Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Parallel Paired Photoelectrochemical Bromination of Alkylarenes with Electrochemical Pinacol Coupling

A paired electrochemical method for paralleling benzylic bromination of alkylarenes under irradiation with reductive pinacol coupling in a divided cell has been developed. A variety of benzyl bromides at the anode and pinacols at the cathode were obtained simultaneously in moderate-to-high faradaic efficiency. This parallel paired electrochemical protocol showed a broad substrate scope and high chemoselectivity as well as high synthetic and faradaic efficiencies.

Ketyl Radical Anion Mediated Radical Polymerization and Anionic Ring-Opening Polymerization to Give Polymers with Low Molecular Weight Distribution

The development of novel polymerization capable of yielding polymers with low molecular weight distribution (Đ) is essential and significant in polymer chemistry, where monofunctional initiator contains only one initiation site in these polymerizations generally. Here, ketyl radical anion species is introduced to develop a novel Ketyl Mediated Polymerization (KMP), which enables radical polymerization at carbon radical site and anionic ring-opening polymerization at oxygen anion site, respectively. Meanwhile, polymerization and corresponding organic synthesis generally couldn't be performed simultaneously in one pot. Through KMP, organic synthesis and polymerization are achieved in one pot, where small molecules (cyclopentane derivates) and polymers with low Đ are successfully prepared under mild condition simultaneously. At the initiation step, both organic synthesis and polymerization are initiated by single electron transfer reaction with ketyl radical anion formation. Cyclopentane derivates are synthesized through 3-3 coupling reaction and cyclization. Polystyrene and polycaprolactone with low Đ and a full monomer conversion are prepared by KMP via radical polymerization and anionic ring-opening polymerization, respectively. This work therefore enables both organic synthesis and two different polymerizations from same initiation system, which saves time, labour, resource and energy and expands the reaction mode and method libraries of organic chemistry and polymer chemistry.

Plasma Electrochemistry for Carbon-Carbon Bond Formation via Pinacol Coupling

The formation of carbon-carbon bonds by pinacol coupling of aldehydes and ketones requires a large negative reduction potential, often realized with a stoichiometric reducing reagent. Here, we use solvated electrons generated via a plasma-liquid process. Parametric studies with methyl-4-formylbenzoate reveal that selectivity over the competing reduction to the alcohol requires careful control over mass transport. The generality is demonstrated with benzaldehydes, benzyl ketones, and furfural. A reaction-diffusion model explains the observed kinetics, and ab initio calculations provide insight into the mechanism. This study opens the possibility of a metal-free, electrically-powered, sustainable method for reductive organic reactions.

Electroreductive coupling of benzaldehyde by balancing the formation and dimerization of the ketyl intermediate

Electroreductive coupling of biomass-derived benzaldehyde offers a sustainable approach to producing value-added hydrobenzoin. The low efficiency of the reaction mainly ascribes to the mismatch of initial formation and subsequent dimerization of ketyl intermediates (Ph-CH = O → Ph-C·-OH → Ph-C(OH)-C(OH)-Ph). This paper describes a strategy to balance the active sites for the generation and dimerization of ketyl intermediates by constructing bimetallic Pd/Cu electrocatalysts with tunable surface coverage of Pd. A Faradaic efficiency of 63.2% and a hydrobenzoin production rate of up to 1.27 mmol mg^-1 h^-1 (0.43 mmol cm^-2 h^-1) are achieved at -0.40 V vs. reversible hydrogen electrode. Experimental results and theoretical calculations reveal that Pd promotes the generation of the ketyl intermediate, and Cu enhances their dimerization. Moreover, the balance between these two sites facilitates the coupling of benzaldehyde towards hydrobenzoin. This work offers a rational strategy to design efficient electrocatalysts for complex reactions through the optimization of specified active sites for different reaction steps.

Water oxidation couples to electrocatalytic hydrogenation of carbonyl compounds and unsaturated carbon–carbon bonds by nickel

Artificial photosynthesis, an umbrella term, is a chemical process that biomimetics natural photosynthesis. In natural photosynthesis, electrons from the water-oxidation reaction are used for carbon dioxide reduction. Herein, we report the reducion of aldehydes and ketones to corresponding alcohols in a simple undivided cell. This reaction utilized inexpensive nickel foam electrodes (1 cm2) and LiClO4 (0.05 M) as a commercially accessible electrolyte in an aqueous medium. Under electrochemical conditions, a series of alcohols (21 examples) produces high selectivity in good yields (up to 100%). Usage the current method, 10 mmol (1060 mg) of benzaldehyde is also successfully reduced to benzyl alcohol (757 mg, 70% isolated yield) without any by‑products. This route to alcohols matched several green chemistry principles: (a) atom economy owing to the use of H2O as the solvent and the source of hydrogen, (b) elimination of the homogeneous metal catalyst, (c) use of smooth reaction conditions, (d) waste inhibition due to low volumetric of by-products, and (e) application of safe EtOH co-solvent. Moreover, the ability of the system to operate with alkyne and alkene compounds enhanced the practical efficiency of this process.

Electrochemical formal homocoupling of sec-alcohols

Electrochemical pinacol coupling of carbonyl compounds in an undivided cell with a sacrificial anode would be a promising approach toward synthetically valuable vic-1,2-diol scaffolds without using low-valent metal reductants. However, sacrificial anodes produce an equimolar amount of metal waste, which may be a major issue in terms of sustainable chemistry. Herein, we report a sacrificial anode-free electrochemical protocol for the synthesis of pinacol-type vic-1,2-diols from sec-alcohols, namely benzyl alcohol derivatives and ethyl lactate. The corresponding vic-1,2-diols are obtained in moderate to good yields, and good to high levels of stereoselectivity are observed for sec-benzyl alcohol derivatives. The present transformations smoothly proceed in a simple undivided cell under constant current conditions without the use of external chemical oxidants/reductants, and transition-metal catalysts.

Electrochemical Reduction of Aldehydes and Ketones for the Synthesis of Alcohols and Diols under Ambient Conditions

A sustainable, practical direct reduction strategy of carbonyl compounds including aldehydes and ketones via an electrochemical pathway is presented, affording a variety of alcohols or diols as major products with decent yields. The reaction proceeds smoothly in the air at ambient temperature with DABCO as the sacrificial reductant. Mechanistic studies reveal that direct electrochemical reduction followed by either protonation or radical-radical homocoupling is the main pathway.

Multiple Mechanisms Mapped in Aryl Alkyl Ether Cleavage via Aqueous Electrocatalytic Hydrogenation over Skeletal Nickel

We present here detailed mechanistic studies of electrocatalytic hydrogenation (ECH) in aqueous solution over skeletal nickel cathodes to probe the various paths of reductive catalytic C-O bond cleavage among functionalized aryl ethers relevant to energy science. Heterogeneous catalytic hydrogenolysis of aryl ethers is important both in hydrodeoxygenation of fossil fuels and in upgrading of lignin from biomass. The presence or absence of simple functionalities such as carbonyl, hydroxyl, methyl, or methoxyl groups is known to cause dramatic shifts in reactivity and cleavage selectivity between sp³ C-O and sp² C-O bonds. Specifically, reported hydrogenolysis studies with Ni and other catalysts have hinted at different cleavage mechanisms for the C-O ether bonds in α-keto and α-hydroxy β-O-4 type aryl ether linkages of lignin. Our new rate, selectivity, and isotopic labeling results from ECH reactions confirm that these aryl ethers undergo C-O cleavage via distinct paths. For the simple 2-phenoxy-1-phenylethane or its alcohol congener, 2-phenoxy-1-phenylethanol, the benzylic site is activated via Ni C-H insertion, followed by beta elimination of the phenoxide leaving group. But in the case of the ketone, 2-phenoxyacetophenone, the polarized carbonyl π system apparently binds directly with the electron rich Ni cathode surface without breaking the aromaticity of the neighboring phenyl ring, leading to rapid cleavage. Substituent steric and electronic perturbations across a broad range of β-O-4 type ethers create a hierarchy of cleavage rates that supports these mechanistic ideas while offering guidance to allow rational design of the catalytic method. On the basis of the new insights, the usage of cosolvent acetone is shown to enable control of product selectivity.