Heterocyclization involving benzylic C(sp3)-H functionalization enabled by visible light photoredox catalysis

Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

Asymmetric Photoaerobic Lactonization and Aza-Wacker Cyclization of Alkenes Enabled by Ternary Selenium-Sulfur Multicatalysis

An adaptable, sulfur-accelerated photoaerobic selenium-π-acid ternary catalyst system for the enantioselective allylic redox functionalization of simple, nondirecting alkenes is reported. In contrast to related photoredox catalytic methods, which largely depend on olefinic substrates with heteroatomic directing groups to unfold high degrees of stereoinduction, the current protocol relies on chiral, spirocyclic selenium-π-acids that covalently bind to the alkene moiety. The performance of this ternary catalytic method is demonstrated in the asymmetric, photoaerobic lactonization and cycloamination of enoic acids and unsaturated sulfonamides, respectively, leading to an averaged enantiomeric ratio (er) of 92:8. Notably, this protocol provides for the first time an asymmetric, catalytic entryway to pharmaceutically relevant 3-pyrroline motifs, which was used as a platform to access a 3,4-dihydroxyproline derivative in only seven steps with a 92:8 er.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Selectfluor-Induced Oxidative Amination of N-Heteroaromatics with Purine

An oxidative coupling reaction between purines and aromatic N-heterocycles was developed to synthesize a series of N-heteroaryl purine derivatives using Selectfluor as an oxidant at room temperature. This process uses a commercial oxidant, uses no base, metal, or other additives, is simple to carry out, and has a broad range of substrates.

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

Regioselective C(sp2)-H imidation of arenes by redox neutral visible-light photocatalysis

We report herein a redox neutral visible light-induced regioselective C(sp²)-H imidation of electron-rich arenes and heteroarenes using conceptually designed redox-active 1 as a source of the N-centered imidyl radical. Structurally diverse aromatic imides were obtained in moderate to good yields. This methodology has been successfully employed for the late stage imidation of complex molecules and has also been applied towards the formal total synthesis of the marine natural products carpatamides A, B and D. It has further been shown that the generated imides can easily be converted to the corresponding anilines in situ directly.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

A general nickel-catalyzed highly regioselective hydroarylation of unactivated alkenes enabled by the picolinamide auxiliary

A practical method for regioselective hydroarylation of unactivated γ- or δ-vinyl alkylamines has been reported, enabling facile preparation of highly value-added ε- or ζ-aryl alkylamines. The protocol employs nickel catalysis, shows high functional group tolerance and can be used for modifying bio-related molecules.

A Metal-Free and Redox-Neutral Benzylic C-O Cyclization for Succinct Access of Helical Chromenes

A redox-neutral benzylic C-O cyclization under beneficial transition-metal-free conditions is reported. Key to the success of this process is the utilization of the Tf anion as the leaving group for achieving the redox-neutral transformation. This protocol delivers a series of captivating helical compounds having various functionality in good-to-excellent yields. It is particularly noteworthy that sterically hindered helical compounds are conformationally stable. In addition to simple helical chromenes, the bihelical multiple-ring systems which are potentially useful in material chemistry are also easily attained by employing this method. DFT calculation revealed that quinone intermediate is the key species, among four possible mechanisms, for accomplishing the desired cyclization via an oxa-6π-electrocyclization pathway.

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.

Direct Arylation of Distal and Proximal C(sp3)-H Bonds of t-Amines with Aryl Diazonium Tetrafluoroborates via Photoredox Catalysis

A visible light-mediated arylation protocol for t-amines has been reported through the coupling of γ- and α-amino alkyl radicals with different aryl diazonium salts using Ru(bpy)₃Cl₂·6H₂O as a photocatalyst. Structurally different 9-aryl-9,10-dihydroacridine, 1-aryl tetrahydroisoquinoline, hexahydropyrrolo[2,1-a]isoquinoline, and hexahydro-2H-pyrido[2,1-a]isoquinoline frameworks with different substitution patterns have been synthesized in good yield using this methodology.

Copper-promoted direct amidation of isoindolinone scaffolds by sodium persulfate

Isoindolinones are ubiquitous structural motifs in natural products and pharmaceuticals. Establishing an efficient method for structural modification of isoindolinones could significantly facilitate new drug development. Herein, we describe copper-promoted direct amidation of isoindolinone scaffolds mediated by sodium persulfate. The method exhibits mild reaction conditions and high site-selectivity, and enables the structural modification of the drug indobufen ester with various amides with yields of 49 to 98%. It is also gram-scalable. Additionally, the reaction mechanism appears to involve a radical and a carbocationic pathway.

Easy access to drug building-blocks through benzylic C-H functionalization of phenolic ethers by photoredox catalysis

A visible light-mediated photocatalyzed C-C-bond forming method for the benzylic C-H functionalization of phenolether containing synthetic building blocks based on a radical-cation/deprotonation strategy is reported. This method allows the mild, selective generation of benzyl radicals in phenolic complex molecules and drug-like compounds, providing new entries in synthetic and medicinal chemistry.

Strategic Use of Visible-Light Photoredox Catalysis in Natural Product Synthesis

Recent progress in the development of photocatalytic reactions promoted by visible light is leading to a renaissance in the use of photochemistry in the construction of structurally elaborate organic molecules. Because of the rich functionality found in natural products, studies in natural product total synthesis provide useful insights into functional group compatibility of these new photocatalytic methods as well as their impact on synthetic strategy. In this review, we examine total syntheses published through the end of 2020 that employ a visible-light photoredox catalytic step. To assist someone interested in employing the photocatalytic steps discussed, the review is organized largely by the nature of the bond formed in the photocatalytic step.

Photoredox-Catalyzed Benzylic Esterification via Radical-Polar Crossover

Photoredox-catalyzed C-O bond formation reactions are reported. The decarboxylative esterification reaction allows the conversion of a variety of arylacetic acids into the corresponding benzyl carboxylates. Furthermore, the use of (diacetoxyiodo)benzene allows the conversion of the benzylic C-H bond through hydrogen atom transfer. The reactions were applied to the divergent transformation of pharmaceuticals via decarboxylative or C-H esterification reactions.

Diastereoselective Construction of trans-2-Alkyl-6-aryl-3,6-dihydro-2H-pyrans via Dehydrogenative Cycloetherification Promoted by DDQ

A diastereoselective synthesis of trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans has been described. Dehydrogenative cycloetherification of (E)-(±)-1-aryl-5-hydroxy-1-alkenes promoted by DDQ proceeded cleanly via 6-endo cyclization to afford trans-2-alkyl-6-aryl-3,6-dihydro-2H-pyrans (32 examples) in good yield (up to 89%) and with moderate to excellent diastereoselectivity (up to 99:1). The synthetic utility of the method was illustrated by the second total synthesis of (±)-(2R,6S)-3,4-dehydro-1,7-bis(4-hydroxy phenyl)-4'-de-O-methyl centrolobine and a total synthesis of (±)-centrolobine.

Recent Advances in the Application of Selectfluor as a "Fluorine-free" Functional Reagent in Organic Synthesis

Selectfluor, [1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-[2.2.2]octane bis(tetrafluoroborate)], is not only an important electrophilic fluorinating agent but also a facile and efficient "fluorine-free" functional reagent in other organic reactions. In this Minireview, we will present a brief history of Selectfluor as a transition metal oxidant, fluorine cation and radical initiator in "fluorine-free" functionalizations over the last five years.

Late-stage trifluoromethylthiolation of benzylic C-H bonds

The benzylic positions in drugs are sites that readily react with cytochrome P450 oxidases via single-electron oxidation. New synthetic methodologies to incorporate a fluoroalkyl group at the benzylic site are continually being developed, and in this paper, we report a metal-free and site-selective organophotoredox-catalyzed trifluoromethylthiolation of benzylic C-H bonds for a wide variety of alkyl arenes and heteroarenes. The precise and predictive regioselectivity among various C(sp³)-H bonds originates from an inner-sphere benzylic radical initiation mechanism, and avoids the use of external oxidants or hydrogen atom abstractors. Its practicality stems from the trifluoromethylthiolation of a series of drugs and complex organic molecules, which is overwhelmingly selective for benzyl groups. This operationally simple protocol can provide a general and practical access to structurally diverse benzylic trifluoromethyl sulfides produced from ubiquitous benzylic C-H bonds. Large scale trifluoromethylthiolation can be achieved with continuous flow photoredox technology.

Fluoroalkylation of C-H bonds is a class of reaction highly sought after in medicinal chemistry. Here, the authors report a regioselective organophotoredox-catalyzed, trifluoromethylthiolation of benzylic C-H bonds for a wide variety of alkyl (hetero)arenes, and demonstrate its utility in continuous flow.

Direct Transformation of Nitroalkanes to Nitriles Enabled by Visible-Light Photoredox Catalysis and a Domino Reaction Process

A mild and convenient process for direct transformation of nitroalkanes to the corresponding nitriles was developed using a visible-light photoredox catalysis strategy with household decorative blue LEDs and the additives of Et₃N and DIPIBA (or DIPEA). Application of the process in secondary nitroalkanes bearing a β-alcohol resulted in a domino process of the retro-Henry reaction and the subsequent acetalization, aldol, cyanohydrin, and ring-contraction reactions with stereoselectivities. The photocatalytic reaction was demonstrated by a continuous flow method.