Photocatalytic Dehydrogenative Cross-Coupling of Alkenes with Alcohols or Azoles without External Oxidant

Electrochemical Oxidative Carbon-Atom Difunctionalization: Towards Multisubstituted Imino Sulfide Ethers

Ethers (C-O/S) are ubiquitously found in a wide array of functional molecules and natural products. Nonetheless, the synthesis of imino sulfide ethers, containing an N(sp² )=C(sp² )-O/S fragment, still remains a challenge because of its sensitivity to acid. Developed here in is an unprecedented electrochemical oxidative carbon-atom difunctionalization of isocyanides, providing a series of novel multisubstituted imino sulfide ethers. Under metal-free and external oxidant-free conditions, isocyanides react smoothly with simple and readily available mercaptans and alcohols. Importantly, the procedure exhibited high stereoselectivities, excellent functional-group tolerance, and good efficiency on large-scale synthesis, as well as further derivatization of the products.

Visible light mediated synthesis of 4-aryl-1,2-dihydronaphthalene derivatives via single-electron oxidation or MHAT from methylenecyclopropanes

A new synthetic strategy of a single-electron oxidation and MHAT of methylenecyclopropanes (MCPs) for the rapid construction of 4-aryl-1,2-dihydronaphthalene derivatives by merging photoredox catalysis and cobalt catalysis has been developed.

Construction of polysubstituted spiro[2.3] or [3.3] cyclic frameworks fused with a tosylated pyrrolidine promoted by visible-light-induced photosensitization

A novel visible-light-induced intramolecular [2 + 2] cycloaddition of methylenecyclopropanes (MCPs) for the rapid construction of polysubstituted spiro[2.3] or [3.3] cyclic frameworks fused with a tosylated pyrrolidine.

Controllable chemoselectivity in the reaction of 2H-indazoles with alcohols under visible-light irradiation: synthesis of C3-alkoxylated 2H-indazoles and ortho-alkoxycarbonylated azobenzenes

A high chemoselectivity in the visible-light-induced reaction of 2H-indazoles with alcohols controlled by the reaction atmosphere was achieved, providing C3-alkoxylated 2H-indazoles and ortho-alkoxycarbonylated azobenzenes.

Photoinduced C(sp2)-H/C(sp2)-H Cross-Coupling of Alkenes: Direct Synthesis of 1,3-Dienes

A highly concise route to substituted 1,3-dienes from vinylarenes and ketene dithioacetals under photoinduced cross-coupling reaction is described. The reaction proceeded in a highly regio- and stereoselective manner and showed broad functional group tolerance. More than 35 substituted 1,3-dienes were synthesized with good to excellent yields through the construction of the Csp²-Csp² bond without using noble metal and external oxidants, and natural sunlight could also induce the reaction to afford gram-scale synthesis under ambient conditions.

Practical and stereoselective electrocatalytic 1,2-diamination of alkenes

The 1,2-diamine motif is widely present in natural products, pharmaceutical compounds, and catalysts used in asymmetric synthesis. The simultaneous introduction of two amino groups across an alkene feedstock is an appealing yet challenging approach for the synthesis of 1,2-diamines, primarily due to the inhibitory effect of the diamine products to transition metal catalysts and the difficulty in controlling reaction diastereoselectivity and regioselectivity. Herein we report a scalable electrocatalytic 1,2-diamination reaction that can be used to convert stable, easily available aryl alkenes and sulfamides to 1,2-diamines with excellent diastereoselectivity. Monosubstituted sulfamides react in a regioselective manner to afford 1,2-diamines bearing different substituents on the two amino groups. The combination of an organic redox catalyst and electricity not only obviates the use of any transition metal catalyst and oxidizing reagent, but also ensures broad reaction compatibility with a variety of electronically and sterically diverse substrates.

Methods to prepare 1,2-diamines are desirable due the importance of these compounds as drug scaffolds and organic ligands for metals. Here, the authors report an electrochemical metal-free 1,2- diamination of aryl alkenes with sulfamides to 1,2-diamines with excellent diastereoselectivity.

Formation of Enol Ethers by Radical Decarboxylation of α-Alkoxy β-Phenylthio Acids

Enol ethers are formed by radical decarboxylation of α-alkoxy β-phenylthio acids via the corresponding Barton esters. The phenylthio acids were usually made by the known regioselective reaction of α,β-epoxy acids with PhSH in the presence of InCl₃, followed by O-alkylation of the resulting alcohol. In one case, thiol addition to an α,β-unsaturated ethoxymethyl ester was used.

Fundamentals and applications of photocatalytic CO2 methanation

The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of recent research efforts, “solar methane” can now be produced through the photocatalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane.

While natural gas and fossil fuels power human activities, increasing concerns over fuel reserves and environmental impacts require finding alternative, renewable resources. Here, authors review the fundamental science and progress on solar-powered conversion of carbon dioxide to methane.

Bimetallic Radical Redox-Relay Catalysis for the Isomerization of Epoxides to Allylic Alcohols

Organic radicals are generally short-lived intermediates with exceptionally high reactivity. Strategically, achieving synthetically useful transformations mediated by organic radicals requires both efficient initiation and selective termination events. Here, we report a new catalytic strategy, namely, bimetallic radical redox-relay, in the regio- and stereoselective rearrangement of epoxides to allylic alcohols. This approach exploits the rich redox chemistry of Ti and Co complexes and merges reductive epoxide ring opening (initiation) with hydrogen atom transfer (termination). Critically, upon effecting key bond-forming and -breaking events, Ti and Co catalysts undergo proton transfer/electron transfer with one another to achieve turnover, thus constituting a truly synergistic dual catalytic system.

Copper-catalyzed intramolecular cross dehydrogenative coupling approach to coumestans from 2'-hydroxyl-3-arylcoumarins

A copper-catalyzed intramolecular cross dehydrogenative C-O coupling reaction of 2'-hydroxyl-3-arylcoumarins was developed. This protocol provided a facile and efficient strategy for the construction of natural coumestans and derivatives in moderate to high yields. This transformation exhibited good functional group compatibility and was amenable to substrates with free phenolic hydroxyl groups.

Recent Advances in Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution via Photo-/Electrochemistry

Photo-/electrochemical catalyzed oxidative R¹-H/R²-H cross-coupling with hydrogen evolution has become an increasingly important issue for molecular synthesis. The dream of construction of C-C/C-X bonds from readily available C-H/X-H with release of H₂ can be facilely achieved without external chemical oxidants, providing a greener model for chemical bond formation. Given the great influence of these reactions in organic chemistry, we give a summary of the state of the art in oxidative R¹-H/R²-H cross-coupling with hydrogen evolution via photo/electrochemistry, and we hope this review will stimulate the development of a greener synthetic strategy in the near future.

Visible light-induced direct α C-H functionalization of alcohols

Considering the synthetic value of introducing active alcoholic hydroxyl group, developing C-H functionalization of alcohols is of significance. Herein, we present a photochemical method that under visible light irradiation, selectfluor can effectively promote the oxidative cross-coupling between alcohols and heteroarenes without the external photocatalysis, achieving the selective α sp³ C-H arylation of alcohol, even in the presence of ether. The N-F activation of selectfluor under blue LEDs irradiation is evidenced by electron paramagnetic resonance (EPR) study, which is the key process for the oxidative activation of α sp³ C-H alcohols. The observed reactivity may have significant implications for chemical transformations.

Visible light-mediated organophotocatalyzed C-H bond functionalization reactions

Over the last decade, a variety of methodologies for the direct functionalization of C-H bonds have been developed. Among others, visible light photoredox reactions have recently emerged as one of the most efficient and highly selective processes for the direct introduction of a functionality into organic molecules. Easy reaction setups, as well as mild reaction conditions, make this approach superior to other methodologies applying transition metals or strong oxidants, in terms of both costs and substrate and functional group tolerance. In this review, the recent developments in organophotocatalyzed C-H bond functionalization reactions are presented.

Photocatalytic Hydrogen-Evolving Cross-Coupling of Arenes with Primary Amines

Herein, we described a cooperative catalyst system consisting of an acridinium photoredox catalyst and a cobalt-based proton-reduction catalyst that is effective for the C-H amination of arenes with concomitant generation of hydrogen. This oxidant-free method allows a variety of amines with diverse functional groups to be converted to aromatic amines. Additionally, this protocol can also be extended to hydrolytically unstable benzophenone imines.

Silver-Catalyzed Olefination of Acetals and Ketals with Diazoesters to β-Alkoxyacrylates

The first silver-catalyzed reaction of acetals or ketals with diazoesters leading to trisubstituted or tetrasubstituted β-alkoxyacrylates is now reported. A broad range of acetals and ketals bearing different substituents is compatible with this protocol and thus provides an attractive approach for the synthesis of complex β-alkoxyacrylates. The power of this method was further demonstrated by the successful synthesis of picoxystrobin, which is one of the most popular agricultural fungicides commercialized by Dupont.

Electrochemically Enabled Carbohydroxylation of Alkenes with H2O and Organotrifluoroborates

Unprecedented hydroxy-alkynylation and -alkenylation reactions of arylalkenes have been developed through electrochemically enabled addition of an organotrifluoroborate reagent and H₂O across the double bond of the alkene. The use of electrochemistry to promote these oxidative alkene 1,2-difunctionalization reactions not only obviates the need for transition-metal catalysts and oxidizing reagents but also ensures high regio- and chemoselectivity to afford homopropargylic or homoallylic alcohols. The possibility of extending the electrochemical alkene difunctionalization strategy to other alkene carbo-heterofunctionalization reactions has been demonstrated.

Photocatalytic Activation of Less Reactive Bonds and Their Functionalization via Hydrogen-Evolution Cross-Couplings

Cross-coupling reactions have been established as potential tools for manufacture of complex molecular frameworks of diversified interests by connecting two simple molecules through the formation of a carbon-carbon (C-C) or a carbon-heteroatom (C-X) bond. Conventional cross-couplings are transition metal-catalyzed reactions between electrophiles and nucleophiles. Generally, the electrophilic partner is an aryl or alkenyl halide, the nucleophile is an organometallic reagent, and both are obtained from prefunctionalization of their corresponding hydrocarbons. During the past decade, transition metal-catalyzed dehydrogenative cross-couplings between two carbon-hydrogen (C-H) bonds and between one C-H bond and one heteroatom-hydrogen (X-H) bond, which build a C-C and a C-X linkage respectively, have emerged as an attractive strategy in synthetic chemistry. Such straightforward couplings allow use of less functionalized reagents, thus reducing the number of steps to the target molecule and minimizing waste production. However, such reactions involve the use of stoichiometric amounts of sacrificial oxidants such as peroxides, high-valent metals, and iodine(III) oxidants. This leads to low atom economy and possible generation of toxic wastes. Recently, visible light photocatalytic dehydrogenative cross-coupling reactions have received much attention due to their potential in utilizing sunlight as a source of energy making the process appealing. In this approach, metal complexes, organic dyes, or semiconductor quantum dots that absorb visible light are employed as photocatalysts. Upon irradiation, photocatalyst initiates single electron transfer with substrate(s) to generate a radical cation or radical anion of the substrate, which undergoes the desired reaction of interest. In this case, molecular oxygen is utilized as the oxidant with the formation of hydrogen peroxide as the only byproduct. These aspects make the process much greener than the corresponding transition metal-catalyzed dehydrogenative cross-coupling reactions. Research efforts from our group have led to the development of an environmentally benign strategy to construct a C-C bond from two different C-H bonds and to construct a C-X bond from one C-H bond and one X-H bond by visible light photocatalysis. Our approach, photocatalytic hydrogen-evolution cross-coupling reactions, combines a photocatalyst with a proton reduction cocatalyst to create a dual catalyst system. The former catalyst uses light energy as the driving force for the cross-coupling, while the latter catalyst may capture electrons from the substrates or reaction intermediates to reduce the protons eliminated from the reactive scaffolds (C-H/C-H or C-H/X-H bonds) into molecular hydrogen (H₂). Thus, without use of any sacrificial oxidant and under mild conditions, our dual catalyst system affords cross-coupling products with excellent yields with generation of an equimolar amount of H₂ as the sole byproduct. The photocatalytic hydrogen-evolution cross-coupling is highly step and atom economical and particularly useful for reactions that involve species sensitive to oxidative conditions. This Account highlights the findings from our laboratories on photocatalytic hydrogen-evolution cross-coupling reactions featuring activation and functionalization of C(sp³)-H bonds adjacent to amino groups and to oxygen atoms in ethers, aromatic C(sp²)-H bonds, and several types of X-H bonds. We expect that this strategy for combining photocatalytic activation of C-H and X-H bonds with proton reduction holds significant potential for development of atom economical and environmentally benign transformations.

Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution

Oxidative R¹-H/R²-H cross-coupling with hydrogen evolution serves as one of the most atom-economical methods for constructing new chemical bonds. This reaction strategy avoids substrate prefunctionalization steps in traditional cross-coupling reactions. Besides, hydrogen gas, which is recognized as a source of green energy, is the only byproduct during the reaction process. The major challenge in this reaction strategy is to achieve selective bond formation and hydrogen evolution at the same time. Over the past few years, novel synthetic techniques especially photochemistry and electrochemistry have provided possibilities for oxidative cross-coupling with H₂ liberation. Both C-C and C-X bonds can be constructed without the use of any sacrificial reagents. In this perspective, we will discuss the concept of this reaction strategy and give an overview of its recent development.

Dehydrogenative reagent-free annulation of alkenes with diols for the synthesis of saturated O-heterocycles

Dehydrogenative annulation reactions are among the most straightforward and efficient approach for the preparation of cyclic structures. However, the applications of this strategy for the synthesis of saturated heterocycles have been rare. In addition, reported dehydrogenative bond-forming reactions commonly employ stoichiometric chemical oxidants, the use of which reduces the sustainability of the synthesis and brings safety and environmental issues. Herein, we report an organocatalyzed electrochemical dehydrogenative annulation reaction of alkenes with 1,2- and 1,3-diols for the synthesis of 1,4-dioxane and 1,4-dioxepane derivatives. The combination of electrochemistry and redox catalysis using an organic catalyst allows the electrosynthesis to proceed under transition metal- and oxidizing reagent-free conditions. In addition, the electrolytic method has a broad substrate scope and is compatible with many common functional groups, providing an efficient and straightforward access to functionalized 1,4-dioxane and 1,4-dioxepane products with diverse substitution patterns.

Dehydrogenative annulation is a valuable approach to heterocycles, however, stoichiometric oxidants are often required. Here, the authors describe the electrochemical dehydrogenative annulation of diols and alkenes to generate dioxanes and dioxepanes under metal- and oxidant-free conditions.