Metal-free fluorination of C(sp3)-H bonds using a catalytic N-oxyl radical

Benzylic Fluorination of Aza-Heterocycles Induced by Single-Electron Transfer to Selectfluor

A selective and mild method for the benzylic fluorination of aromatic azaheterocycles with Selectfluor is described. These reactions take place by a previously unreported mechanism, in which electron transfer from the heterocyclic substrate to the electrophilic fluorinating agent Selectfluor eventually yields a benzylic radical, thus leading to the desired C-F bond formation. This mechanism enables high intra- and intermolecular selectivity for aza-heterocycles over other benzylic components with similar C-H bond-dissociation energies.

Nondirected Copper-Catalyzed Sulfoxidations of Benzylic C-H Bonds

A copper-catalyzed sulfoxidation of benzylic C-H bonds by nondirected oxidative C(sp³)-H activation was developed. The process proceeds via sulfenate anions, which are generated by base-triggered elimination of β-sulfinyl esters and benzyl radicals. The functional group tolerance is high, and the product yields are good.

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.

p-Toluenesulfonic acid catalysed fluorination of α-branched ketones for the construction of fluorinated quaternary carbon centres

A catalytic and concise fluorination of α-branched ketones for the construction of the challenging quaternary C–F bond could be achieved.

Site-selective 18F fluorination of unactivated C-H bonds mediated by a manganese porphyrin

A direct aliphatic C–H ¹⁸F labeling method using [¹⁸F]fluoride ion at inaccessible and unreactive sites is reported.

The first direct C–H ¹⁸F fluorination reaction of unactivated aliphatic sites using no-carrier-added [¹⁸F]fluoride is reported. Under the influence of a manganese porphyrin/iodosylbenzene system, a variety of unactivated aliphatic C–H bonds can be selectively converted to C–¹⁸F bonds. The mild conditions, broad substrate scope and generally inaccessible regiochemistry make this radio-fluorination a powerful alternate to established nucleophilic substitution for the preparation of ¹⁸F labeled radio tracers.

Ketones as directing groups in photocatalytic sp3 C-H fluorination

The ubiquitous ketone carbonyl group generally deactivates substrates toward radical-based fluorinations, especially sites closest to it. Herein, ketones are used instead to direct aliphatic fluorination using Selectfluor, catalytic benzil, and visible light. Selective β- and γ-fluorination are demonstrated on rigid mono-, di-, tri-, and tetracyclic (steroidal) substrates employing both cyclic and exocyclic aliphatic ketones as directing groups.

Ketone-catalyzed photochemical C(sp3)-H chlorination

Photoexcited arylketones catalyze the direct chlorination of C(sp3)-H groups by N-chlorosuccinimide. Acetophenone is the most effective catalyst for functionalization of unactivated C-H groups while benzophenone provides better yields for benzylic C-H functionalization. Activation of both acetophenone and benzophenone can be achieved by irradiation with a household compact fluorescent lamp. This light-dependent reaction provides a better control of the reaction as compared to the traditional chlorination methods that proceed through a free radical chain propagation mechanism.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Mechanistic insights for the photoredox organocatalytic fluorination of aliphatic carbons by anthraquinone using time-resolved and DFT studies

Nanosecond TAS and DFT calculations reveal mechanistic insights for photoredox fluorination of aliphatic C–H bonds and evidence of an anthraquinone–Selectfluor® exciplex.

Radical fluorination reactions by thermal and photoinduced methods

Radical fluorination reactions of aliphatic Csp³ and Csp² atoms, decarboxylative fluorination, and fluorination of (hetero)aromatics can be accomplished with electrophilic fluorinating reagents such as Selectfluor and NFSI.

Recent progress in mild Csp3–H bond dehydrogenative or (mono-) oxidative functionalization

This review summarizes recent advances in mild and green dehydrogenative and mono-oxidative Csp³–H bond functionalization reactions, considering both new approaches and the re-elaboration of known methodologies.

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C-H activation

Since our initial report in 1976, the oxygen rebound mechanism has become the consensus mechanistic feature for an expanding variety of enzymatic C-H functionalization reactions and small molecule biomimetic catalysts. For both the biotransformations and models, an initial hydrogen atom abstraction from the substrate (R-H) by high-valent iron-oxo species (Feⁿ=O) generates a substrate radical and a reduced iron hydroxide, [Fe^n-1-OH ·R]. This caged radical pair then evolves on a complicated energy landscape through a number of reaction pathways, such as oxygen rebound to form R-OH, rebound to a non-oxygen atom affording R-X, electron transfer of the incipient radical to yield a carbocation, R⁺, desaturation to form olefins, and radical cage escape. These various flavors of the rebound process, often in competition with each other, give rise to the wide range of C-H functionalization reactions performed by iron-containing oxygenases. In this review, we first recount the history of radical rebound mechanisms, their general features, and key intermediates involved. We will discuss in detail the factors that affect the behavior of the initial caged radical pair and the lifetimes of the incipient substrate radicals. Several representative examples of enzymatic C-H transformations are selected to illustrate how the behaviors of the radical pair [Fe^n-1-OH ·R] determine the eventual reaction outcome. Finally, we discuss the powerful potential of "radical rebound" processes as a general paradigm for developing novel C-H functionalization reactions with synthetic, biomimetic catalysts. We envision that new chemistry will continue to arise by bridging enzymatic "radical rebound" with synthetic organic chemistry.

Iron-Catalyzed, Fluoroamide-Directed C-H Fluorination

This communication describes a mild, amide-directed fluorination of benzylic, allylic, and unactivated C-H bonds mediated by iron. Upon exposure to a catalytic amount of iron(II) triflate (Fe(OTf)₂), N-fluoro-2-methylbenzamides undergo chemoselective fluorine transfer to provide the corresponding fluorides in high yield. The reaction demonstrates broad substrate scope and functional group tolerance without the use of any noble metal additives. Mechanistic and computational experiments suggest that the reaction proceeds through short-lived radical intermediates with F-transfer mediated directly by iron.

Xenon Difluoride Mediated Fluorodecarboxylations for the Syntheses of Di- and Trifluoromethoxyarenes

XeF2 is demonstrated to be a more proficient fluorine-transfer reagent than either NFSI or Selectfluor in fluorodecarboxylations of both mono- and difluoroaryloxy acetic acid derivatives. This method efficiently converts a wide range of neutral and electron-poor substrates to afford the desired di- and trifluoromethyl aryl ethers in good to excellent yields. The purifications are facile, and the reaction times are less than 5 min, which makes these fluorodecarboxylations promising for future PET-imaging applications.

A visible-light-promoted radical reaction system for azidation and halogenation of tertiary aliphatic C-H bonds

A highly tunable radical-mediated reaction system for the functionalization of tertiary aliphatic C-H bonds was developed. Reactions of various substrates with the Zhdankin azidoiodane reagent 1, Ru(bpy)3Cl2, and visible light irradiation at room temperature gave C-H azidated or halogenated products in an easily controllable fashion. These reactions are efficient, selective, and compatible with complex substrates. They provide a potentially valuable tool for selectively labeling tertiary C-H bonds of organic and biomolecules with tags of varied chemical and biophysical properties for comparative functional studies.

Direct photocatalytic fluorination of benzylic C-H bonds with N-fluorobenzenesulfonimide

The late-stage fluorination of common synthetic building blocks and drug leads is an appealing reaction for medicinal chemistry. In particular, fluorination of benzylic C-H bonds provides a means to attenuate drug metabolism at this metabolically labile position. Here we report two complimentary strategies for the direct fluorination of benzylic C-H bonds using N-fluorobenzenesulfonimide and either a decatungstate photocatalyst or AIBN-initiation.

Selectfluor-mediated highly selective radical dioxygenation of alkenes

Selectfluor mediated highly selective radical dioxygenation of alkenes using hydroxylamines and O₂ was achieved under mild conditions.

Metal-free intermolecular C–O cross-coupling reactions: synthesis of N-hydroxyimide esters

Selectfluor-mediated intermolecular C–O cross coupling reaction for the synthesis of N-hydroxyimide esters was achieved under metal-free conditions.

A Visible-Light-Mediated Radical Smiles Rearrangement and its Application to the Synthesis of a Difluoro-Substituted Spirocyclic ORL-1 Antagonist

A visible-light-mediated radical Smiles rearrangement has been developed to address the challenging synthesis of the gem-difluoro group present in an opioid receptor-like 1 (ORL-1) antagonist that is currently in development for the treatment of depression and/or obesity. This method enables the direct and efficient introduction of the difluoroethanol motif into a range of aryl and heteroaryl systems, representing a new disconnection for the synthesis of this versatile moiety. When applied to the target compound, the photochemical step could be conducted on 15 g scale using industrially relevant [Ru(bpy)3Cl2] catalyst loadings of 0.01 mol %. This transformation is part of an overall five-step route to the antagonist that compares favorably to the current synthetic sequence and demonstrates, in this specific case, a clear strategic benefit of photocatalysis.

Unstrained C-C bond activation and directed fluorination through photocatalytically-generated radical cations

Expanding the repertoire of controlled radical fluorination techniques, we present a photosensitized unstrained C-C bond activation/directed monofluorination method using Selectfluor and 9-fluorenone. The reaction is amenable to the opening of multiple 1-acetal-2-aryl substituted rings to yield ω-fluoro carboxylic acids, esters, alcohols, and ketones with relative ease. Initial mechanistic insight suggests radical ion intermediates.

Stereoselective Synthesis of Chiral β-Fluoro α-Amino Acids via Pd(II)-Catalyzed Fluorination of Unactivated Methylene C(sp(3))-H Bonds: Scope and Mechanistic Studies

The synthesis of fluorinated complex molecules via direct C(sp(3))-H fluorination is attractive yet remains challenging. Here we describe the Pd(II)-catalyzed fluorination of unactivated methylene C(sp(3))-H bonds by an inner-sphere mechanism. This method allows the site- and diastereoselective fluorination of β-methylene C(sp(3))-H bonds of α-amino acid derivatives. A range of substrates containing both aliphatic and benzylic C(sp(3))-H bonds were compatible with this protocol, leading to an array of β-fluorinated α-amino acids. Stoichiometric fluorination of an isolated palladacycle intermediate takes place rapidly under very mild reaction conditions (room temperature, 5-10 min). Data from preliminary mechanistic studies are consistent with direct C-F reductive elimination from a high-valent intermediate.

Manganese Catalyzed C-H Halogenation

The remarkable aliphatic C-H hydroxylations catalyzed by the heme-containing enzyme, cytochrome P450, have attracted sustained attention for more than four decades. The effectiveness of P450 enzymes as highly selective biocatalysts for a wide range of oxygenation reactions of complex substrates has driven chemists to develop synthetic metalloporphyrin model compounds that mimic P450 reactivity. Among various known metalloporphyrins, manganese derivatives have received considerable attention since they have been shown to be versatile and powerful mediators for alkane hydroxylation and olefin epoxidation. Mechanistic studies have shown that the key intermediates of the manganese porphyrin-catalyzed oxygenation reactions include oxo- and dioxomanganese(V) species that transfer an oxygen atom to the substrate through a hydrogen abstraction/oxygen recombination pathway known as the oxygen rebound mechanism. Application of manganese porphyrins has been largely restricted to catalysis of oxygenation reactions until recently, however, due to ultrafast oxygen transfer rates. In this Account, we discuss recently developed carbon-halogen bond formation, including fluorination reactions catalyzed by manganese porphyrins and related salen species. We found that biphasic sodium hypochlorite/manganese porphyrin systems can efficiently and selectively convert even unactivated aliphatic C-H bonds to C-Cl bonds. An understanding of this novel reactivity derived from results obtained for the oxidation of the mechanistically diagnostic substrate and radical clock, norcarane. Significantly, the oxygen rebound rate in Mn-mediated hydroxylation is highly correlated with the nature of the trans-axial ligands bound to the manganese center (L-Mn(V)═O). Based on the ability of fluoride ion to decelerate the oxygen rebound step, we envisaged that a relatively long-lived substrate radical could be trapped by a Mn-F fluorine source, effecting carbon-fluorine bond formation. Indeed, this idea led to the discovery of the first Mn-catalyzed direct aliphatic C-H fluorination reactions utilizing simple, nucleophilic fluoride salts. Mechanistic studies and DFT calculations have revealed a trans-difluoromanganese(IV) species as the key fluorine transfer intermediate. In addition to catalyzing normal (19)F-fluorination reactions, manganese salen complexes were found to enable the incorporation of radioactive (18)F fluorine via C-H activation. This advance represented the first direct Csp(3)-H bond (18)F labeling with no-carrier-added [(18)F]fluoride and facilitated the late-stage labeling of drug molecules for PET imaging. Given the high reactivity and enzymatic-like selectively of metalloporphyrins, we envision that this new Heteroatom-Rebound Catalysis (HRC) strategy will find widespread application in the C-H functionalization arena and serve as an effective tool for forming new carbon-heteroatom bonds at otherwise inaccessible sites in target molecules.

Decarboxylative Fluorination of Aliphatic Carboxylic Acids via Photoredox Catalysis

The direct conversion of aliphatic carboxylic acids to the corresponding alkyl fluorides has been achieved via visible light-promoted photoredox catalysis. This operationally simple, redox-neutral fluorination method is amenable to a wide variety of carboxylic acids. Photon-induced oxidation of carboxylates leads to the formation of carboxyl radicals, which upon rapid CO2-extrusion and F(•) transfer from a fluorinating reagent yield the desired fluoroalkanes with high efficiency. Experimental evidence indicates that an oxidative quenching pathway is operable in this broadly applicable fluorination protocol.