Recent Developments in the Electroreductive Functionalization of Carbon–Halogen Bonds

Abstract

Electrochemical organic transformations have made enormous progress over the past decades owing to the idiosyncratic redox nature. Electrochemistry is globally acknowledged for its sustainability and environ friendliness and several well-known redox processes get a new exquisite touch without expelling chemical waste and toxic by-products. Apart from this, electrochemistry has adequate potential to steer numerous non-spontaneous reactions like cross-coupling, C−C bond cleavage, radical generation, directed C–H functionalization reactions etc. in a straightforward manner. Beyond the electrochemical oxidation reactions, electrochemical reductive transformations are also enriching in the last few years. Electrochemical reduction can be facilitated by using different strategies like using a sacrificial electrode, sacrificial reagent or can be accomplished in a divided cell. In this short review, different methods for the functionalization of C−halogen bonds, including detailed mechanistic approaches, are discussed. 1 Introduction 2 Different strategies for the electrochemical reduction 3 Functionalization of C−halogen bonds through electrochemical reduction 3.1 Electro-reductive hydrodehalogenation 3.2 Electro-reductive C−C coupling of organohalides 3.2.1 Aryl-aryl C−C coupling 3.2.2 Aryl-alkenyl C−C coupling 3.2.3 Aryl-alkyl C−C coupling 3.2.4 Alkyl-alkenyl C−C coupling 3.2.5 Alkyl-alkyl C−C coupling 3.3 Electro-reductive coupling of organohalides with carbonyls (C=O) 3.4 Electro-reductive coupling of organohalides with organoborones 4 Conclusion

Electroreductive Borylation of Unactivated (Hetero)Aryl Chlorides Without Light Using Cumulene Electrocatalysts

“…Heteroaryl halides, including those bearing carbazole (10), benzothiazole (12), and pyrrole (16) groups, react smoothly to afford the corresponding heteroaryl boronates in good yields. Notably, the reaction works well with the biologically active coumarin substrate (5), which has a very negative redox potential (Ep/2 = −2.86 V vs Fc/Fc + ). Using 1.0 equiv.…”

“…However, the radical XEC of (hetero)aryl halides is largely restricted to substrates with weak carbon-halogen bonds (e.g., aryl iodides) or low-magnitude redox potentials (e.g., electron-deficient arenes). [4][5][6][7][8][9] Among (hetero)aryl halides, chloroarenes are desirable substrates due to their broad availability, but they are also the most challenging to reduce due to their very negative redox potentials (<−2 V vs Fc/Fc + ) and high C-Cl bond dissociation energies (>97 kcal/mol) (Figure 1). 10 Although deeply reducing conditions can be accessed using stoichiometric amounts of strong reductants, 8 the functional group tolerance of these methods is generally low.…”

Electrosynthesis of Flavanones via oxa-Michael Addition Using Sacrificial Electrodes

“…Additionally, the electrochemical reduction is facilitated by the transfer of electrons from the cathode to the substrate, promoting oxidation of the sacrificial reductant at the anode, maintaining the charge balance. 55 In conclusion, we have developed an attractive alternative to organic reactions applying sacrificial electrodes in electrosynthesis, offering a new approach to using silvercatalyzed reactions. As a model for this methodology, the respective flavanones were obtained with good to excellent yields using silver electrodes on the anode and graphite on the cathode, in the presence of TBAPF 6 as electrolyte and methanol as solvent.…”

“…Additionally, the electrochemical reduction is facilitated by the transfer of electrons from the cathode to the substrate, promoting oxidation of the sacrificial reductant at the anode, maintaining the charge balance. 55…”

Reductive Hydrodehalogenation of Halogenated Carboxylic Acid Derivatives Using a DMSO/HCOONa·2H2O System

“…Due to these important applications, the development of facile and efficient methods for hydrodehalogenation has attracted considerable attention from organic chemists over the past several decades. Conventional methods for hydrodehalogenation of aryl halides generally involve radical reductive dehalogenation using AIBN as an initiator; transition metal-catalyzed reductions using reducing agents such as H 2 , hydrosilanes, hydrides, and alcohols; electrolysis-promoted dehalogenation of aryl or alkyl halides using a metal electrode to supply an electron or using amines as terminal reductants and hydrogen atom donors; , and visible-light-induced dehalogenation in the presence of organic or inorganic photocatalysts, together with a variety of additives, including strong bases, sodium formate, thiols, disulfides, amides, and amines. , However, these methods suffer from their own limitations, such as the poor selectivity leading to low yields, harsh reaction conditions, the use of an expensive noble metal together with an inert atmosphere and essential ligands, excess amounts of potentially hazardous radical initiators, and special photo/electrochemical reactors. Although the metal-free hydrodehalogenation of aryl halides through a radical chain pathway has also been developed, additives covering strong bases (e.g., t -BuOK and NaH) together with electron donors (e.g., 1,10-phenanthroline) and hydrogen sources (e.g., alcohol) are still required .…”