we report the discovery of a simple organic system (2-azaallyl anions) that enables radical coupling reactions under transition-metal-free conditions. Deprotonation of N-benzyl ketimines generates semistabilized 2-azaallyl anions that behave as "super-electron-donors" (SEDs) and reduce aryl iodides and alkyl halides to aryl and alkyl radicals. The SET process converts the 2-azaallyl anions into persistent 2-azaallyl radicals, which capture the aryl and alkyl radicals to form C-C bonds. The radical coupling of aryl and alkyl radicals with 2-azaallyl radicals makes possible the synthesis of functionalized amine derivatives without the use of exogenous radical initiators or transition metal catalysts. Radical clock studies and 2-azaallyl anion coupling studies provide mechanistic insight for this unique reactivity.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Decarboxylative Alkynylation

The development of a new decarboxylative cross-coupling method that affords terminal and substituted alkynes from various carboxylic acids is described using both nickel- and iron-based catalysts. The use of N-hydroxytetrachlorophthalimide (TCNHPI) esters is crucial to the success of the transformation, and the reaction is amenable to in situ carboxylic acid activation. Additionally, an inexpensive, commercially available alkyne source is employed in this formal homologation process that serves as a surrogate for other well-established alkyne syntheses. The reaction is operationally simple and broad in scope while providing succinct and scalable avenues to previously reported synthetic intermediates.

Transforming Olefins into γ,δ-Unsaturated Nitriles through Copper Catalysis

We have developed a strategy to transform olefins into homoallylic nitriles through a mechanism that combines copper catalysis with alkyl nitrile radicals. The radicals are easily generated from alkyl nitriles in the presence of the mild oxidant di-tert-butyl peroxide. This cross-dehydrogenative coupling between simple olefins and alkylnitriles bears advantages over the conventional use of halides and toxic cyanide reagents. With this method, we showcase the facile synthesis of a flavoring agent, a natural product, and a polymer precursor from simple olefins.

Enantioselective Total Synthesis of (-)-Pavidolide B

The enantioselective synthesis of (-)-pavidolide B (1) was achieved in a linear sequence of 10 steps. The key steps are (a) an enantioselective organocatalytic cyclopropanation; (b) a radical-based cascade annulation for the regio- and diastereo-selective synthesis of the highly functionalized lactone 3 bearing the characteristic tricyclic core and seven contiguous stereocenters;

Late-Stage C-H Alkylation of Heterocycles and 1,4-Quinones via Oxidative Homolysis of 1,4-Dihydropyridines

Under oxidative conditions, 1,4-dihydropyridines (DHPs) undergo a homolytic cleavage, forming exclusively a Csp3-centered radical that can engage in the C-H alkylation of heterocyclic bases and 1,4-quinones. DHPs are readily prepared from aldehydes, and considering that aldehydes normally require harsh reaction conditions to take part in such transformations, with mixtures of alkylated and acylated products often being obtained, this net decarbonylative alkylation approach becomes particularly useful. The present method takes place under mild reaction conditions and requires only persulfate as a stoichiometric oxidant, making the procedure suitable for the late-stage C-H alkylation of complex molecules. Notably, structurally complex pharmaceutical agents could be functionalized or prepared with this protocol, such as the antimalarial Atovaquone and antitheilerial Parvaquone, thus evidencing its applicability. Mechanistic studies revealed a likely radical chain process via the formation of a dearomatized intermediate, providing a deeper understanding of the factors governing the reactivity of these radical forebears.

Unified Total Synthesis of Polyoxins J, L, and Fluorinated Analogues on the Basis of Decarbonylative Radical Coupling Reactions

Polyoxins J (1 a) and L (1 b) are important nucleoside antibiotics. The complex and densely functionalized dipeptide structures of 1 a and 1 b contain thymine and uracil nucleobases, respectively. Herein we report the unified total synthesis of 1 a, 1 b, and their artificial analogues 1 c and 1 d with trifluorothymine and fluorouracil structures. Decarbonylative radical coupling between α-alkoxyacyl tellurides and a chiral glyoxylic oxime ether led to chemo- and stereoselective construction of the ribonucleoside α-amino acid structures of 1 a-d without damaging the preinstalled nucleobases. The high applicability of the radical-based methodology was further demonstrated by preparation of the trihydroxynorvaline moiety of 1 a-d. The two amino acid fragments were connected and elaborated into 1 a-d (longest linear sequence: 11 steps). Compounds 1 a and 1 b assembled in this way exhibited potent activity against true fungi, while only 1 d was active against Gram-positive bacteria.

Enantioselective Hydrogen Atom Transfer: Discovery of Catalytic Promiscuity in Flavin-Dependent 'Ene'-Reductases

Flavin has long been known to function as a single electron reductant in biological settings, but this reactivity has rarely been observed with flavoproteins used in organic synthesis. Here we describe the discovery of an enantioselective radical dehalogenation pathway for α-bromoesters using flavin-dependent 'ene'-reductases. Mechanistic experiments support the role of flavin hydroquinone as a single electron reductant, flavin semiquinone as the hydrogen atom source, and the enzyme as the source of chirality.

Cooperative Light-Activated Iodine and Photoredox Catalysis for the Amination of Csp3 -H Bonds

An unprecedented method that makes use of the cooperative interplay between molecular iodine and photoredox catalysis has been developed for dual light-activated intramolecular benzylic C-H amination. Iodine serves as the catalyst for the formation of a new C-N bond by activating a remote Csp3 -H bond (1,5-HAT process) under visible-light irradiation while the organic photoredox catalyst TPT effects the reoxidation of the molecular iodine catalyst. To explain the compatibility of the two involved photochemical steps, the key N-I bond activation was elucidated by computational methods. The new cooperative catalysis has important implications for the combination of non-metallic main-group catalysis with photocatalysis.