Electrochemical Benzylic C(sp3)-H Acyloxylation

Electrochemical Benzylic C(sp3)-H Imidation Enabled by Benzoic Acid Derived Radicals

We developed an electrochemical approach for benzylic C(sp3)-H imidation by virtue of the in situ generated oxygen-centered radicals (OCRs). The electrochemical imidation provides a complementary approach to giving distinct imide products compared with previous acyloxylation products. This protocol exhibits good site selectivity and broad substrate generality. Moreover, the utility of the OCR-mediated protocol was extended to the electrochemical oxidation of silane, and its robustness was also highlighted by the imidation of complex substrates, which would otherwise be inaccessible for previous approaches. A plausible reaction mechanism was proposed to rationalize the experimental observations.

Electrochemical Benzylic C-H Amination via N-Aminopyridinium

An electrochemical protocol for benzylic C(sp3)-H aminopyridylation via direct C-H/N-H cross-coupling of alkylarenes with N-aminopyridinium triflate has been developed. This method features excellent site-selectivity, broad substrate scope, redox reagent-free and facile scalability. The generated benzylaminopyridiniums can be readily converted to benzylamines via electroreductive N-N bond cleavage.

α-Acyloxylation of Ketones/Cyclic Ethers Mediated by Hypervalent Iodine(III) Reagents as Oxidants and Nucleophilic Sources

This study describes a catalyst-free α-acyloxylation of ketones and a KBr-mediated α-acyloxylation of cyclic ethers. These conversions are effectively mediated by hypervalent iodine(III) reagents serving dual roles as the oxidant and nucleophilic source. Consequently, esters are produced directly in moderate to excellent yields. The proposed method features good functional group compatibility, a broad substrate scope, and high synthetic efficiency and is remarkably environmentally friendly.

Pulsed electrolysis: enhancing primary benzylic C(sp3)-H nucleophilic fluorination

Electrosynthesis is an efficient and powerful tool for the generation of elusive reactive intermediates. The application of alternative electrolysis waveforms provides a new level of control for dynamic redox environments. Herein, we demonstrate that pulsed electrolysis provides a favourable environment for the generation and fluorination of highly unstable primary benzylic cations from C(sp3)-H bonds. By introduction of a toff period, we propose this waveform modulates the electrical double layer to improve mass transport and limit over-oxidation.

Electrochemical Benzylic C(sp3)-H Direct Amidation

Amide bonds are ubiquitous and found in a myriad of functional molecules. Although formed in a reliable and robust fashion, alternative amide bond disconnections provide flexibility and synthetic control. Herein we describe an electrochemical method to form the non-amide C-N bond from direct benzylic C(sp3)-H amidation. Our approach is applied toward the synthesis of secondary amides by coupling secondary benzylic substrates with substituted primary benzamides. The reaction has been scaled up to a multigram scale in flow.

Continuous Flow Electrochemistry Enables Practical and Site-Selective C-H Oxidation

The selective oxygenation of ubiquitous C(sp3 )-H bonds remains a highly sought-after method in both academia and the chemical industry for constructing functionalized organic molecules. However, it is extremely challenging to selectively oxidize a certain C(sp3 )-H bond to afford alcohols due to the presence of multiple C(sp3 )-H bonds with similar strength and steric environment in organic molecules, and the alcohol products being prone to further oxidation. Herein, we present a practical and cost-efficient electrochemical method for the highly selective monooxygenation of benzylic C(sp3 )-H bonds using continuous flow reactors. The electrochemical reactions produce trifluoroacetate esters that are resistant to further oxidation but undergo facile hydrolysis during aqueous workup to form benzylic alcohols. The method exhibits a broad scope and exceptional site selectivity and requires no catalysts or chemical oxidants. Furthermore, the electrochemical method demonstrates excellent scalability by producing 115 g of one of the alcohol products. The high site selectivity of the electrochemical method originates from its unique mechanism to cleave benzylic C(sp3 )-H bonds through sequential electron/proton transfer, rather than the commonly employed hydrogen atom transfer (HAT).

Cell Voltage-Dependent Structural Dichotomy: Electrochemical C-H Acyloxylation and N-Acylation of 2H-Indazoles

A simple and efficient electrochemical method that utilizes modulation of the cell voltage to cause structural alterations in 2H-indazole is introduced. This method enables the C-3 acyloxylation of 2H-indazole and promotes the transfer of the acyl group from C-3 to N-1, allowing the N-acylation of 2H-indazoles. Additionally, the application of the μ-electro flow reactor was demonstrated, showcasing its effectiveness in achieving gram-scale production of 3x within a short residence time.

Electrochemical Difunctionalization of gem-Difluoroalkenes: A Metal-Free Synthesis of α-Difluoro(alkoxyl/azolated) Methylated Ethers

A scalable electrochemical difunctionalization of gem-difluoroalkenes to structurally versatile difluoro motifs was achieved. This methodology features reagent-free conditions, good functional group tolerance, and a relatively broad substrate scope. Meanwhile, the electrolysis protocol is easy to handle, and the products show good regio- and chemoselectivity. The reaction mechanism was also preliminarily studied.

eFluorination Using Cheap and Readily Available Tetrafluoroborate Salts

A practical electrochemical method for the rapid, safer, and mild synthesis of tertiary hindered alkyl fluorides from carboxylic acids has been developed without the need for hydrofluoric acid salts or non-glass reactors. In this anodic fluorination, collidinium tetrafluoroborate acts as both the supporting electrolyte and fluoride donor. A wide range of functional groups has been shown to be compatible, and the possibility of scale-up using flow electrochemistry has also been demonstrated.

Electrochemical C(sp3)-H Lactonization of 2-Alkylbenzoic Acids toward Phthalides

Herein, we report direct electrochemical C(sp³)-H lactonization of 2-alkylbenzoic acids toward phthalides. The reaction provides a wide substrate scope of 2-alkylbenzoic acids bearing primary to tertiary C(sp³)-H bonds by utilizing a graphite anode, dichloromethane (DCM) solvent, hexafluoroisopropanol (HFIP) cosolvent, and n-Bu₄NClO₄ electrolyte. Our synthetic approach offers a simple, intuitive, and atom-economical protocol to synthesize various phthalides (25 examples, up to 92% yield) and obtain other 5- and 6-membered lactones (10 examples, up to 83% yield).

Study of the Electrochemical Behavior of N-Substituted-4-Piperidones Curcumin Analogs: A Combined Experimental and Theoretical Approach

The electrochemical behavior of N-methyl- and N-benzyl-4-piperidone curcumin analogs were studied experimentally and theoretically. The studied compounds present different substituents at the para position in the phenyl rings (-H, -Br, -Cl, -CF₃, and -OCH₃). We assessed their electrochemical behavior by differential pulse and cyclic voltammetry, while we employed density functional theory (DFT) M06 and M06-2x functionals along with 6-311+G(d,p) basis set calculations to study them theoretically. The results showed that compounds suffer a two-electron irreversible oxidation in the range of 0.72 to 0.86 V, with surface concentrations ranging from 1.72 × 10^-7 to 5.01 × 10^-7 mol/cm². The results also suggested that the process is diffusion-controlled for all compounds. M06 DFT calculations showed a better performance than M06-2x to obtain oxidation potentials. We found a good correlation between the experimental and theoretical oxidation potential for N-benzyl-4-piperidones (R² = 0.9846), while the correlation was poor for N-methyl-4-piperidones (R² = 0.3786), suggesting that the latter suffer a more complex oxidation process. Calculations of the BDEs for labile C-H bonds in the compounds suggested that neither of the two series of compounds has a different tendency for a proton-coupled electron transfer (PCET) oxidation process. It is proposed that irreversible behavior is due to possible dimerization of the compounds by Shono-type oxidation.

Benzylic C(sp3)-H Bonds Play the Dual Role of Starting Material and Oxidation Inhibitor for Hydrazides in the Electrochemical Synthesis of Hydrazones

The electrooxidation of benzylic C(sp³)-H bonds to produce hydrazones as an alternate for conventional pathways has an enormous dignity. Under the aegis of electricity, instead of hazardous metal catalysts and external oxidants, we unveil an electrochemical process for electrooxidation of various benzylic C(sp³)-H bonds in aqueous media in all pH ranges that subsequently produce hydrazones with further reactions. This electrooxidative reaction strategy provides an acceptable condition for synthesizing hydrazones with various functional groups in good efficiency and amenable to gram-scale synthesis. The electrochemical oxidation condition proves an excellent level of compatibility with super cheap electrolyte NaCl for the oxidation of benzylic C(sp³)-H position despite the highly oxidizable hydrazide group remaining intact in the reaction.