Photocatalytic Oxidative Iodination of Electron‐Rich Arenes

PIDA as an Iodinating Reagent: Visible-Light-Induced Iodination of Pyrazolo[1,5-a]pyrimidines and Other Heteroarenes

We have developed a visible-light-mediated convenient and efficient strategy for the iodination of heteroarenes using diacetoxyiodobenzene (PIDA) under photocatalyst-free conditions. This unique approach is the first report on photocatalytic C-H iodination employing PIDA as the iodinating agent. The new photocatalyst-free strategy is applicable to a wide range of pyrazolo[1,5-a]pyrimidine derivatives with various functionalities. Iodination of other electron-rich heterocycles like imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine, imidazo[2,1-b]thiazole, benzo[d]imidazo[2,1-b]thiazole, and pyrazoles has been accomplished employing this benign protocol. The usefulness of 3-iodo pyrazolo[1,5-a]pyrimidine as a synthetic intermediate in synthesizing various functionalized pyrazolo[1,5-a]pyrimidines has been demonstrated.

Benzoimidazolyl Organoseleniums: Antioxidant Activity and Catalysts for Selective Iodination of Arenes and Nitro-Michael Reaction

Here, the synthesis and catalytic activities of benzoimidazole-derived organoselenium compounds have been explored. The synthesized bis(2-benzoimidazolyl) diselenide, having increased Lewis acidity on the selenium center, outperforms simple phenyl and N-phenyl benzamide-based diselenides when compared for thiol peroxidase hydrogen peroxide decomposing antioxidant activity with a reduction rate of 18.6 ± 1.9 μM/s. The synthesized diselenide also acted as an efficient catalyst for the activation of N-iodo-succinimide toward the regioselective, monoiodination of electron-rich arenes and activation of nitro-alkene for nitro-Michael reactions for the first time.

Efficient Synthesis and Iodine-Functionalization of Pyrazoles via KIO3/PhSeSePh System under Acidic Conditions

This manuscript reports a novel method for the direct iodination of the C-4 pyrazole ring generated in situ by the reaction of 1,1,3,3-tetramethoxypropane and various hydrazines. In this approach, potassium iodate (KIO3) is used as the iodinating agent and (PhSe)2 is used as catalysts under acidic conditions. This protocol provides a convenient method for the efficient synthesis of 4-iodo-1-aryl-1H-pyrazoles, valuable intermediates for several different coupling reactions.

Visible light-mediated halogenation of organic compounds

The use of visible light and photoredox catalysis emerged as a powerful and sustainable tool for organic synthesis, showing high value for distinctly different ways of bond creation. Halogenated compounds are the cornerstone of contemporary organic synthesis: it is almost impossible to develop a route towards a pharmaceutical reagent, agrochemical, natural product, etc. without the involvement of halogen-containing intermediates. Moreover, the halogenated derivatives as final products became indispensable for drug discovery and materials science. The idea of this review is to understand and summarise the impact of visible light-promoted chemistry on halogenation and halofunctionalisation reactions.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

A Green Alternative for the Direct Aerobic Iodination of Arenes Using Molecular Iodine and a POM@MOF Catalyst

Iodoarenes are important precursors for fine chemicals and pharmaceuticals. The direct iodination of arenes using molecular iodine (I₂) has emerged as an attractive green synthesis method. Most of the direct iodination protocols are still homogeneous systems that require harsh conditions and use or produce toxic products. We report a new heterogeneous catalytic route for the direct aerobic iodination of arenes under mild conditions using a PMoV₂ polyoxometalate (POM) embedded in the metal-organic framework (MOF) MIL-101 (PMoV₂@MIL-101). The catalyst shows full yield for the conversion of mesitylene to 2-iodomesitylene at a rate that is similar to the homogeneous POM system. Moreover, the catalyst is applicable for a wide range of substrates in an oxygen atmosphere without using any co-catalysts or sacrificial agents. To the best of our knowledge, this is the first report on designing a sustainable and green MOF-based heterogeneous catalytic system for the direct iodination reaction using molecular oxygen and iodine.

Modular Generation of (Iodinated) Polyarenes Using Triethylgermane as Orthogonal Masking Group

While the modular construction of molecules from suitable building blocks is a powerful means to more rapidly generate a diversity of molecules than through customized syntheses, the further evolution of the underlying coupling methodology is key to realize widespread applications. We herein disclose a complementary modular coupling approach to the widely employed Suzuki coupling strategy of boron containing precursors, which relies on organogermane containing building blocks as key orthogonal functionality and an electrophilic (rather than nucleophilic) unmasking event paired with air-stable PdI dimer based bond construction. This allows to significantly shorten the reaction times for the iterative coupling steps and/or to close gaps in the accessible compound space, enabling straightforward access also to iodinated compounds.

Transition-metal-free synthesis of trifluoromethylated benzoxazines via a visible-light-promoted tandem difunctionalization of o-vinylanilides with trifluoromethylsulfinate

A Umemoto's reagent-free and cost-effective method for synthesis of trifluoromethylated benzoxazines by 9,10-phenanthrenedione visible-light photocatalysis is described in this article.

Recent Progress in Organophotoredox Reaction

In the past decade, visible light photoredox catalysis has been established as a gentle and powerful strategy for the activation of organic molecules. As an important part of it, organic...

Visible-Light-Induced Catalytic Selective Halogenation with Photocatalyst

Halide moieties are essential structures of compounds in organic chemistry due to their popularity and wide applications in many fields such as natural compounds, agrochemicals, and pharmaceuticals. Thus, many methods have been developed to introduce halides into various organic molecules. Recently, visible-light-driven reactions have emerged as useful methods of organic synthesis. Particularly, halogenation strategies using visible light have significantly improved the reaction efficiency and reduced toxicity, as well as promoted reactions under mild conditions. In this review, we have summarized recent studies in visible-light-mediated halogenation (chlorination, bromination, and iodination) with photocatalysts.

A Metal-Free, Photocatalytic Method for Aerobic Alkane Iodination

Halogenation is an important alkane functionalization strategy, but O₂ is widely considered the most desirable terminal oxidant. Here, the aerobic iodination of alkanes, including methane, was performed using catalytic [ⁿBu₄N]Cl and light irradiation (390 nm). Up to 10 turnovers of CH₃I were obtained from CH₄ and air, using a stop-flow microtubing system. Mechanistic studies using cyclohexane as the substrate revealed important details about the iodination reaction. Iodine (I₂) serves multiple roles in the catalysis: (1) as the alkyl radical trap, (2) as a precursor for the light absorber, and (3) as a mediator of aerobic oxidation. The alkane activation is attributed to Cl^• derived from photofragmentation of the electron donor-acceptor complex of I₂ and Cl^-. The kinetic profile of cyclohexane iodination showed that aerobic oxidation of I₃^- to produce I₂ in CH₃CN is turnover-limiting.

A practical route to 2-iodoanilines via the transition-metal-free and base-free decarboxylative iodination of anthranilic acids under oxygen

A new approach for synthesizing 2-iodoanilines via the transition-metal-free and base-free decarboxylative ortho-C–H iodination of anthranilic acids with a combination of KI and I₂ as the halogen donor and catalyst under oxygen has been developed.

Tracking down the brominated single electron oxidants in recent organic red-ox transformations: photolysis and photocatalysis

A wide range of organic and inorganic brominated compounds including molecular bromine have been extensively used as oxidants in many organic photo-redox transformations in recent years, an area of ever growing interest because of greener and milder approaches. The oxidation power of these compounds is utilized through both mechanistic pathways (by hydrogen atom transfer or HAT in the absence of a photocatalyst and a combination of single electron transfer or SET and/or HAT in the presence of a photocatalyst). Not only as terminal oxidants for regeneration of photocatalysts, but brominated reactants have also contributed to the oxidation of the reaction intermediate(s) to carry on the radical chain process in several reactions. Here in this review mainly the non-brominative oxidative product formations are discussed, carried out since the last two decades, skipping the instances where they acted as terminal oxidants only to regenerate photocatalysts. The reactions are used to generate natural products, pharmaceuticals and beyond.

2-Bromoanthraquinone as a highly efficient photocatalyst for the oxidation of sec-aromatic alcohols: experimental and DFT study

Anthraquinones are recognized as high efficiency photocatalysts which can perform various redox reactions in aqueous or organic phases. We have experimentally proven that 2-BrAQ can undergo hydrogen transfer with an alpha-aromatic alcohol under light conditions, thereby efficiently oxidizing the aromatic alcohol to the corresponding product. The yield of 1-phenethanol to acetophenone can reach more than 96%. In subsequent catalyst screening experiments, it was found that the electronegativity of the substituent at the 2 position of the anthraquinone ring and the acidity of the solvent affect the photocatalytic activity of anthraquinones. After using various aromatic alcohol substrates, 2-BrAQ showed good conversion and selectivity for most aromatic alcohols, but showed C-C bond cleavage and low selectivity with non-α-position aromatic alcohols. In order to explore the mechanism of the redox reaction of 2-BrAQ in acetonitrile solution, the corresponding free radical reaction pathway was proposed and verified by density functional theory (DFT). Focusing on calculations for 2-BrAQ during the reaction and the first-step hydrogen transfer reaction between the 2-BrAQ triplet molecule and the 1-phenylethanol molecule, we recognized the changes that occurred during the reaction and thus have a deeper understanding of the redox reaction of anthraquinone compounds in organic systems.