Late-stage azolation of benzylic C‒H bonds enabled by electrooxidation

Electrochemically Direct Fluorination Functionalization of Styrenes with Different Fluorine Source: Access to Fluoroalkyl Derivatives

A mild protocol for electrochemically oxidative fluorodifunctionalization of styrenes has been demonstrated. The reaction proceeds under metal, external oxidant, and catalyst free conditions, allowing tunable access to a wide variety of synthetically useful fluoroalkyl derivatives, such as β-fluorosulfone/fluoromethyl, fluorothiocyanation, and vinylsulfonyl derivatives. Moreover, CsF was shown to be the proper fluorine source for this electrochemical fluorodifunctionalization transformation.

Electrochemical synthesis of peptide aldehydes via C‒N bond cleavage of cyclic amines

Peptide aldehydes are crucial biomolecules essential to various biological systems, driving a continuous demand for efficient synthesis methods. Herein, we develop a metal-free, facile, and biocompatible strategy for direct electrochemical synthesis of unnatural peptide aldehydes. This electro-oxidative approach enabled a step- and atom-economical ring-opening via C‒N bond cleavage, allowing for homoproline-specific peptide diversification and expansion of substrate scope to include amides, esters, and cyclic amines of various sizes. The remarkable efficacy of the electro-synthetic protocol set the stage for the efficient modification and assembly of linear and macrocyclic peptides using a concise synthetic sequence with racemization-free conditions. Moreover, the combination of experiments and density functional theory (DFT) calculations indicates that different N-acyl groups play a decisive role in the reaction activity.

Al(OTf)3-Catalyzed Regioselective N2-Arylation of Tetrazoles with Diazo Compounds

Regioselective methods to access alkylated tetrazoles still remain a challenging goal. Herein, we describe a novel regioselective protocol for N2-arylation of tetrazoles with diazo compounds using inexpensive Al(OTf)3. This reaction could be conducted under mild conditions to access a diverse array of alkylated tetrazoles with 2-substituted tetrazoles as the major products, demonstrating a comprehensive range of substrate compatibility and excellent functional group compatibility. Mechanistic studies revealed a carbene-free process in this reaction procedure. Furthermore, the scale-up reaction and transformations of the N2-arylation of tetrazole products demonstrated the potential of this strategy.

Electrochemical Fluorination Functionalization of gem-Difluoroalkenes with CsF as a Fluorine Source: Access to Fluoroalkyl Building Blocks

Using easily handled CsF as a fluorine source, an electrochemically metal-free protocol for chemo- and regioselective synthesis of various types of long-chain perfluoroalkyl aromatics with gem-difluoroalkene as a substrate and an alcohol or azole as an additional nucleophile was developed. The eletrochemical transformation could tolerate several functional groups, such as halogens, cyanos, benzyls, and heterocycles, and is amenable to gram-scale. The application of this electrochemical method in radiofluorination was also tested.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

The electrochemically enabled α-C(sp3)-H azolation of ketones

C-H/N-H cross-coupling has become a key technology for the selective conjugation of azole drug molecules. However, the development of new synthetic models and green chemical methods is imperative to enhance the construction of multi-functional compounds and compounds with unique functional groups. We herein reported an electrochemical synthesis of α-tetrazolyl ketones with excellent yields and broad substrate scope, encompassing electron-donating and electron-withdrawing groups of aryl ketones, heterocycles, and alkyl and various ketone drugs. It was further proved that α-iodoketone was involved in this transformation of the reaction as a critical intermediate.

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.

Electrochemical Selenylation of Sulfoxonium Ylides for the Synthesis of gem-Diselenides as Antimicrobials against Fungi

Organoselenium compounds are important scaffolds in pharmaceutical molecules. Herein, we report metal-free, electrochemical, highly chemo- and regioselective synthesis of gem-diselenides through the coupling of α-keto sulfoxonium ylides with diselenides. The versatility of the electrochemical manifold enabled the selenylation with ample scope and broad functional group tolerance, as well as setting the stage for modification of complex bioactive molecules. Detailed mechanistic studies revealed that the key C-Se bond was constructed using n-Bu₄NI as an electrolyte and catalyst through the electrosynthetic protocol. Finally, the desired α-keto gem-diselenides showed excellent antimicrobial activity against Candida albicans, which can be identified as the lead compounds for further exploration.

Azolation of Benzylic C-H Bonds via Photoredox-Catalyzed Carbocation Generation

A visible-light photoredox-catalyzed method is reported that enables the coupling between benzylic C-H substrates and N-H azoles. Classically, medicinally relevant N-benzyl azoles are produced via harsh substitution conditions between the azole and a benzyl electrophile in the presence of strong bases at high temperatures. Use of C-H bonds as the alkylating partner streamlines the preparation of these important motifs. In this work, we report the use of N-alkoxypyridinium salts as a critically enabling reagent for the development of a general C(sp3)-H azolation. The platform enables the alkylation of electron-deficient, -neutral, and -rich azoles with a range of C-H bonds, most notably secondary and tertiary partners. Moreover, the protocol is mild enough to tolerate benzyl electrophiles, thus offering an orthogonal approach to existing SN2 and cross-coupling methods.

An electrochemical multicomponent reaction toward C-H tetrazolation of alkyl arenes and vicinal azidotetrazolation of alkenes

The widespread use of tetrazoles in medicine, biology, and materials science continuously promotes the development of their efficient and selective syntheses. Despite the prosperous development of multicomponent reactions, the use of the most abundant and inexpensive chemical feedstocks, i.e., alkanes and alkenes, toward the preparation of diverse tetrazoles remains elusive. Herein, we developed an electrochemical multicomponent reaction (e-MCR) for highly efficient and selective C-H tetrazolation of alkyl arenes. When applied to alkenes, the corresponding vicinal azidotetrazoles were readily obtained, which were further demonstrated to be versatile building blocks and potential high-energy materials.

Enantioselective Synthesis of N-Benzylic Heterocycles by Ni/Photoredox Dual Catalysis

An asymmetric cross-coupling of α-N-heterocyclic trifluoroborates with aryl bromides using Ni/photoredox dual catalysis has been developed. This C(sp2)-C(sp3) cross-coupling provides access to pharmaceutically relevant chiral N-benzylic heterocycles in good to excellent enantioselectivity when bioxazolines (BiOX) are used as the chiral ligand. High-throughput experimentation significantly streamlined reaction development by identifying BiOX ligands for further investigation and by allowing for rapid optimization of conditions for new trifluoroborate salts.

Direct Electrooxidative Selenylation/Cyclization of Alkynes: Access to Functionalized Benzo[b]furans

A mild, practical, metal and oxidant-free methodology for the synthesis of various C-3 selenylated benzo[b]furan derivatives was developed through the intramolecular cyclization of alkynes promoted with diselenides via electrooxidation. A wide range of selenium-substituted benzo[b]furan derivatives were obtained in good to excellent yields with high regioselectivity under constant current in an undivided cell equipped with carbon and platinum plates as the anode and cathode, respectively. Moreover, the convergent approach exhibited good functional group tolerance and could be easily scaled up with good efficiency, providing rapid access to a diverse range of selenylated benzo[b]furans derivatives from simple, readily available starting materials.

Site-Selective Synthesis of N-Benzyl 2,4,6-Collidinium Salts by Electrooxidative C-H Functionalization

N-alkylpyridinium salts are versatile pseudohalides for SET-mediated cross couplings. However, the common 2,4,6-triphenylpyridinium salt is plagued by poor atom economy and high cost of synthesis. Thus, there is a growing need for more practical scaffolds and innovative strategies for pyridinium salt formation. Herein, we report the synthesis of benzylic 2,4,6-collidinium salts via electrooxidative C-H functionalization. This method provides a complementary approach to tradtional strategies relying on substitution and condensation of prefunctionalized substrates.

Metal-Free Regioselective N2-Arylation of 1H-Tetrazoles with Diaryliodonium Salts

We describe a simple, metal-free regioselective N²-arylation strategy for 5-substituted-1H-tetrazoles with diaryliodonium salts to access 2-aryl-5-substituted-tetrazoles. Diaryliodonium salts with a wide range of both electron-rich and previously challenged electron-deficient aryl groups are applicable in this method. Diversely functionalized tetrazoles are tolerable also. We have devised a one-pot system to synthesize 2,5-diaryl-tetrazoles directly from nitriles. The synthetic utility of this method is furthered extended to late-stage arylation of two biologically active molecules.

Ritter-type amination of C(sp3)-H bonds enabled by electrochemistry with SO42

By merging electricity with sulfate, the Ritter-type amination of C(sp³)-H bonds is developed in an undivided cell under room temperature. This method features broad substrate generality (71 examples, up to 93% yields), high functional-group compatibility, facile scalability, excellent site-selectivity and mild conditions. Common alkanes and electron-deficient alkylbenzenes are viable substrates. It also provides a straightforward protocol for incorporating C-deuterated acetylamino group into C(sp³)-H sites. Application in the synthesis or modification of pharmaceuticals or their derivatives and gram-scale synthesis demonstrate the practicability of this method. Mechanistic experiments show that sulfate radical anion, formed by electrolysis of sulfate, served as hydrogen atom transfer agent to provide alkyl radical intermediate. This method paves a convenient and flexible pathway for realizing various synthetically useful transformations of C(sp³)-H bonds mediated by sulfate radical anion generated via electrochemistry.

The amination of C(sp3)–H bonds is an appealing and challenging task in organic synthesis. Here, by using an electrogenerated sulfate radical an HAT agent, the authors report a practical Ritter-type amination of C(sp3)–H bonds.

Access to 3-Amino-[1,2,4]-triazolo Pyridines and Related Heterocycles via Electrochemically Induced Desulfurative Cyclization

An efficient, one-pot approach has been established for synthesizing a wide range of 3-amino-[1,2,4]-triazolo pyridines and related heterocycles from the electrochemically induced desulfurative cyclization of 2-hydrazinopyridines with isothiocyanates. The protocol allows for the formation of C-N bonds under simple conditions without transition metals or external oxidants. The practicability of this strategy is demonstrated by its broad substrate scope, good functional group compatibility, and gram-scale synthesis. The late-stage modification of 3-amino-[1,2,4]-triazolo pyridines enables us to obtain several molecules with potent anticancer activity.

Electrochemical Benzylic C-H Functionalization with Isocyanides

We report the challenging direct carbamoylation or cyanation of benzylic C(sp³)-H bonds with an isocyanide via an electrochemical process giving rise to structures that are encountered in several biologically relevant compounds and drugs. This transformation proceeds under mild conditions without the need for any external oxidant and avoids the necessity to start from a prefunctionalized benzylic substrate or the deployment of the cation pool method. The anodic oxidation of the benzylic position and the subsequent addition of the isocyanide lead to the formation of a C-C bond and to a nitrilium cation that hydrolyzes to yield α-aryl acetamide derivatives, whereas the elimination of a t-butyl cation delivers α-aryl acetonitrile derivatives.

Electrochemical Benzylic C(sp3)-H Isothiocyanation

Selective C(sp³)-H isothiocyanation represents a significant strategy for the synthesis of isothiocyanate derivatives. We report herein an electrochemical benzylic isothiocyanation in a highly chemo- and site-selective manner under external oxidant-free conditions. The high chemoselectivity is attributed to the facile in situ isomerization of benzylic thiocyanates to isothiocyanates. Notably, the method exhibits high functional group compatibility and is suitable for late-stage functionalization of bioactive molecules.

Anodic C(sp3)-H Acyloxylation of Indolin-3-ones Enabled by Oxidant-Free Cross-Dehydrogenative C(sp3)-O Coupling

An efficient anodic C(sp³)-H acyloxylation protocol has been established via intermolecular cross-dehydrogenative C(sp³)-O coupling. The protocol provides various C2-acyloxy indolin-3-ones without the addition of metal catalysts and external oxidants because indolin-3-ones can be directly oxidized at the anode. The effective application of several medical drugs and the realization of the gram-scale experiment have proven the practicality of this protocol.

Electricity-driven redox-neutral C(sp3)–H amidation with N-alkoxyamide as an amidating reagent

An electrochemical redox-neutral C(sp3)–H amidation was developed with N-alkoxyamide as an amidating reagent. Under sequential paired electrolysis, N-alkoxyamides showed higher reactivity compared to the direct reaction of primary amides.

Electrochemical regioselective C-H selenylation of 2H-indazole derivatives

Selenium-substituted heteroarenes are biologically active compounds and useful building blocks. In this study, we have developed a metal- and oxidant-free, environmentally friendly protocol for the regioselective selenylation of 2H-indazole derivatives by an electrochemical strategy. A number of selenylated 2H-indazoles with a wide range of functional groups have been synthesized in moderate to good yields under mild and environment-friendly reaction conditions.

Intermolecular CDC amination of remote and proximal unactivated Csp3 -H bonds through intrinsic substrate reactivity - expanding towards a traceless directing group

An intermolecular radical based distal selectivity in appended alkyl chains has been developed. The selectivity is maximum when the distal carbon is γ to the appended group and decreases by moving from γ → δ → ε positions. In –COO– linked alkyl chains, the same distal γ-selectivity is observed irrespective of its origin, either from the alkyl carboxy acid or alkyl alcohol. The appended groups include esters, N–H protected amines, phthaloyl, sulfone, sulfinimide, nitrile, phosphite, phosphate and borate esters. In borate esters, boron serves as a traceless directing group, which is hitherto unprecedented for any remote C_sp³–H functionalization. The selectivity order follows the trend: 3° benzylic > 2° benzylic > 3° tertiary > α to keto > distal methylene (γ > δ > ε). Computations predicted the radical stability (thermodynamic factors) and the kinetic barriers as the factors responsible for such trends. Remarkably, this strategy eludes any designer catalysts, and the selectivity is due to the intrinsic substrate reactivity.

An intermolecular amination at the distal methylene carbon has been realized in an appended alkyl chain with electron withdrawing groups. Traceless remote C_sp³–H functionalization has been accomplished using borate esters.

Electro-Oxidative C-N Bond Formation through Azolation of Indole Derivatives: An Access to 3-Substituent-2-(Azol-1-yl)indoles

A practical protocol to synthesize 3-substituent-2-(azol-1-yl)indole derivatives has been developed via an electrochemical oxidative cross coupling process under mild conditions. This electro-oxidative C-N bond formation strategy tolerates a range of functional groups and is amenable to gram scale synthesis. Moreover, this method was applied to the late-stage functionalization of bioactive molecules.

Metal-Free Cascade Formation of Intermolecular C-N Bonds Accessing Substituted Isoindolinones under Cathodic Reduction

An electrochemical protocol for the construction of substituted isoindolinones via reduction/amidation of 2-carboxybenzaldehydes and amines has been realized. Under metal-free and external-reductant-free electrolytic conditions, the reaction achieves the cascade formation of intermolecular C-N bonds and provides a series of isoindolinones in moderate to good yields. The deuterium-labeling experiment proves that the hydrogen in the methylene of the product is mainly provided by H₂O in the system.

Electro-oxidative cyclization: access to quinazolinones via K2S2O8 without transition metal catalyst and base

A K₂S₂O₈-promoted oxidative tandem cyclization of primary alcohols with 2-aminobenzamides to synthesize quinazolinones was successfully achieved under undivided electrolytic conditions without a transition metal and base. The key feature of this protocol is the utilization of K₂S₂O₈ as an inexpensive and easy-to-handle radical surrogate that can effectively promote the reaction via a simple procedure, leading to the formation of nitrogen heterocycles via direct oxidative cyclization at room temperature in a one-pot procedure under constant current. Owing to the use of continuous-flow electrochemical setups, this green, mild and practical electrosynthesis features high efficiency and excellent functional group tolerance and is easy to scale up.

A K₂S₂O₈-promoted oxidative tandem cyclization of primary alcohols with 2-aminobenzamides to synthesize quinazolinones was successfully achieved under undivided electrolytic conditions without a transition metal and base.

Copper-Catalyzed Cross-Coupling of Benzylic C-H Bonds and Azoles with Controlled N-Site Selectivity

Azoles are important motifs in medicinal chemistry, and elaboration of their structures via direct N-H/C-H coupling could have broad utility in drug discovery. The ambident reactivity of many azoles, however, presents significant selectivity challenges. Here, we report a copper-catalyzed method that achieves site-selective cross-coupling of pyrazoles and other N-H heterocycles with substrates bearing (hetero)benzylic C-H bonds. Excellent N-site selectivity is achieved, with the preferred site controlled by the identity of co-catalytic additives. This cross-coupling strategy features broad scope for both the N-H heterocycle and benzylic C-H coupling partners, enabling application of this method to complex molecule synthesis and medicinal chemistry.

Direct Electrochemical Selenylation/Cyclization of Alkenes: Access to Functionalized Benzheterocycles

A catalyst-free, environmentally friendly, and efficient electrochemical selenylation/cyclization of alkenes has been developed with moderate to excellent yields. This selenylated transformation proceeds smoothly and tolerates a wide range of synthetically useful groups to deliver diverse functionalized benzheterocycles, including iminoisobenzofuran, lactones, oxindoles, and quinolinones. Moreover, the present synthetic route could also be readily scaled up to gram quantity with convenient operation in an undivided cell.

Organocatalytic electrochemical amination of benzylic C–H bonds

An organocatalytic site-selective electrochemical method for the benzylic C–H amination of alkylarenes with azoles through hydrogen evolution has been developed.