Recent Advances in Organic Electrochemical C-H Functionalization

Chemistry with Electrochemically Generated N-Centered Radicals

N-centered radicals are versatile reaction intermediates that can react with various π systems to construct C-N bonds. Current methods for generating N-centered radicals usually involve the cleavage of an N-heteroatom bond; however, similar strategies that are applicable to N-H bonds prove to be more challenging to develop and therefore are attracting increasing attention. In this Account, we summarize our recent efforts in the development of electrochemical methods for the generation and synthetic utilization of N-centered radicals. In our studies, N-aryl amidyl radical, amidinyl radical and iminyl radical cation intermediates are generated from N-H precursors through direct electrolysis or indirect electrolysis assisted by a redox catalyst. In addition, an electrocatalytic method that converts oximes to iminoxyl radicals has also been developed. The electrophilic amidyl radical intermediates can participate in 5-exo or 6-exo cyclization with alkenes and alkynes to afford C-centered radicals, which can then undergo various transformations such as H atom abstraction, single-electron transfer oxidation to a carbocation, cyclization, or aromatic substitution, leading to a diverse range of N-heterocyclic products. Furthermore, amidinyl radicals, iminyl radical cations, and iminoxyl radicals can undergo intramolecular aromatic substitution to afford various N-heteroaromatic compounds. Importantly, the electrochemical reaction can be channeled toward a specific product despite the presence of other competing pathways. For a successful electrosynthesis, it is important to take into consideration of both the electron transfer steps associated with the electrode and the nonelectrode related processes. A unique feature of electrochemistry is the simultaneous occurrence of anodic oxidation and cathodic reduction, which, as this Account demonstrates, allows the dehydrogenative transformations to proceed through H₂ evolution without the need for chemical oxidants. In addition, cathodic solvent reduction can continuously generate a low concentration of base, which facilitates anodic substrate oxidation. Such a mechanistic paradigm obviates the need for stoichiometric strong bases and avoids base-promoted decomposition of sensitive substrates or products. Furthermore, electrode materials can also be adjusted to control the reaction outcome, as demonstrated by the synthesis of N-heteroaromatics and the corresponding N-oxides from biaryl ketoximes.

Electrochemical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols: synthesis of β-CF3-substituted ketones

Electrochemical oxidative radical trifluoromethylation/semipinacol rearrangement sequences of alkenyl alcohols were developed in this study. This approach is environmentally benign and uses the shelf-stable Langlois reagent as a trifluoromethyl radical precursor and electrons as the oxidizing reagents. The present protocol offers a facile route to prepare β-trifluoromethylated ketone derivatives.

Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization

Synergistic use of electrochemistry and organometallic catalysis has emerged as a powerful tool for site-selective C-H functionalization, yet this type of transformation has thus far mainly been limited to arene C-H functionalization. Herein, we report the development of electrochemical vinylic C-H functionalization of acrylic acids with alkynes. In this reaction an iridium catalyst enables C-H/O-H functionalization for alkyne annulation, affording α-pyrones with good to excellent yields in an undivided cell. Preliminary mechanistic studies show that anodic oxidation is crucial for releasing the product and regeneration of an Ir(III) intermediate from a diene-Ir(I) complex, which is a coordinatively saturated, 18-electron complex. Importantly, common chemical oxidants such as Ag(I) or Cu(II) did not give significant amounts of the desired product in the absence of electrical current under otherwise identical conditions.

Scalable Rhodium(III)-Catalyzed Aryl C-H Phosphorylation Enabled by Anodic Oxidation Induced Reductive Elimination

Transition metal catalyzed C-H phosphorylation remains an unsolved challenge. Reported methods are generally limited in scope and require stoichiometric silver salts as oxidants. Reported here is an electrochemically driven Rh^III -catalyzed aryl C-H phosphorylation reaction that proceeds through H₂ evolution, obviating the need for stoichiometric metal oxidants. The method is compatible with a variety of aryl C-H and P-H coupling partners and particularly useful for synthesizing triarylphosphine oxides from diarylphosphine oxides, which are often difficult coupling partners for transition metal catalyzed C-H phosphorylation reactions. Experimental results suggest that the mechanism responsible for the C-P bond formation involves an oxidation-induced reductive elimination process.

Electrochemical Difluoromethylation of Electron-Deficient Alkenes

Electrochemical 1,2-hydroxydifluoromethylation and C-H difluoromethylation of acrylamides were developed by using CF₂ HSO₂ NHNHBoc as the source of the CF₂ H group. These electricity-powered oxidative alkene functionalization reactions do not need transition-metal catalysts or chemical oxidants. The reaction outcome, 1,2-difuntionalization or C-H functionalization, is determined by the substituents on the amide nitrogen atom of the acrylamides instead of by the reaction conditions.

Continuous-Flow Electrosynthesis of Benzofused S-Heterocycles by Dehydrogenative C-S Cross-Coupling

Reported herein is the synthesis of benzofused six-membered S-heterocycles by intramolecular dehydrogenative C-S coupling using a modular flow electrolysis cell. The continuous-flow electrosynthesis not only ensures efficient product formation, but also obviates the need for transition-metal catalysts, oxidizing reagents, and supporting electrolytes. Reaction scale-up is conveniently achieved through extended electrolysis without changing the reaction conditions and equipment.

A diastereoselective approach to axially chiral biaryls via electrochemically enabled cyclization cascade

A diastereoselective approach to axially chiral imidazopyridine-containing biaryls has been developed. The reactions proceed through a radical cyclization cascade to construct the biaryls with good to excellent central-to-axial chirality transfer.

Merging Photochemistry with Electrochemistry: Functional-Group Tolerant Electrochemical Amination of C(sp3 )-H Bonds

Direct amination of C(sp³ )-H bonds is of broad interest in the realm of C-H functionalization because of the prevalence of nitrogen heterocycles and amines in pharmaceuticals and natural products. Reported here is a combined electrochemical/photochemical method for dehydrogenative C(sp³ )-H/N-H coupling that exhibits good reactivity with both sp² and sp³ N-H bonds. The results show how use of iodide as an electrochemical mediator, in combination with light-induced cleavage of intermediate N-I bonds, enables the electrochemical process to proceed at low electrode potentials. This approach significantly improves the functional-group compatibility of electrochemical C-H amination, for example, tolerating electron-rich aromatic groups that undergo deleterious side reactions in the presence of high electrode potentials.

Electrochemical Umpolung of Bromide: Transition-Metal-Free Bromination of Indole C⁻H Bond

A facile and sustainable electrochemical umpolung of bromide ion protocol was developed under mild reaction conditions. Transition metal catalysts and exogenous chemical oxidants were obviated for the bromination of C⁻H bond. Notably, graphite rod, which is commercially available at supermarkets and is inexpensive, was employed as the electrode material. This operationally easy and environmentally friendly approach accomplished the synthesis of 3-bromoindole in excellent yield and regioselectivity.

Catalyst-Free, Direct Electrochemical Tri- and Difluoroalkylation/Cyclization: Access to Functionalized Oxindoles and Quinolinones

The catalyst-free electrochemical di- and trifluoromethylation/cyclization of N-substituted acrylamides was realized under external oxidant-free conditions. The strategy provides expedient access to fluoroalkylated oxindoles and 3,4-dihydroquinolin-2(1 H)-ones with ample scope and broad functional group tolerance by mild, direct electrolysis of sodium sulfinates in an undivided cell. Detailed mechanistic studies provided strong support for a SET-based reaction manifold.