Direct Access to Nitrogen Bi-heteroarenes via Iridium-Catalyzed Hydrogen-Evolution Cross-Coupling Reaction

5-Aminopyrazole Dimerization: Cu-Promoted Switchable Synthesis of Pyrazole-Fused Pyridazines and Pyrazines via Direct Coupling of C-H/N-H, C-H/C-H, and N-H/N-H Bonds

A Cu-promoted highly chemoselective dimerization of 5-aminopyrazoles to produce pyrazole-fused pyridazines and pyrazines is reported. The protocol generates switchable products via the direct coupling of C-H/N-H, C-H/C-H and N-H/N-H bonds, with the merits of broad substrate scope and high functional group compatibility. Gram-scale experiments demonstrated the potential applications of this reaction. Moreover, the preliminary fluorescence results uncovered that dipyrazole-fused pyridazines and pyrazines may have some potential applications in materials chemistry.

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.

Mn-catalyzed electrooxidative radical phosphorylation of 2-isocyanobiaryls

As an efficient and green synthesis method, the electrocatalysis hydrogen evolution coupling reaction has been widely used by chemists to realize the combining of two nucleophiles. In this work, an alternative method to synthesize 6-phosphorylated phenanthridines has been developed by synergistically utilizing electrocatalysis and Mn catalysis, with moderate to relatively good yields achieved. Mild and oxidant-free conditions make this synthetic method applicable in various settings.

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Rhodium-catalysed direct formylmethylation using vinylene carbonate and sequential dehydrogenative esterification

A rhodium-catalysed direct formylmethylation adopting vinylene carbonate as an ethynol equivalent is reported. The developed catalytic system is further utilised for the oxidant-free production of esters with the liberation of hydrogen gas. Some control experiments are conducted to elucidate the reaction mechanism.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Ruthenium-Catalyzed Hydrogen Evolution o-Aminoalkylation of Phenols with Cyclic Amines

Herein, we present a ruthenium-catalyzed new hydrogen evolution ortho-aminoalkylation of phenolic derivatives with cyclic amines as the coupling agents. The developed cross-coupling reaction offers a practical platform for direct access to a variety of functionalized phenols with the features of good substrate and functional group compatibility, readily available catalyst system and feedstocks, no need for additional sacrificial oxidants, and high atom efficiency.

Hydrogen Transfer-Mediated Multicomponent Reaction for Direct Synthesis of Quinazolines by a Naphthyridine-Based Iridium Catalyst

Selective linkage of renewable alcohols and ammonia into functional products would not only eliminate the prepreparation steps to generate active amino agents but also help in the conservation of our finite fossil carbon resources and contribute to the reduction of CO2 emission. Herein the development of a novel 2-(4-methoxyphenyl)-1,8-naphthyridine-based iridium (III) complex is reported, which exhibits excellent catalytic performance toward a new hydrogen transfer-mediated annulation reaction of 2-nitrobenzylic alcohols with alcohols and ammonia. The catalytic transformation proceeds with the striking features of good substrate and functional group compatibility, high step and atom efficiency, no need for additional reductants, and liberation of H2O as the sole by-product, which endows a new platform for direct access to valuable quinazolines. Mechanistic investigations suggest that the non-coordinated N-atom in the ligand serves as a side arm to significantly promote the condensation process by hydrogen bonding.

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.

Copper-catalysed dehydrogenative α-C(sp3)–H amination of tetrahydroquinolines with O-benzoyl hydroxylamines

A copper-catalysed dehydrogenative α-C(sp³)–H amination of tetrahydroquinolines with O-benzoyl hydroxylamines has been demonstrated, which enables direct access to 2-alkylaminoquinolines in efficient manner.

Transition-metal-catalyst-free synthesis of anthranilic acid derivatives by transfer hydrogenative coupling of 2-nitroaryl methanols with alcohols/amines

A transition metal catalyst-free synthesis of anthranilic acid derivatives, featuring operational simplicity, broad substrate scope, good functional tolerance, high step efficiency, is presented.

Selective synthesis of nitrogen bi-heteroarenes by a hydrogen transfer-mediated direct α,β-coupling reaction

By an external hydrogen transfer-mediated activation mode, we herein demonstrate a new palladium-catalyzed direct α,β-coupling of different types of N-heteroarenes. Such a selective coupling reaction proceeds with the advantages of operational simplicity, high atom-economical efficiency, and use of safe and abundant i-propanol as the activating agent, offering a practical way to access nitrogen bi-heteroarenes. Preliminary exploration has revealed that the obtained bis-1,10-phenanthroline 2qq' as a ligand is capable of improving a copper catalyst for C-C bond formation. The work reported in this paper has built an important basis for the creation of extended π-conjugated systems that are of high significance in biological, medicinal, materials and synthetic organic chemistry as well as catalysis.