Chemo- and Site-Selective Electro-Oxidative Alkane Fluorination by C(sp3 )-H Cleavage

A General Strategy for C(sp3)─H Bond Etherification via Quinoline Derivative-Mediated Electrolysis

Electrooxidative coupling of C(sp3)─H bonds with nucleophiles offers an attractive method for constructing C─C and C─X bonds without sacrificial oxidants. However, the direct electrochemical approach requires the nucleophilic reagent to have a higher potential than the C(sp3)─H coupling partners, which restricts the substrate scope. In this study, a quinoline derivative is introduced as an electrochemical mediator, enabling efficient C─H bond etherification with reduced reliance on the electronic properties of substrates. The catalytic system demonstrates broad substrate compatibility, extending to C(sp3)─H coupling partners featuring a diverse range of C─H bonds, including tertiary benzylic C─H bonds and unactivated C(sp3)─H bonds. Mechanistic investigations confirm the role of the electrocatalyst in the hydrogen atom transfer (HAT) process. This method provides a versatile and efficient strategy for the late-stage functionalization of bioactive molecules.

Electrochemical Fluorination of Organic Compounds Using a Hexafluorosilicate Salt as an Inexpensive and Widely Available Fluorine Source

The introduction of fluorine into organic molecules is of the utmost importance in the preparation of active pharmaceutical ingredients (APIs). While a wide range of fluorine sources for organic synthesis have been used over the past decades, the associated safety risks, cost, or environmental impact are still serious limitations. Hexafluorosilicate salts are one of the most inexpensive and readily available sources of nucleophilic fluorine, but they have so far not been used in organic synthesis. Herein we report the first example of the use of a hexafluorosilicate salt as a reagent for the formation of C-F bonds. We have selected as the model reaction an electrochemical decarboxylative fluorination procedure. The synthesis of bis(5-ethyl-2-methylpyridin-1-ium) hexafluorosilicate(IV) was key to obtaining a soluble and reactive hexafluorosilicate salt. This protocol enabled the synthesis of a wide range of primary, secondary, and tertiary aliphatic fluorides (22 examples) in up to 85% yield. The electrochemical method was also successfully transferred to a flow electrolysis cell, demonstrating its robustness and scalability. Finally, we extended the scope of the fluorine source by demonstrating its applicability to electrochemical benzylic C-H fluorination.

Pt/IrOx enables selective electrochemical C-H chlorination at high current

Employing electrochemistry for the selective functionalization of liquid alkanes allows for sustainable and efficient production of high-value chemicals. However, the large potentials required for C(sp3)-H bond functionalization and low water solubility of such alkanes make it challenging. Here we discover that a Pt/IrOx electrocatalyst with optimized Cl binding energy enables selective generation of Cl free radicals for C-H chlorination of alkanes. For instance, we achieve monochlorination of cyclohexane with a current up to 5 A, Faradaic efficiency (FE) up to 95% and stable performance over 100 h in aqueous KCl electrolyte. We further demonstrate that our system can directly utilize concentrated seawater derived from a solar evaporation reverse osmosis process, achieving a FE of 93.8% towards chlorocyclohexane at a current of 1 A. By coupling to a photovoltaic module, we showcase solar-driven production of chlorocyclohexane using concentrated seawater in a membrane electrode assembly cell without any external bias. Our findings constitute a sustainable pathway towards renewable energy driven chemicals manufacture using abundant feedstock at industrially relevant rates.

Benzylic C(sp3)-H fluorination

The selective fluorination of C(sp3)-H bonds is an attractive target, particularly for pharmaceutical and agrochemical applications. Consequently, over recent years much attention has been focused on C(sp3)-H fluorination, and several methods that are selective for benzylic C-H bonds have been reported. These protocols operate via several distinct mechanistic pathways and involve a variety of fluorine sources with distinct reactivity profiles. This review aims to give context to these transformations and strategies, highlighting the different tactics to achieve fluorination of benzylic C-H bonds.

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

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 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.

Amide-Ligand-Promoted Silver-Catalyzed C-H Fluorination via Radical/Polar Crossover

We describe an efficient method for benzylic C-H fluorination via sequential hydrogen-atom transfer (HAT) and oxidative radical-polar crossover utilizing the Ag(I)/Selectfluor system. Amide ligands, such as benzamide and sulfonamide, substantially facilitate the processes leading to a carbocation intermediate, which subsequently reacts with nucleophilic fluorinating reagent to form a C-F bond. This protocol is applicable to the fluorination of all 1°, 2°, and 3° C-H bonds as well as to late-stage C-H fluorination of bioactive molecules.

Late-Stage Modification of Drugs via Alkene Formal Insertion into Benzylic C-F Bond

C-F insertion of carbon-atom units is underdeveloped although it poses significant potential applications in both drug discovery and development. Herein, we report a photocatalytic protocol for late-stage modification of trifluoromethyl aromatic drugs involving formal insertion of abundant alkene feedstocks into a benzylic C-F bond selectively. This redox-neutral transformation features mild conditions and extraordinary functional group tolerance. Preliminary studies are consistent with this transformation involving a radical-polar crossover pathway. Additionally, it offers an alternative strategy for difunctionalization of alkenes via quenching of the carbocation intermediate with nucleophiles other than external fluoride.

Palladium-Catalyzed Electrooxidative Hydrofluorination of Aryl-Substituted Alkenes with a Nucleophilic Fluorine Source

Herein, we report an electrocatalytic hydrofluorination of aryl-substituted alkenes with a nucleophilic fluorine source. The merger of palladium catalysis with electrooxidation enables the transformation of various substrates ranging from styrenes to more challenging α,β-unsaturated carbonyl derivatives to the corresponding benzylic fluorides. This method can also be applied to the late-stage modification of pharmaceutical derivatives. Mechanistic studies suggest that the generation of a high-valent palladium intermediate via anodic oxidation is the crucial step in this electrocatalytic hydrofluorination.