External Oxidant-Free Regioselective Cross Dehydrogenative Coupling of 2-Arylimidazoheterocycles and Azoles with H2 Evolution via Photoredox Catalysis

Visible-light-mediated C-H amidation of imidazoheterocycles with N-amidopyridiniums

C-3 amidated imidazoheterocycles were synthesized via a visible light-promoted reaction of imidazoheterocycles with N-amidopyridinium salts catalyzed by 4CzIPN under mild conditions. For imidazoheterocycles and N-amidopyridinium salts with various substituents, the reaction proceeded smoothly to give the corresponding products in moderate to good yields. The reaction provides a new strategy for the synthesis of secondary amides with the imidazo[1,2-a]pyridine core.

Photoredox-Cobaloxime Catalysis for Selective Oxidative Dehydrogenative [4+2] Annulation of Imidazo-Fused Heterocycles with Alkenes

A selective oxidative [4+2] annulation of alkenes with imidazo-fused heterocycles has been developed by using the synergistic combination of photoredox and cobaloxime catalysts. It allows facile access to various imidazole-fused polyaromatic scaffolds accompanied by H2 evolution. This protocol features high regioselectivity as well as a broad substrate scope. Detailed mechanistic studies indicate that twice the electron/H transfer processes facilitated by this catalytic system achieve the annulation π-extension of imidazo-fused heterocycles with alkenes.

Metal-free C-3 selective C(sp2)-C(sp3) heteroarylation of anilines with imidazo[1,2-a]pyridine derivatives via cross-dehydrogenative coupling

A general and straightforward method for the regioselective construction of C-3 heteroaryl-containing imidazo[1,2-a]pyridines via cross-dehydrogenative coupling under transition-metal-free conditions has been reported, utilizing N,N-dimethylaniline as the methylenation source and furnishing the C(sp²)-C(sp³) functionalized products in good to excellent yields. Mechanism studies indicate that a radical pathway is responsible for this transformation.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.

One-Step Visible Light Photoredox-Catalyzed Purine C8 Alkoxylation with Alcohol

A cross-dehydrogenation coupling reaction between purines and alcohols, induced by visible light, using an acridinium photocatalyst and air as the sole oxidant, to synthesize a series of C8-alkoxy purine derivatives was developed. This protocol is a green and novel method to construct the C₈-O bond on a purine ring with high step and atom economy.

Recent Developments in the Photochemical Synthesis of Functionalized Imidazopyridines

Imidazopyridines constitute one of the most important scaffolds in medicinal chemistry, as their skeleton could be found in a myriad of biologically active molecules. Although numerous strategies were elaborated for imidazopyridine preparation in the 2010s, novel eco-compatible synthetic approaches have emerged, conscious of climate change concerns. In this framework, photochemical methods have been promoted to conceive this heterocyclic motif over the last decade. This review covers the recently published works on synthesizing highly functionalized imidazopyridines by light induction.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Organophotoredox-Catalyzed C–H Alkylation of Imidazoheterocycles with Malonates: Total Synthesis of Zolpidem

Organophotocatalytic C–H bond functionalization has attracted a lot of attention in the past several years due to the possibility of catalyzing reactions in a metal- and peroxide-free environment. Continuing on these lines, an organophotoredox-catalyzed C–H functionalization of imidazo[1,2-a]pyridines and related heterocycles with bromomalonates under mild conditions is reported, providing excellent yields of the products at room temperature. This is the first report involving malonates as coupling partners leading to the synthesis of a range of functionalized products including total synthesis of zolpidem, a sedative­-hypnotic drug molecule.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Catalyst and additive-free oxidative dual C-H sulfenylation of imidazoheterocycles with elemental sulfur using DMSO as a solvent and an oxidant

Dual C-H sulfenylation was used to obtain 3-vulcanized imidazoheterocycles using odorless elemental sulfur under catalyst- and additive-free conditions. C-H activation of both imidazoheterocycles and arylamines/arenols/indoles was realized by a practical protocol in which DMSO served as both a solvent and an internal oxidant.

Visible light-induced photocatalytic C–H ethoxycarbonylmethylation of imidazoheterocycles with ethyl diazoacetate

A visible-light-promoted regioselective ethoxycarbonylmethylation of imidazoheterocycles has been developed using an α-diazoester via a radical pathway.

Peroxide-mediated site-specific C–H methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones under metal-free conditions

An effective approach to realize the direct methylation of imidazo[1,2-a]pyridines and quinoxalin-2(1H)-ones with peroxides under metal-free conditions is described.

Visible light promoted C–H functionalization of imidazoheterocycles

The present review summarizes the visible light mediated direct C–H functionalizations of pharmacologically active imidazo[1,2-a]pyridines and other related heterocycles.

Electrooxidative and Regioselective C-H Azolation of Phenol and Aniline Derivatives

A general and practical protocol was developed for the regioselective C-H azolation of phenol and aniline derivatives by electrooxidative cross-coupling. The reaction occurs under metal-, oxidant-, and reagent-free conditions, allowing access to a wide variety of synthetically useful heteroarene derivatives. The reaction also tolerates a broad range of functional groups and is amenable to gram-scale synthesis. Finally, a preliminary mechanistic study indicated that a radical-radical-combination pathway might be involved in the coupling reaction.

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.

Oxidative dual C–H selenation of imidazoheterocycles with ethers or alkanes using selenium powder via a radical pathway

A DCP-mediated oxidative dual C–H selenation radical reaction of imidazoheterocycles with ethers or alkanes using selenium powder was developed.

Electrochemical oxidative C–H/N–H cross-coupling for C–N bond formation with hydrogen evolution

We herein report an electrochemical oxidative C–H/N–H cross-coupling reaction in an undivided cell.

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.