Carbon Radicals Research Articles

Iron-catalysed reductive coupling for the synthesis of polyfluorinated compounds.

Herein we reported the use of Earth-abundant iron as the catalytic metal in the presence of Mn to induce difluorobromoacetates to form carbon radicals, which reacted with trifluoromethyl olefins followed by β-F elimination to generate the corresponding gem-difluoroolefins. The cross-electrophile coupling displayed excellent functional group tolerance and broad substrate scope under mild reductive conditions, affording a large number of polyfluorinated compounds, which could be further transformed to other valuable molecules.

Determining the inherent selectivity for carbon radical hydroxylation versus halogenation with high-spin oxoiron(iv)-halide complexes: a concerted rebound step.

A synthetic iron model can process both halogenation and hydroxylation with vague selectivity, which is different from halogenase even though these structures are used for the simulation of halogenase. The key factor of the synthetic oxo-iron model mediated hydroxylation or the halogenation is still under debate. Herein density functional theory calculation is used to investigate the hydroxylation versus halogenation of propylene by the complex [FeIV(O)(TQA)(X)]+ (X = F, Cl, Br). Our results suggest that a concerted rebound mechanism (between the -X and the hydroxyl ligands after the hydrogen abstraction) leads to the formation of two different kinds of products.

Incorporation of a boryl pendant as the trigger in a methacrylate polymer for backbone degradation.

Alkenylboronate units were incorporated into poly(methyl methacrylate) as trigger sites for backbone degradation. The complexation of a fluoride anion to the boron pendant and subsequent activation by a manganese salt triggered the degradation through the generation of a main-chain carbon radical and β-scission in an adjacent methacrylate unit. The addition of catechol was required, and presumably the chelate coordination to the Mn center was crucial for the degradation.

Radical Addition-Triggered Remote Migratory Isomerization of Unactivated Alkenes to Difluoromethylene-Containing Alkenes Enabled by Bimetallic Catalysis

A fascinating alkene remote migratory isomerization engendered by carbon radical addition to C═C bond in alkenes via bimetallic catalysis has been disclosed. A diverse array of alkenes bearing distantly incorporated the difluoromethylene (RCF2R′) functionality have been expediently obtained. The retainment of C═C bonds in products could serve as an useful synthetic platform furnishing otherwise difficult to access value-added densely functionalized RCF2R′-containing molecules. In addition, some experimental studies have been implemented to shed light on the probable mechanism.

Effect of Diamond Polishing and Thermal Treatment on Carbon Paramagnetic Centers' Nature and Structure.

Diamonds contain carbon paramagnetic centers (stable carbon radicals) in small concentrations (at the level of ~1 × 1012 spins/mg) that can help in elucidating the structure of the nitrogen atoms’ contaminants in the diamond crystal. All diamonds that undergo polishing are exposed to high temperatures, owing to the friction force during the polishing process, which may affect the carbon-centered radicals’ concentration and structure. The temperature is increased appreciably; consequently, the black body radiation in the visible range turns orange. During polishing, diamonds emit an orange light (at a wavelength of about 600 nm) that is typical of a black body temperature of 900 °C or higher. Other processes in which color-enhanced diamonds are exposed to high temperatures are thermal treatments or the high-pressure, high-temperature (HPHT) process in which the brown color (resulting from plastic deformation) is bleached. The aim of the study was to examine how thermal treatment and polishing influence the paramagnetic centers in the diamond. For this purpose, four rough diamonds were studied: two underwent a polishing process, and the other two were thermally treated at 650 °C and 1000 °C. The diamonds were analyzed pre- and post-treatment by EPR (Electron Paramagnetic resonance), FTIR (Fourier transform infrared, fluorescence, and their visual appearance. The results indicate that the polishing process results in much more than just thermal heating the paramagnetic centers.

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Direct fluorination of nanographene molecules with fluorine gas

The transformation of graphene into sp3-bonded carbon films through ultra-high pressure, hydrogenation or fluorination has been studied for years. It is considered that only few layers of graphenes on metal surfaces can realize this transformation and the electron orbitals hybridization between metal substrate and graphene layer is thought to be the drving force and stabilize the formed sp3 carbon structure. Here, we use nanographene molecule, hexa-peri-hexabenzocoronene (HBC), as the model compound to exploit the fluorination reaction between graphene and fluorine gas. MALDI-TOF mass, XPS and XRD results show that the direct fluorination realized the intermolecular cross-linking of nanographene molecules without the assistence of mental substrate. Combining with theoretical calculation and EPR test results, we proved that the fluorination of nanographene molecules with F2 follows a free-radical mechanism, and the carbon radicals formed during fluorination caused the covalent cross-linking between graphene molecules which have a tight π-π stacking. Meanwhile, the fluorination of other small nanographene molecules, pyrene and perylene, further confirmed the universality of this free-radical cross-linking mechanism. This would provide a further understanding for the chemical transformation from graphene layers to 2D diamond structure under chemical adsorption process.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis.

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

A biomimetic SH2 cross-coupling mechanism for quaternary sp3-carbon formation.

Bimolecular homolytic substitution (SH2) is an open-shell mechanism that is implicated across a host of biochemical alkylation pathways. Surprisingly, however, this radical substitution manifold has not been generally deployed as a design element in synthetic C–C bond formation. We found that the SH2 mechanism can be leveraged to enable a biomimetic sp3-sp3 cross-coupling platform that furnishes quaternary sp3-carbon centers, a long-standing challenge in organic molecule construction. This heteroselective radical-radical coupling uses the capacity of iron porphyrin to readily distinguish between the SH2 bond-forming roles of open-shell primary and tertiary carbons, combined with photocatalysis to generate both radical classes simultaneously from widely abundant functional groups. Mechanistic studies confirm the intermediacy of a primary alkyl–Fe(III) species prior to coupling and provide evidence for the SH2 displacement pathway in the critical quaternary sp3-carbon bond formation step.

Spin-dependent vibronic response of a carbon radical ion in two-dimensional WS2

Atomic spin centers in 2D materials are a highly anticipated building block for quantum technologies. Here, we demonstrate the creation of an effective spin-1/2 system via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides. Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping are activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state, the carbon impurity is computed to have a magnetic moment of 1 μB resulting from an unpaired electron populating a spin-polarized in-gap orbital. We show that the CRI defect states couple to a small number of local vibrational modes. The vibronic coupling strength critically depends on the spin state and differs for monolayer and bilayer WS2. The carbon radical ion is a surface-bound atomic defect that can be selectively introduced, features a well-understood vibronic spectrum, and is charge state controlled.

Mechanism of the photoreduction of carbon dioxide catalyzed by the benchmarking rhenium dimethylbipyridine complexes; operando measurements by XAFS and FT-IR

The photoreduction of CO2 catalyzed by [fac-Re(dmbpy)(CO)3Cl] was precisely investigated through operando measurements of X-ray absorption fine structure spectroscopy (XAFS) and laser-flash FT-IR transient spectroscopy. In the early stage of the photoreaction, Re0-species assigned as penta-coordinated complex, [Re0(dmbpy)(CO)3], appeared and a delayed rise of ReII-species, [ReII(dmbpy)(CO)3CO2] (3)/[ReII(dmbpy)(CO)3COOH]+ (3′), were observed in XAFS measurements. The valence state of Re-center in the catalytic cycle changed as ReI → Re0 → ReII to be a clear evidence of an oxidative addition of CO2 to the Re0 center. In the laser-flash induced FT-IR measurements, a rise and decay of 3 followed by a delayed rise and decay of 3′ was observed. The second electron transfer from the carbon radical of triethylamine (TEA•) to 3′ induced the formation of [ReI(dmbpy)(CO)4]+ through [ReI(dmbpy)COOH]. The rate determining step of the catalytic cycle was revealed to be the protonation process of forming 3′ from 3.

Asymmetric radical carboesterification of dienes

The straightforward strategy of building a chiral C-O bond directly on a general carbon radical center is challenging and stereocontrol of the reactions of open-chain hydrocarbon radicals remains a largely unsolved problem. Advance in this elementary step will spur the development of asymmetric radical C-O bond construction. Herein, we report a copper-catalyzed regioselective and enantioselective carboesterification of substituted dienes using alkyl diacyl peroxides as the source of both the carbon and oxygen substituents. The participation of external acids in this reaction substantially extends its applicability and leads to structurally diverse allylic ester products. This work represents the advance in the key elementary reaction of intermolecular enantioselective construction of C-O bond on open-chain hydrocarbon radicals and may lead to the discovery of other asymmetric radical reactions.

An Expanded SET Model Associated with the Functional Hindrance Dominates the Amide-Directed Distal sp3 C-H Functionalization.

The mechanistic understanding of catalytic radical reactions currently lags behind the flourishing development of new types of catalytic activation. Herein, an innovative single electron transfer (SET) model has been expanded by using the nonadiabatic crossing integrated with the rate-determining step of 1,5-hydrogen atom transfer (HAT) reaction to provide the control mechanism of radical decay dynamics through calculating excited-state relaxation paths of a paradigm example of the amide-directed distal sp3 C-H bond alkylation mediated by Ir-complex-based photocatalysts. The stability of carbon radical intermediates, the functional hindrance associated with the back SET, and the energy inversion between the reactive triplet and closed-shell ground states were verified to be key factors in improving catalytic efficiency via blocking radical inhibition. The expanded SET model associated with the dynamic behaviors and kinetic data could guide the design and manipulation of visible-light-driven inert bond activation by the utilization of photocatalysts bearing more or less electron-withdrawing groups and the comprehensive considerations of kinetic solvent effects and electron-withdrawing effects of substrates.

Photoinduced Atom Transfer Radical Addition/Cyclization Reaction between Alkynes or Alkenes with Unsaturated α-Halogenated Carbonyls.

We have developed a photochemical ATRA/ATRC reaction that is mediated by halogen bonding interactions. This reaction is caused by the reaction of malonic acid ester derivatives containing bromine or iodine with unsaturated compounds such as alkenes and alkynes in the presence of diisopropylethylamine under visible light irradiation. As a result of various control experiments, it was found that the formation of complexes between amines and halogens by halogen-bonding interaction occurs in the reaction system, followed by the cleavage of the carbon–halogen bonds by visible light, resulting in the formation of carbon radicals. In this reaction, a variety of substrates can be used, and the products, cyclopentenes and cyclopentanes, were obtained by intermolecular addition and intramolecular cyclization.

N‐Hydroxyphthalimide‐Catalyzed Epoxidation of Inactive Aliphatic Olefins with Air at Room Temperature

AbstractMetal‐free aerobic epoxidation of aliphatic alkenes, especially terminal alkenes, remains a significant challenge. Herein, a novel and practical synthetic method with N‐hydroxyphthalimide (NHPI) as catalyst, isobutyraldehyde as sacrificial reductant and air as oxidant has been developed for the epoxidation of olefins in excellent to moderate conversions and selectivity at room temperature. Meanwhile, this synthetic methodology could efficiently convert inactive light olefins to corresponding epoxides which are highly desired in industry. Moreover, several free radicals in the reaction were identified and characterized by in situ electron paramagnetic resonance spectroscopy (in situ EPR) and high‐resolution mass spectrometry (HRMS). Mechanistic studies showed that the phthalimide N‐oxyl radical (PINO⋅) generated from NHPI would promote the homolysis of isobutyraldehyde to carbonyl carbon radical by the extraction of hydrogen atoms.

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles.

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an AgI-pyrrole (η2-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C6H4)3C• and 1 gave k = 1.34(3) × 103 M-1 s-1 at 23 °C and 2.2(2) M-1 s-1 at -60 °C, ΔH⧧ = +9.8(3) kcal mol-1, and ΔS⧧ = -14(1) cal mol-1 K-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 105 M-1 s-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F- > Cl- > OH- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Cover Feature: Photoredox‐Induced Deaminative Radical‐Cationic Three‐Component Couplings with N‐Alkylpyridinium Salts and Alkenes (ChemPhotoChem 11/2021)

The Cover Feature shows a radical three-component coupling driven by visible light photoredox catalysis. Single electron transfer (SET) reduction of N-alkylpyridinium salts liberates nucleophilic carbon radicals that successively couple with an acceptor and a donor alkene, enabling a stereoselective synthesis of new succinimide derivatives. Cover image made by Marcin Henk Kowalski. More information can be found in the Communication by Malte Brasholz and co-workers.

Photocatalytic Anti-Markovnikov Radical Hydro- and Aminooxygenation of Unactivated Alkenes Tuned by Ketoxime Carbonates.

A tunable photocatalytic method is reported for anti-Markovnikov hydro- and aminooxygenation of unactivated alkenes using readily accessible ketoxime carbonates as the diverse functionalization reagents. Mechanistic studies reveal that this reaction is initiated through an energy-transfer-promoted N-O bond homolysis of ketoxime carbonates leading to alkoxylcarbonyloxyl and iminyl radicals under visible-light photocatalysis, followed by the addition of alkoxylcarbonyloxyl radical to alkenes. By taking advantage of the different stability of the iminyl radicals, the generated carbon radical either abstracts a hydrogen atom from the media to form the anti-Markovnikov hydrooxygenation product, or it is trapped by the persistent iminyl radical to furnish the aminooxygenation product. Notably, this is the first example of direct hydrooxygenation of unactivated olefins with anti-Markovnikov regioselectivity involving an oxygen-centered radical.

Photocatalytic Anti‐Markovnikov Radical Hydro‐ and Aminooxygenation of Unactivated Alkenes Tuned by Ketoxime Carbonates

AbstractA tunable photocatalytic method is reported for anti‐Markovnikov hydro‐ and aminooxygenation of unactivated alkenes using readily accessible ketoxime carbonates as the diverse functionalization reagents. Mechanistic studies reveal that this reaction is initiated through an energy‐transfer‐promoted N−O bond homolysis of ketoxime carbonates leading to alkoxylcarbonyloxyl and iminyl radicals under visible‐light photocatalysis, followed by the addition of alkoxylcarbonyloxyl radical to alkenes. By taking advantage of the different stability of the iminyl radicals, the generated carbon radical either abstracts a hydrogen atom from the media to form the anti‐Markovnikov hydrooxygenation product, or it is trapped by the persistent iminyl radical to furnish the aminooxygenation product. Notably, this is the first example of direct hydrooxygenation of unactivated olefins with anti‐Markovnikov regioselectivity involving an oxygen‐centered radical.

Visible-light photoredox-catalyzed selective carboxylation of C(sp3)−F bonds with CO2

Visible-light photoredox-catalyzed selective carboxylation of C(sp3)−F bonds with CO2