Direct Single Electron Research Articles

Electroredox N-Heterocyclic Carbene-Catalyzed Enantioselective (3 + 3) Annulation of Enals with 2-Naphthols.

Developing asymmetric transformations using electroredox and N-heterocyclic carbene (NHC)-catalyzed radical pathways is still desirable and challenging. Herein, we report an iodide-promoted β-carbon activation (LUMO-lowering process) of enals via electroredox carbene catalysis coupled with a hydrogen evolution reaction (HER). This strategy offers an environmentally friendly and sustainable route for rapidly assembling synthetically useful chiral naphthopyran-3-one in good to excellent yield and enantioselectivity using traceless electrons as inexpensive and greener oxidants. The mechanistic studies and cyclic voltammetry suggest that the reaction proceeds via direct single electron transfer (SET) of the in situ-generated Breslow intermediate.

Synthesis of 1‐Azabicyclo[2.1.1]hexanes via Formal Single Electron Reduction of Azabicyclo[1.1.0]butanes under Photochemical Conditions

AbstractC(sp3)‐rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen‐containing heterocycles are in high demand. Herein, we report a general synthesis of 1‐azabicyclo[2.1.1]hexanes (1‐aza‐BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar‐radical‐polar relay strategy that leverages a fast acid‐mediated ring‐opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB‐ketones and we demonstrate the 1‐aza‐BCH products can be further functionalized to access larger saturated, conformationally rigid heterocycles.

Synthesis of 1-azabicyclo[2.1.1]hexanes via formal single electron reduction of azabicyclo[1.1.0]butanes under photochemical conditions.

C(sp3)-rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen-containing heterocycles are in high demand. Herein, we report a general synthesis of 1-azabicyclo[2.1.1]hexanes (1-aza-BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar-radical-polar relay strategy that leverages a fast acid-mediated ring-opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB-ketones and we demonstrate the 1-aza-BCH products can be further functionalised to access larger saturated, conformationally rigid heterocycles.

Electrochemical oxidative C-C bond cleavage of methylenecyclopropanes with alcohols.

Herein, an electrochemical approach toward the ring opening functionalization of methylenecyclopropanes (MCPs) via C-C bond cleavage in the presence of alcohols is reported. The methodology avoids the usage of external oxidants and shows good functional group tolerance. The mechanistic studies suggest that the reaction proceeds via direct single electron oxidation of the C-C bond of MCPs followed by ring opening to form the desired product.

Light‐Promoted Chlorine‐Radical‐Mediated Oxidation of Benzylic C(sp3)−H Bonds utilizing Air as Oxidant

AbstractA photocatalyzed metal‐free method for benzylic C(sp3)−H bond oxidation has been developed. Achieved under the mild conditions, a variety of methylarenes and benzyl derivatives could be converted into corresponding carboxylic acids and ketones, and isolated yields of 31–98% are obtained. The mechanistic investigations showed that chlorine radical could be generated via the direct single electron transfer (SET) between chloride anion and excited heteroarene under light irradiation, which could be used as hydrogen atom transfer agent to facilitate benzylic C(sp3)−H bond oxygenation. The merits of this catalytic system could be further demonstrated by the degradation of plastic to form benzoic acids.magnified image

Recent Progress on Transition-Metal-Mediated Reductive C(sp3)–O Bond Radical Addition and Coupling Reactions

AbstractIn this short review, we summarize the recent developments on thermo-driven C(sp3)–O bond radical scission methods and their applications in the construction of C(sp3)–C bonds via conjugate addition with activated double bonds and reductive coupling mediated by economic 3d metals, in particular nickel. We have arranged the review based on three approaches for C(sp3)–O bond radical scission (vide infra). After generating the radical intermediates, their subsequent transformation into C(sp3)–C bonds enabled by C(sp3)–O cross-electrophile coupling with carbon electrophiles is discussed in detail.1 Introduction2 Direct Single-Electron Transfer to a C(sp3)–O Bond3 Radical Scission of Activated C(sp3)–O Bonds via Single-Electron Transfer to Protecting Groups4 In Situ Activation of Alcohols5 Summary and Outlook

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp 3 )–H Bond Activation

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp ³ )–H Bond Activation

Enhanced ciprofloxacin degradation by electrochemical activation of persulfate using iron decorated carbon membrane cathode: Promoting direct single electron transfer to produce 1O2

In this work, electrochemical activation of persulfate (E-PS) for the energy efficient treatment of ciprofloxacin (CIP) was performed using a novel iron-decorated carbon membrane (FeCM) cathode and SnO2-Bi/Ti anode. Synergistic effect of electrochemical oxidation and PS activation resulted in superior CIP removal. The FeCM cathode outperforms various cathodes (Pt, Ti, graphite, and stainless steel), owing to the existence of C = O, Fe2+/Fe3+, and F on FeCM, which can facilitate 1O2 generation and utilization in the E-PS system. Moreover, the continuous supply of negative charges endows rapid redox cycle of Fe2+/Fe3+ on FeCM cathode, which not only prevents iron leaching, but also reduces the consumption of active sites, and thus, enables FeCM with high stability over a wide pH range and improved cyclic performance. The potential of E-FeCM-PS for outdoor application were evaluated by removing various persistent organic pollutants, and treating actual hospital effluent. The CIP removal mechanism in the E-FeCM-PS process and the possible CIP degradation pathways were proposed. Overall, this work provides a plausible route for the continuous production of 1O2 for effective abatement of organic contaminants via electro-activation of PS and gives an insight into the intrinsic role of iron-doped carbon membrane for persulfate cathodic activation.

Highly selective α-aryloxyalkyl C-H functionalisation of aryl alkyl ethers.

We report highly selective photocatalytic functionalisations of alkyl groups in aryl alkyl ethers with a range of electron-poor alkenes using an acridinium catalyst with a phosphate base and irradiation with visible light (456 nm or 390 nm). Experiments indicate that the reaction operates via direct single-electron oxidation of the arene substrate ArOCHRR' to its radical cation by the excited state organic photocatalyst; this is followed by deprotonation of the ArOC-H in the radical cation to yield the radical ArOC˙RR'. This radical then attacks the electrophile to form an intermediate alkyl radical that is reduced to complete the photocatalytic cycle. The oxidation step is selective for activated arenes (ArOR) over their non-activated counterparts and the subsequent deprotonation of the methoxy group affords the α-aryloxyalkyl radical that leads to a wide range of functionalised products in good to excellent yield.

Reductive Activation and Hydrofunctionalization of Olefins by Multiphoton Tandem Photoredox Catalysis

The conversion of olefin feedstocks to architecturally complex alkanes represents an important strategy in the expedient generation of valuable molecules for the chemical and life sciences. Synthetic approaches are reliant on the electrophilic activation of unactivated olefins, necessitating functionalization with nucleophiles. However, the reductive functionalization of unactivated and less activated olefins with electrophiles remains an ongoing challenge in synthetic chemistry. Here, we report the nucleophilic activation of inert styrenes through a photoinduced direct single electron reduction to the corresponding nucleophilic radical anion. Central to this approach is the multiphoton tandem photoredox cycle of the iridium photocatalyst [Ir(ppy)2(dtbbpy)] PF6, which triggers in situ formation of a high-energy photoreductant that selectively reduces styrene olefinic π bonds to radical anions without stoichiometric reductants or dissolving metals. This mild strategy enables the chemoselective reduction and hydrofunctionalization of styrenes to furnish valuable alkane and tertiary alcohol derivatives. Mechanistic studies support the formation of a styrene olefinic radical anion intermediate and a Birch-type reduction involving two sequential single electron transfers. Overall, this complementary mode of olefin activation achieves the hydrofunctionalization of less activated alkenes with electrophiles, adding value to abundant olefins as valuable building blocks in modern synthetic protocols.

Prompting direct single electron transfer to produce non-radical 1O2/H* by electro-activating peroxydisulfate process with core-shell cathode

A novel heteroatomic N, P and S co-doped core-shell material (MnFe3O4@PZS) was synthesized by a simple polycondensation hydro-thermal method, and used as the cathode to cooperate with electron-catalysis to activate persulfate (S2O82−) (E-MnFe3O4@PZS-PDS) for tetracycline (TTC) degradation. Radical scavenger studies demonstrated that non-radicals including atomic H* and singlet oxygen (1O2) rather than sulfate and hydroxyl radicals were the crucial reactive oxygen species (ROS). Electrochemical analysis indicated that Mn doping could promote electro-catalytic process via diverting pathway from four/two-electron to one-electron to generate non-radical H*/1O2 at the cathode, including one-electron oxygen reduction reaction (1e-ORR) (O2→1O2), and one-electron hydrogen reduction reaction (1e-HRR) (H2O+e−→H∗), as evidenced by the lowest onset potential (0.072 V) together with electron transfer number (n = 1.65). Besides, the regeneration/reduction of FeⅡ/Ⅲ/MnⅡ/Ⅲ/Ⅳ and persulfate will not cause excessive consumption of electron and chemicals due to that could directly get the electron individually from the cathode and anode, and finally TTC could be completely degraded with low energy consumption (0.655 kWh m−3). This study provides new insights into the direct single electron activating PDS to produce non-radical H*/1O2 via core-shell catalytic MnFe3O4@PZS, and displays a promising application in wastewater treatment.

Low-Temperature, Transition-Metal-Free Cross-Dehydrogenative Coupling Protocol for the Synthesis of 3,3-Disubstituted Oxindoles

A new low-temperature procedure for the synthesis of 3,3-disubstituted 2-oxindoles via cross-dehydrogenative coupling (CDC) is reported. The use of a strong, nonreversible base in these reactions has been found to effect a dramatic drop in reaction temperature (to room temperature) relative to the current state-of-the-art (>100 °C) procedure. When employing iodine as an "oxidant", new evidence suggests that this transformation may occur via a transiently stable iodinated intermediate rather than by direct single-electron oxidation.

A pnCCD-based, fast direct single electron imaging camera for TEM and STEM

We report on a new camera that is based on a pnCCD sensor for applications in scanning transmission electron microscopy. Emerging new microscopy techniques demand improved detectors with regards to readout rate, sensitivity and radiation hardness, especially in scanning mode. The pnCCD is a 2D imaging sensor that meets these requirements. Its intrinsic radiation hardness permits direct detection of electrons. The pnCCD is read out at a rate of 1,150 frames per second with an image area of 264 x 264 pixel. In binning or windowing modes, the readout rate is increased almost linearly, for example to 4000 frames per second at 4× binning (264 x 66 pixel). Single electrons with energies from 300 keV down to 5 keV can be distinguished due to the high sensitivity of the detector. Three applications in scanning transmission electron microscopy are highlighted to demonstrate that the pnCCD satisfies experimental requirements, especially fast recording of 2D images. In the first application, 65536 2D diffraction patterns were recorded in 70 s. STEM images corresponding to intensities of various diffraction peaks were reconstructed. For the second application, the microscope was operated in a Lorentz-like mode. Magnetic domains were imaged in an area of 256 x 256 sample points in less than 37 seconds for a total of 65536 images each with 264 x 132 pixels. Due to information provided by the two-dimensional images, not only the amplitude but also the direction of the magnetic field could be determined. In the third application, millisecond images of a semiconductor nanostructure were recorded to determine the lattice strain in the sample. A speed-up in measurement time by a factor of 200 could be achieved compared to a previously used camera system.

Results of a pnCCD Based Ultrafast Direct Single Electron Imaging Camera for Transmission Electron Microscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

EUV spectra of Xe xvii–Xe xxi produced in charge-exchange collisions

Charge-state-specific spectra produced from charge exchange collisions between xenon ions and helium target atoms using an ECR (electron cyclotron resonance) source have been recorded in extreme ultraviolet (EUV). At low target gas pressures, the spectra were produced after a single collision event involving electron capture by the projectile ion and in each case intense emission near 11 nm was observed. The spectra of Xe${}^{16+}$ to Xe${}^{20+}$ are compared with atomic structure calculations and the possible mechanisms for electron capture are discussed. There is some evidence that for lower charged ions the spectra in the 10--15 nm region result mainly from transfer excitation or ionization processes rather than direct single electron capture into high $nl$ states.

Cofactor-independent oxygenases go it alone

Oxygenases have long been thought to require a cofactor for catalysis. The structure of a vancomycin biosynthetic enzyme in complex with a substrate analog, and with molecular oxygen bound in its active site, supports the idea that cofactor-independent oxygenases act by mediating direct single-electron transfer from a substrate anion to dioxygen.

Direct single electron detection with a CMOS detector for electron microscopy

We report the results of an investigation into the use of a monolithic active pixel sensor (MAPS) for electron microscopy. MAPS, designed originally for astronomers at the Rutherford Appleton Laboratories, was installed in a 120 kV electron microscope (Philips CM12) at the MRC Laboratory in Cambridge for tests which included recording single electrons at 40 and 120 keV, and measuring signal-to-noise ratio (SNR), spatial resolution and radiation sensitivity. Our results show that, due to the excellent SNR and resolution, it is possible to register single electrons. The radiation damage to the detector is apparent with low doses and gets progressively greater so that its lifetime is limited to 600,000–900,000 electrons/pixel (very approximately 10–15 krad). Provided this detector can be radiation hardened to reduce its radiation sensitivity several hundred fold and increased in size, it will provide excellent performance for all types of electron microscopy.

Cobalt‐Mediated Mizoroki–Heck‐Type Reaction of Epoxide with Styrene

AbstractTreatment of a mixture of epoxide and styrene with trimethylsilylmethylmagnesium bromide in the presence of a cobalt‐phosphine complex afforded homocinnamyl alcohol in good yield. The reaction should begin with ring opening of the epoxide by the action of magnesium bromide which leads to the formation of a magnesium 2‐bromoethoxide derivative and not with a direct single electron transfer from a cobalt complex to the epoxide.

Dioxygenases without Requirement for Cofactors and Their Chemical Model Reaction: Compulsory Order Ternary Complex Mechanism of 1H-3-Hydroxy-4-oxoquinaldine 2,4-Dioxygenase Involving General Base Catalysis by Histidine 251 and Single-Electron Oxidation of the Substrate Dianion

1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod) is a cofactor-less dioxygenase belonging to the alpha/beta hydrolase fold family, catalyzing the cleavage of 1H-3-hydroxy-4-oxoquinaldine (I) and 1H-3-hydroxy-4-oxoquinoline (II) to N-acetyl- and N-formylanthranilate, respectively, and carbon monoxide. Bisubstrate steady-state kinetics and product inhibition patterns of HodC, the C69A protein variant of Hod, suggested a compulsory-order ternary-complex mechanism, in which binding of the organic substrate precedes dioxygen binding, and carbon monoxide is released first. The specificity constants, k(cat)/K(m,A) and k(cat)/K(m,O)()2, were 1.4 x 10(8) and 3.0 x 10(5) M(-1) s(-1) with I and 1.2 x 10(5) and 0.41 x 10(5) M(-1) s(-1) with II, respectively. Whereas HodC catalyzes formation of the dianion of its organic substrate prior to dioxygen binding, HodC-H251A does not, suggesting that H251, which aligns with the histidine of the catalytic triad of the alpha/beta hydrolases, acts as general base in catalysis. Investigation of base-catalyzed dioxygenolysis of I by electron paramagnetic resonance (EPR) spectroscopy revealed formation of a resonance-stabilized radical upon exposure to dioxygen. Since in D(2)O spectral properties are not affected, exchangeable protons are not involved, confirming that the dianion is the reactive intermediate that undergoes single-electron oxidation. We suggest that in the ternary complex of the enzyme, direct single-electron transfer from the substrate dianion to dioxygen may occur, resulting in a radical pair. Based on the estimated spin distribution within the radical anion (observed in the model reaction of I), radical recombination may produce a C4- or C2-hydroperoxy(di)anion. Subsequent intramolecular attack would result in the 2,4-endoperoxy (di)anion that may collapse to the reaction products.

State-selective and total single-electron capture for impact of slow He2+ (E ≤ 1 keV) on H2, O2 and CO

Projectile state-selective single-electron capture for impact of slow He2+ (impact energy≤1keV) on H2, O2 and CO has been studied experimentally by means of translational energy spectroscopy. Comparative measurements were made with a Ne gas target for absolute calibration of the electron capture cross-sections for the molecular targets. We compare the importance of “direct” single-electron capture (transition of one electron only without excitation or dissociation of the ionised target molecule) versus single-electron capture accompanied by target excitation (transfer excitation, possibly leading to molecular dissociation). Our measurements are extended to lower impact energies than earlier ones and are qualitatively interpreted on the basis of Landau–Zener transitions in the collisional quasimolecules. Similar as for direct single-electron capture, transfer excitation proceeds preferentially within a “reaction window” for the involved reaction energy defect.