Electrochemistry and catalytic properties of amphiphilic vitamin B12 derivatives in nonaqueous media

Cobalamin-Mediated Electrocatalytic Reduction of Ethyl Chloroacetate in Dimethylformamide

The catalytic reduction of ethyl chloroacetate (ECA) by hydroxocobalamin (HOCbl) in dimethylformamide was studied electrochemically and spectroelectrochemically to identify initial steps in the reaction between the electrogenerated Co(I) center of cobalamin (cob(I)alamin) and ECA. Cyclic voltammograms of HOCbl in the presence of ECA show a small increase in current related to reduction of Co(II) to Co(I), and a new peak at more negative potentials related to reduction of an ethyl carboxymethyl-Cbl intermediate. The oxidation state of HOCbl during catalysis was monitored by means of spectroelectrochemical controlled-potential bulk electrolysis. Addition of ECA to electrogenerated cob(I)alamin initially generates the Co(II) form (cob(II)alamin) followed by a gradual formation of an ethyl carboxymethyl-Cbl intermediate. Controlled-potential bulk electrolysis was performed to identify products formed from catalytic reduction of ECA by electrogenerated cob(I)alamin and quantify the number of electrons transferred per molecule of ECA. Product distributions and coulometric results, together with the results of voltammograms and spectroelectrochemical controlled-potential bulk electrolysis, were interpreted to propose a reaction mechanism.

Cobalt Catalyst Determines Regioselectivity in Ring Opening of Epoxides with Aryl Halides

Ring-opening of epoxides furnishing either linear or branched products belongs to the group of classic transformations in organic synthesis. However, the regioselective cross-electrophile coupling of aryl epoxides with aryl halides still represents a key challenge. Herein, we report that the vitamin B₁₂/Ni dual-catalytic system allows for the selective synthesis of linear products under blue-light irradiation, thus complementing methodologies that give access to branched alcohols. Experimental and theoretical studies corroborate the proposed mechanism involving alkylcobalamin as an intermediate in this reaction.

Bioinspired Electrolysis for Green Molecular Transformations of Organic Halides Catalyzed by B12 Complex

Naturally-occurring B₁₂ -dependent enzymes catalyze various molecular transformations that are of particular interest from the viewpoint of biological chemistry as well as synthetic organic chemistry. Inspired by the unique property of the B₁₂ -dependent enzymes, various catalytic reactions have been developed using its model complex. Among the B₁₂ model complexes, heptamethyl cobyrinate, synthesized from natural vitamin B₁₂ , is highly soluble in various organic solvents and a redox active cobalt complex with an excellent catalysis in electroorganic synthesis. The electrochemical dechlorination of pollutant organic chlorides, such as DDT, was effectively catalyzed by the B₁₂ complex. Modification of the electrode surface by the sol-gel method to immobilize the B₁₂ complex was also developed. The B₁₂ modified electrodes were effective for the dehalogenation of organic halides with high turnover numbers based on the immobilized B₁₂ complex. Electrolysis of an organic halide catalyzed by the B₁₂ complex provided dechlorinated products under anaerobic conditions, while the electrolysis under aerobic conditions afforded oxygen incorporated products, such as an ester and amide along with dechlorination. Benzotrichloride was transformed into ethylbenzoate or N,N-diethylbenzamide in the presence of ethanol or diethylamine, respectively. This amide formation was further expanded to a unique paired electrolysis. Electrochemical reductions of an alkene and alkyne were also catalyzed by the B₁₂ complex. A cobalt-hydrogen complex should be formed as a bioinspired intermediate. Using the B₁₂ complex, light-assisted electrosynthesis was also developed to save the applied energy.

Electrocatalytic Dechlorination of Dichloromethane in Water Using a Heterogenized Molecular Copper Complex

The remediation of organohalides from water is a challenging process in environment protection and water treatment. Herein, we report a molecular copper(I) complex with two triazole units, CuT2, in a heterogeneous aqueous system that is capable of dechlorinating dichloromethane (CH₂Cl₂) to afford hydrocarbons (methane, ethane, and ethylene). The catalytic performance is evaluated in water and presented high Faradaic efficiency (average 70% CH₄) across a range of potentials (-1.1 to -1.6 V vs Ag/AgCl) and high activity (maximum -25.1 mA/cm² at -1.6 V vs Ag/AgCl) with a turnover number of 2.0 × 10⁷. The CuT2 catalyst also showed excellent stability for 14 h of constant exposure to CH₂Cl₂ and 10 h of CH₂Cl₂ exposure cycling. The control compound, a copper-free triazole unit (T1), was also investigated under the same condition and showed inferior catalytic activity, indicating the importance of the copper center. Plausible catalytic mechanisms are proposed for the formation of C₁ and C₂ products via radical intermediates. Computational studies provided additional insight into the reaction mechanism and the selectivity toward the CH₄ formation. The findings in this study demonstrate that complex CuT2 is an efficient and stable catalyst for the dehalogenation of CH₂Cl₂ and could potentially be used for the exploration of the removal of halogenated species from aqueous systems.

Electropolymerized metal-protoporphyrin electrodes for selective electrochemical reduction of CO2

Developing catalysts that exhibit high efficiencies for the electrochemical CO₂ reduction reaction (CO₂RR) in aqueous media is vital both for a healthier environment and practical implementation to produce value-added fuels from energy waste.

Polarity-Reversal Strategy for the Functionalization of Electrophilic Strained Molecules via Light-Driven Cobalt Catalysis

Strain-release-driven methodology is a powerful tool for accessing structural motifs, highly desirable by the pharmaceutical industry. The reactivity of spring-loaded cyclic reagents is dominated by transformations relying on their inherent electrophilic reactivity. Herein, we present a polarity-reversal strategy based on light-driven cobalt catalysis, which enables the generation of nucleophilic radicals through strain release. The applicability of this methodology is demonstrated by the design of two distinct types of reactions: Giese-type addition and Co/Ni-catalyzed cross-coupling. Moreover, a series of electrochemical, spectroscopic, and kinetic experiments as well as X-ray structural analysis of the intermediate alkylcobalt(III) complex give deeper insight into the mechanism of the reaction.

Evans–Showell-type polyoxometalate-based metal–organic complexes with novel 3D structures constructed from flexible bis-pyrazine–bis-amide ligands and copper metals: syntheses, structures, and fluorescence and catalytic properties

Two 3D Evans–Showell-type POM-based copper–bis-pyrazine–bis-amide complexes show good heterogeneous catalytic activity for the oxidation of benzyl alcohol.

Learning from B12 enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis

Cobalamins (B₁₂) play various important roles in vivo. Most B₁₂-dependent enzymes are divided into three main subfamilies: adenosylcobalamin-dependent isomerases, methylcobalamin-dependent methyltransferases, and dehalogenases. Mimicking these B₁₂ enzyme functions under non-enzymatic conditions offers good understanding of their elaborate reaction mechanisms. Furthermore, bio-inspiration offers a new approach to catalytic design for green and eco-friendly molecular transformations. As part of a study based on vitamin B₁₂ derivatives including heptamethyl cobyrinate perchlorate, we describe biomimetic and bioinspired catalytic reactions with B₁₂ enzyme functions. The reactions are classified according to the corresponding three B₁₂ enzyme subfamilies, with a focus on our recent development on electrochemical and photochemical catalytic systems. Other important reactions are also described, with a focus on radical-involved reactions in terms of organic synthesis.

Stereoselective cobalt-catalyzed halofluoroalkylation of alkynes

Stereoselective additions of highly functionalized reagents to available unsaturated hydrocarbons are an attractive synthetic tool due to their high atom economy, modularity, and rapid generation of complexity. We report efficient cobalt-catalyzed (E)-halofluoroalkylations of alkynes/alkenes that enable the construction of densely functionalized, stereodefined fluorinated hydrocarbons. The mild conditions (2 mol% cat., 20 °C, acetone/water, 3 h) tolerate various functional groups, i.e. halides, alcohols, aldehydes, nitriles, esters, and heteroarenes. This reaction is the first example of a highly stereoselective cobalt-catalyzed halo-fluoroalkylation. Unlike related cobalt-catalyzed reductive couplings and Heck-type reactions, it operates via a radical chain mechanism involving terminal halogen atom transfer which obviates the need for a stoichiometric sacrificial reductant.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Evans–Showell-type polyoxometalate constructing novel 3D inorganic architectures with alkaline earth metal linkers: syntheses, structures and catalytic properties

Two 3D frameworks and two 2D networks with an excellent catalytic effect of cyanosilylation were successfully obtained, originating from Evans–Showell-type polyoxoanions [Co₂Mo₁₀H₄O₃₈]⁶⁻ and alkaline earth metal cations (Sr²⁺, Ba²⁺).