Outer-sphere Single-electron Transfer Research Articles

In situ copper photocatalysts triggering halide atom transfer of unactivated alkyl halides for general C(sp3)-N couplings

Direct reduction of unactivated alkyl halides for C(sp3)-N couplings under mild conditions presents a significant challenge in organic synthesis due to their low reduction potential. Herein, we introduce an in situ formed pyridyl-carbene-ligated copper (I) catalyst that is capable of abstracting halide atom and generating alkyl radicals for general C(sp3)-N couplings under visible light. Control experiments confirmed that the mono-pyridyl-carbene-ligated copper complex is the active species responsible for catalysis. Mechanistic investigations using transient absorption spectroscopy across multiple decades of timescales revealed ultrafast intersystem crossing (260 ps) of the photoexcited copper (I) complexes into their long-lived triplet excited states (>2 μs). The non-Stern-Volmer quenching dynamics of the triplets by unactivated alkyl halides suggests an association between copper (I) complexes and alkyl halides, thereby facilitating the abstraction of halide atoms via inner-sphere single electron transfer (SET), rather than outer-sphere SET, for the formation of alkyl radicals for subsequent cross couplings.

(Invited) Chemical Reactions of Ionic Metallofullerenes: An Alternative Route for Exohedral Functionalization

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M3N@Ih -C80 (M = Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the typical Bingel-Hirsch reaction that fails with neutral Sc3N@Ih -C80. Two isomeric Sc3N@Ih -C80 cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared via a Bingel-Hirsch reaction with neutral clusterfullerenes. Density functional theory (DFT) calculations indicated that the oxidized M3N@Ih -C80 was much more reactive than the neutral compound upon addition of the diethyl bromomalonate anion. The Bingel-Hirsch reaction of M3N@Ih -C80 cations occurred via an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M3N@C80(C2H5COO)2CBr]•. Remarkably, the diethyl bromomalonate anion was found to act as both a nucleophile and an electron donor. Cationic forms of clusterfullerenes exhibited chemical properties that are different from those of the corresponding neutral and anionic forms. These findings promote a better understanding of EMFs cations and offer a complementary strategy to activate and functionalize EMFs.

(Invited) Chemical Reactions of Ionic Metallofullerenes: An Alternative Route for Exohedral Functionalization

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M3N@Ih -C80 (M = Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the reactions that fail with neutral Sc3N@Ih -C80. For example, two isomeric Sc3N@Ih -C80 cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared via a Bingel-Hirsch reaction with neutral clusterfullerenes. Density functional theory (DFT) calculations indicated that the oxidized M3N@Ih -C80 was much more reactive than the neutral compound upon addition of the nucleophiles. The Bingel-Hirsch reaction of M3N@Ih -C80 cations occurred via an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M3N@C80(C2H5COO)2CBr]•. Cationic forms of clusterfullerenes exhibited chemical properties that are different from those of the corresponding neutral and anionic forms. These findings promote a better understanding of EMFs cations and offer a complementary strategy to activate and functionalize EMFs.

Chemical Reactions of Cationic Metallofullerenes: An Alternative Route for Exohedral Functionalization.

The chemistry of cationic forms of clusterfullerenes remain less explored than that of the corresponding neutral or anionic species. In the present work, M3 N@Ih -C80 (M=Sc or Lu) cations were generated by both electrochemical and chemical oxidation methods. The as-obtained cations successfully underwent the typical Bingel-Hirsch reaction that fails with neutral Sc3 N@Ih -C80 . Two isomeric Sc3 N@Ih -C80 cation derivatives, [5,6]-open and [6,6]-open adducts, were synthesized, and the former has never been prepared by means of a Bingel-Hirsch reaction with neutral clusterfullerenes. In the case of the Lu3 N@Ih -C80 cation, however, only a [6,6]-open adduct was obtained. Density functional theory (DFT) calculations indicated that the oxidized M3 N@Ih -C80 was much more reactive than the neutral compound upon addition of the diethyl bromomalonate anion. The Bingel-Hirsch reaction of M3 N@Ih -C80 cations occurred by means of an unusual outer-sphere single-electron transfer (SET) process from the diethyl bromomalonate anion to the stable intermediate [M3 N@C80 (C2 H5 COO)2 CBr]. . Remarkably, the diethyl bromomalonate anion was found to act as both a nucleophile and an electron donor.

Oxidation kinetics of ferrocene derivatives with dibenzoyl peroxide

Chemical oxidation of ferrocene and related derivatives by dibenzoyl peroxide in acetonitrile solution produces ferrocenium and benzoic acid after acidification. The rate law is first order in oxidant and in reductant. Steric effects and activation parameters are consistent with a rate-controlling outer-sphere single-electron transfer (ET) step, and reorganization energies are obtained using Marcus theory with B3LYP calculations. Energetics, optimized structures, and solvent effects indicate that rate is affected more by anion than cation solvation and that oxidation of decamethylferrocene by 3-chloroperoxybenzoic acid does not occur by ET.

Palladium-Catalyzed Intermolecular Heck-Type Reaction of Epoxides

The palladium-catalyzed intermolecular Heck-type reaction of both cyclic and acyclic epoxides is reported with tolerance of typical polar groups and acidic protons. Suitable alkenes include styrenes, conjugate dienes, and some electron-deficient olefins. In reactions of aliphatic terminal epoxides, ring opening occurs selectively at terminal positions, and stereocenters of epoxides are fully retained. Mechanistic studies provide evidence for in situ conversion of epoxides to β-halohydrins, generation of alkyl radicals, and radical addition to alkenes as key steps. Cyclovoltammetric determination of reduction potentials suggests that during activation of alkyl iodides by palladium(0) complexes, inner-sphere halogen abstraction is more likely than outer-sphere single electron transfer.

Recent Developments in the Synthesis of Biomacromolecules and their Conjugates by Single Electron Transfer-Living Radical Polymerization.

Single electron transfer-living radical polymerization (SET-LRP) represents a robust and versatile tool for the synthesis of vinyl polymers with well-defined topology and chain end functionality. The crucial step in SET-LRP is the disproportionation of the Cu(I)X generated by activation with Cu(0) wire, powder, or nascent Cu(0) generated in situ into nascent, extremely reactive Cu(0) atoms and nanoparticles and Cu(II)X2. Nascent Cu(0) activates the initiator and dormant chains via a homogeneous or heterogeneous outer-sphere single-electron transfer mechanism (SET-LRP). SET-LRP provides an ultrafast polymerization of a plethora of monomers (e.g., (meth)-acrylates, (meth)-acrylamides, styrene, and vinyl chloride) including hydrophobic and water insoluble to hydrophilic and water soluble. Some advantageous features of SET-LRP are (i) the use of Cu(0) wire or powder as readily available catalysts under mild reaction conditions, (ii) their excellent control over molecular weight evolution and distribution as well as polymer chain ends, (iii) their high functional group tolerance allowing the polymerization of commercial-grade monomers, and (iv) the limited purification required for the resulting polymers. In this Perspective, we highlight the recent advancements of SET-LRP in the synthesis of biomacromolecules and of their conjugates.

Spectroelectrochemistry of Vanadium Acetylacetonate and Chromium Acetylacetonate for Symmetric Nonaqueous Flow Batteries

Chromium acetylacetonate, or Cr(acac)3, is a promising active species for high-energy-density symmetric redox flow batteries because the neutral complex supports multiple charge-transfer reactions with widely separated redox potentials. Voltammetric and spectroelectrochemical measurements were performed to probe the mechanism of the first electrochemical disproportionation of Cr(acac)3 – i.e., the cell reaction associated with the two redox couples immediately adjacent to the equilibrium potential of a freshly prepared nonaqueous Cr(acac)3 solution. Substantially different limiting currents are observed for the positive and negative half-reactions, suggesting that at least one deviates from the similar outer-sphere single-electron transfer mechanisms proposed earlier. Spectroelectrochemical chronoamperometry suggests ligand dissociation in the negative reaction, and consequent structural reorganization of the Cr(acac)3 complex. Vanadium acetylacetonate was investigated for comparison, and no ligand dissociation was observed. A negative half-reaction mechanism consistent with the voltammetric and spectroelectrochemical data is proposed, and used to rationalize observations of charge/discharge behavior in cycling Cr(acac)3 cells.

Dioxygen reactivity with a ferrocene-Lewis acid pairing: reduction to a boron peroxide in the presence of tris(pentafluorophenyl)borane.

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C6 F5 )3 , a Lewis acid unreactive to O2 , to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO2 B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described.

Dioxygen Reactivity with a Ferrocene-Lewis Acid Pairing: Reduction to a Boron Peroxide in the Presence of Tris(pentafluorophenyl)borane

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C6F5)3, a Lewis acid unreactive to O2, to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO2B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described.

Direct, Electrocatalytic Oxygen Reduction by Laccase on Anthracene-2-methanethiol-Modified Gold

Laccase, a multicopper oxidase, catalyses the four electron reduction of oxygen to water. Upon adsorption to an electrode surface, laccase is known to reduce oxygen at overpotentials lower than the best noble metal electrocatalysts usually employed. While the electrocatalytic activity of laccase is well established on carbon electrodes, laccase does not typically adsorb to better defined noble metal surfaces in an orientation that allows for efficient electrocatalysis. In this work, we utilized anthracene-2-methanethiol (AMT) to modify the surface of Au electrodes and examined the electrocatalytic activity of adsorbed laccase. AMT facilitated the adsorption of laccase, and the onset of electrocatalytic oxygen reduction was observed as high as 1.13 V(RHE). We observed linear Tafel behavior with a 144 mV/dec slope, consistent with an outer sphere single electron transfer from the electrode to a Cu site in the enzyme as the rate determining step of the oxygen reduction mechanism.

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

AbstractA quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007