HCl/DMSO/HFIP-Mediated Chlorination of Pyrrolo[2,1-a]isoquinolines and Other Electron-Rich Heteroarenes

An efficient oxidative chlorination of pyrrolo[2,1-a]isoquinolines has been established using HCl (aq) as the chlorine source and DMSO as the terminal oxidant in HFIP at ambient temperature. A variety of chlorinated pyrrolo[2,1-a]isoquinoline derivatives have been prepared readily in 23 to 99% yields. This chlorination strategy can be expanded to the functionalization of other electron-rich heteroarenes including substituted pyrroles, indoles, and naphthols.

Recent advances in oxidative chlorination

Considering the wide occurrence and extensive application of organic chlorides in many research fields, the development of easy, practical and green chlorination methodologies is much needed. In the oxidative chlorination strategy, active chlorinating species can be in situ formed by the interaction of easily accessible chlorides such as NaCl, HCl, KCl, CHCl3, etc. and suitable oxidants. Among the established chlorination approaches, this strategy is an attractive one as it features the use of readily available, cheap and safe inorganic or organic chlorides, good atom economy of chlorine, and multiple choices of oxidants. This review summarizes the representative methodologies in the field of oxidative chlorination, covering 2013 to 2023.

Parallel Paired Photoelectrochemical Bromination of Alkylarenes with Electrochemical Pinacol Coupling

A paired electrochemical method for paralleling benzylic bromination of alkylarenes under irradiation with reductive pinacol coupling in a divided cell has been developed. A variety of benzyl bromides at the anode and pinacols at the cathode were obtained simultaneously in moderate-to-high faradaic efficiency. This parallel paired electrochemical protocol showed a broad substrate scope and high chemoselectivity as well as high synthetic and faradaic efficiencies.

Selective Mono- and Diamination of Ketones in a Combined Copper-Organocatalyst System

Herein, we report a simple and mild protocol for the chemoselective mono- and diamination of ketone using pyrazole as the amine source in a combined copper-organocatalyst system. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy gives an efficient and convenient solution for the synthesis of α-pyrazole and α,α-dipyrazole ketone derivatives. The control experiment demonstrates that in situ generated hydrazone is a key intermediate in the transformation.

Electrochemical oxidative Z-selective C(sp2)-H chlorination of acrylamides

An electrochemical method for the oxidative Z-selective C(sp²)-H chlorination of acrylamides has been developed. This catalyst and organic oxidant free method is applicable across various substituted tertiary acrylamides, and provides access to a broad range of synthetically useful Z-β-chloroacrylamides in good yields (22 examples, 73% average yield). The orthogonal derivatization of the products was demonstrated through chemoselective transformations and the electrochemical process was performed on gram scale in flow.

Electrohalogenation of organic compounds

In this review we target sp, sp² and sp³ carbon fluorination, chlorination, bromination and iodination reactions using electrolysis as a redox medium. Mechanistic insights and substrate reactivity are also discussed.

3d metallaelectrocatalysis for resource economical syntheses

Resource economy constitutes one of the key challenges for researchers and practitioners in academia and industries, in terms of rising demand for sustainable and green synthetic methodology. To achieve ideal levels of resource economy in molecular syntheses, novel avenues are required, which include, but are not limited to the use of naturally abundant, renewable feedstocks, solvents, metal catalysts, energy, and redox reagents. In this context, electrosyntheses create the unique possibility to replace stoichiometric amounts of oxidizing or reducing reagents as well as electron transfer events by electric current. Particularly, the merger of Earth-abundant 3d metal catalysis and electrooxidation has recently been recognized as an increasingly viable strategy to forge challenging C-C and C-heteroatom bonds for complex organic molecules in a sustainable fashion under mild reaction conditions. In this review, we highlight the key developments in 3d metallaelectrocatalysis in the context of resource economy in molecular syntheses until February 2020.

Catalyzing Electrosynthesis: A Homogeneous Electrocatalytic Approach to Reaction Discovery

Electrochemistry has been used as a tool to drive chemical reactions for over two centuries. With the help of an electrode and a power source, chemists are bestowed with an imaginary reagent whose potential can be precisely dialed in. The theoretically infinite redox range renders electrochemistry capable of oxidizing or reducing some of the most tenacious compounds (e.g., F- to F2 and Li+ to Li0). Meanwhile, a granular level of control over the electrode potential allows for the chemoselective differentiation of functional groups with minute differences in potential. These features make electrochemistry an attractive technique for the discovery of new modes of reactivity and transformations that are not readily accessible with chemical reagents alone. Furthermore, the use of an electrical current in place of chemical redox agents improves the cost-efficiency of chemical processes and reduces byproduct generation. Therefore, electrochemistry represents an attractive approach to meet the prevailing trends in organic synthesis and has seen increasingly broad use in the synthetic community over the past several years.While electrochemical oxidation or reduction can provide access to reactive intermediates, redox-active molecular catalysts (i.e., electrocatalysts) can also enable the generation of these intermediates at reduced potentials with improved chemoselectivity. Moreover, electrocatalysts can impart control over the chemo-, regio-, and stereoselectivities of the chemical processes that take place after electron transfer at electrode surfaces. Thus, electrocatalysis has the potential to significantly broaden the scope of organic electrochemistry and enable a wide range of new transformations. Our initial foray into electrocatalytic synthesis led to the development of two generations of alkene diazidation reactions, using transition-metal and organic catalysis, respectively. In these reactions, the electrocatalysts play two critical roles; they promote the single-electron oxidation of N3- at a reduced potential and complex with the resultant transient N3• to form persistent reactive intermediates. The catalysts facilitate the sequential addition of 2 equiv of azide across the alkene substrates, leading to a diverse array of synthetically useful vicinally diaminated products.We further applied this electrocatalytic radical mechanism to the heterodifunctionalization of alkenes. Anodically coupled electrolysis enables the simultaneous anodic generation of two distinct radical intermediates, and the appropriate choice of catalyst allowed the subsequent alkene addition to occur in a chemo- and regioselective fashion. Using this strategy, a variety of difunctionalization reactions, including halotrifluoromethylation, haloalkylation, and azidophosphinoylation, were successfully developed. Importantly, we also demonstrated enantioselective electrocatalysis in the context of Cu-promoted cyanofunctionalization reactions by employing a chiral bisoxazoline ligand. Finally, by introducing a second electrocatalyst that mediates oxidatively induced hydrogen atom transfer, we expanded scope of electrocatalysis to hydrofunctionalization reactions, achieving hydrocyanation of conjugated alkenes in high enantioselectivity. These developments showcase the generality of our electrocatalytic strategy in the context of alkene functionalization reactions. We anticipate that electrocatalysis will play an increasingly important role in the ongoing renaissance of synthetic organic electrochemistry.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Cobalt-catalyzed electrooxidative C-H/N-H [4+2] annulation with ethylene or ethyne

Ethylene and ethyne are among the simplest two-carbon building blocks. However, quite limited methods can be applied to incorporate ethylene or ethyne into fine chemicals. Here we demonstrate a cobalt-catalyzed dehydrogenative C–H/N–H [4+2] annulation of aryl/vinyl amides with ethylene or ethyne by using an electrochemical reaction protocol. Significantly, this work shows an example of electrochemical recycling of cobalt catalyst in oxidative C–H functionalization reactions, avoiding the use of external chemical oxidants and co-oxidants. The electrochemical method provides a reliable and safe way for incorporating gas-phase ethylene or ethyne into fine chemicals. High reaction efficiency and good functional group tolerance are observed under divided electrolytic conditions.

Catalytic methodologies that incorporate commodity chemicals ethylene and ethyne into fine chemicals are quite limited. Here, the authors show a cobalt-catalyzed electrochemical C-H/N–H [4+2] annulation of aryl/vinyl amides with ethylene or ethyne to produce N-heterocycles in absence of oxidants.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Heteromultimetallic catalysis for sustainable organic syntheses

Fully complementary bimetallic catalysis has been identified as an increasingly powerful tool for molecular transformations, which was largely inspired by early examples of sequential catalytic transformations.

C-H bond halogenation catalyzed or mediated by copper: an overview

Carbon-halogen (C-X) bonds are amongst the most fundamental groups in organic synthesis, they are frequently and widely employed in the synthesis of numerous organic products. The generation of a C-X bond, therefore, constitutes an issue of universal interest. Herein, the research advances on the copper-catalyzed and mediated C-X (X = F, Cl, Br, I) bond formation via direct C-H bond transformation is reviewed.