Photocatalyst-Free Transformation of C(sp3)-H Bonds to Oxime Ethers via Photoinduced Hydrogen Atom Transfer

Herein, a direct transformation of aliphatic C-H bonds to oxime ethers has been developed via light-promoted hydrogen atom transfer (HAT) in the absence of a photocatalyst. Singlet oxygen and chlorine radical are complementary C(sp3)-H bond cleaving agents in this reaction, enabling the extraction of hydrogen atoms from a diverse range of compounds, like cycloalkanes, ethers, amines, amides, and cyclic sulfides. This method excels in transforming common aliphatic C-H bonds into valuable oxime ethers featuring abundant chemical feedstocks, good functional group tolerance, and catalyst free conditions.

C-H Labeling with [18F]Fluoride: An Emerging Methodology in Radiochemistry

Fluorine-18 is the most routinely employed radioisotope for positron emission tomography, a dynamic nuclear imaging modality. The radiolabeling of C-H bonds is an attractive method for installing fluorine-18 into organic molecules since it can preclude the cumbersome prefunctionalization of requisite precursors. Although electrophilic "F+" reagents (e.g., [18F]F2) are effective for C-H radiolabeling, state-of-the-art methodologies predominantly leverage high molar activity nucleophilic [18F]fluoride sources (e.g., [18F]KF) with substantial (pre)clinical advantages. Reflecting this, multiple nucleophilic C-H radiolabeling techniques of high utility have been disclosed over the past decade. However, the adoption of (pre)clinical C-H radiolabeling has been slow, and PET imaging agents are still routinely prepared via methods that, despite a high level of practicality, are limited in scope (e.g., SNAr, SN2 radiofluorinations). By addressing the drawbacks inherent to these strategies, C-H radiofluorination and radiofluoroalkylation carry the potential to complement and supersede state-of-the-art labeling methods, facilitating the expedited production of PET agents used in disease staging and drug development. In this Outlook, we showcase recent C-H labeling developments with fluorine-18 and discuss the merits, potential, and barriers to adoption in (pre)clinical settings. In addition, we highlight trends, challenges, and directions in this emerging field of study.

Metal-free Borylation of α-Naphthamides and Phenylacetic Acid Drug

Site-selective C-H borylation is an important strategy for constructing molecular diversity in arenes and heteroarenes. Although transition-metal-catalyzed borylation is well explored, developing metal-free strategies remains scarce. Herein, we developed a straightforward approach for BBr3-mediated selective C-H borylation of naphthamide and phenyl acetamide derivatives under metal-free conditions. This methodology appears to be economical and cost-effective. Successful borylation of drug molecules such as ibuprofen and indoprofen demonstrates the versatility and utility of this metal-free borylation. An exclusive monoselectivity was observed without a trace of diboration. Despite the possibility of forming a 5-membered boronated intermediate at the ortho-position, the selectively 6-membered intermediate paved the way for the formation of the peri-product, which was further supported by detailed computational investigation.

Catalyst Design for Rh-Catalyzed Arene and Alkane C-H Borylation: The NHC Affects the Induction Period, and Indenyl is Superior to Cp

In order to establish design criteria for Rh C-H borylation catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)] (Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene, and COE = cis-cyclooctene) were synthesized by changing the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C-C activated, cyclometalated byproduct [RhCp(κ2CAr,Ccarbene-SIDipp')(iPr)] (rac-2; SIDipp' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). Computational modeling of COE dissociation showed that both C-C and C-H activation of the SIDipp aryl group is thermally attainable and reversible under experimental conditions, with the C-C activation products being the more thermodynamically stable species. Oxidative addition of 1 with SiH(OEt)3 gave the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm-1, cf. 1944 cm-1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)] (6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) were also characterized, but attempts to synthesize Rh carbene complexes with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For the catalytic C-H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all other complexes tested due to the shortest induction period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane C-H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.

Chlorobenzene-driven palladium-catalysed lactonisation of benzoic acids

Herein, we developed a palladium-catalysed C-H cyclisation of benzoic acids in chlorobenzene without additional oxidants. The key to the success of these reactions is the use of chlorobenzene, which serves a dual role as a solvent and an oxidant, thus providing a simple and efficient method for synthesising phthalides.

Recent advances in metal directed C-H amidation/amination using sulfonyl azides and phosphoryl azides

Transition metal-catalyzed C-N bond formation reactions have gained popularity as a method for selectively transforming common C-H bonds into N-functionalized molecules. This approach is particularly useful for synthesizing aminated molecules, which require aminating reagents and amidated building blocks. Over the past two decades, significant advancements have been achieved in transition-metal-catalyzed C-H functionalization, with organic azides emerging as promising amino sources and internal oxidants. This review focuses on recent developments in utilizing sulfonyl and phosphoryl azides as building blocks for directed intra- and intermolecular C-H functionalization reactions. Specifically, it discusses methods for synthesizing sulfonamidates and phosphoramidates using sulfonyl and phosphoryl azides, respectively. The article highlights the potential of C-H functionalization reactions with organic azides for efficiently and sustainably synthesizing N-functionalized molecules, providing valuable insights into the latest advancements in this field.

C-H Bond Functionalization of N-Heteroarenes Mediated by Selectfluor

Herein, recent developments for Selectfluor-mediated C-H functionalization of N-heteroarenes are described. This type of C-H bond activation is an attractive and competitive alternative to traditional methodologies, allowing the functionalization of a variety of chemical functions. In addition, Selectfluor is a more sustainable and economically accessible oxidant compared with expensive/toxic metals or hazardous peroxides. For a practical understanding, the current review classified systematically the reported strategies in four subsections as follows: (1) carbon-carbon formation, (2) carbon-nitrogen bond formation, (3) carbon-chalcogen bond, and (4) carbon-halogen bond formation. Mechanistic aspects and reaction conditions are fully discussed to provide an understanding of the aspects that govern C-H functionalization in N-heteroarenes mediated by Selectfluor.

Strategies Toward the Catalyst-Free α-C-H Functionalizations of Tertiary Amines

α-C-H functionalization of tertiary amines has been a highly studied field for the past two decades because several important nitrogen containing heterocycles or compounds can be synthesized through this strategy. Though transition metal catalysts and some metal-free catalysts are mainly used for these reactions, a few catalyst-free reactions have recently been efficiently performed. Catalyst-free reactions are cost-effective, less sensitive to air/moisture, easier to operate, have a simple purification process, and are relatively environment-friendly. In this article, we have summarized all the α-C-H functionalization reactions of tertiary amines performed without using any external catalysts. The content of this article will undoubtedly encourage readers to do more work in this area.

Time-resolved EPR revealed C(sp3)–H activation through a photo-enhanced phthalimide-N-oxyl (PINO) radical

The time-resolved EPR technique under operando photochemical conditions as an efficient strategy to investigate the fast formation of abundant long-lived PINO radicals and their activation of the C(sp3)–H reaction is reported.

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal- or enzyme-based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition-metal-based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition-metal-free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

Oxidase‐Type C−H/C−H Coupling Using an Isoquinoline‐Derived Organic Photocatalyst

Oxidase‐type oxidation is an attractive strategy in organic synthesis due to the use of O₂ as the terminal oxidant. Organic photocatalysis can effect metal‐free oxidase chemistry. Nevertheless, current methods are limited in reaction scope, possibly due to the lack of suitable photocatalysts. Here we report an isoquinoline‐derived diaryl ketone‐type photocatalyst, which has much enhanced absorption of blue and visible light compared to conventional diaryl ketones. This photocatalyst enables dehydrogenative cross‐coupling of heteroarenes with unactivated and activated alkanes as well as aldehydes using air as the oxidant. A wide range of heterocycles with various functional groups are suitable substrates. Transient absorption and excited‐state quenching experiments point to an unconventional mechanism that involves an excited state “self‐quenching” process to generate the N‐radical cation form of the sensitizer, which subsequently abstracts a hydrogen atom from the alkane substrate to yield a reactive alkyl radical.

C−H Methylation Using Sustainable Approaches

C−H methylation of sp2 and sp3 carbon centers is significant in many biological processes. Methylated drug candidates show unique properties due to the change in solubility, conformation and metabolic activities. Several photo-catalyzed, electrochemical, mechanochemical and metal-free techniques that are widely utilized strategies in medicinal chemistry for methylation of arenes and heteroarenes have been covered in this review.

A Unified Mechanism for the PhCOCOOH-mediated Photochemical Reactions: Revisiting its Action and Comparison to Known Photoinitiators

Since 2014, we have introduced in literature the use of phenylglyoxylic acid (PhCOCOOH), a small and commercially available organic molecule, as a potent promoter in a variety of photochemical processes. Although PhCOCOOH has a broad scope of photochemical reactions that can promote, the understanding of its mode of action in our early contributions was moderate. Herein, we are restudying and revisiting the mechanism of action of PhCOCOOH in most of these early contributions, providing a unified mechanism of action. Furthermore, the understanding of its action as a photoinitiator opened a new comparison study with known and commercially available photoinitiators.

Copper-catalyzed aerobic benzylic C(sp3)-H lactonization of 2-alkylbenzamides via N-centered radicals

Herein, we describe the copper-catalyzed aerobic C(sp³)-H functionalization of 2-alkylbenzamides for the synthesis of benzolactones. This reaction proceeds via 1,5-hydrogen atom transfer of N-centered radicals directly generated by N-H bond cleavage and does not require the synthesis of pre-functionalized N-centered radical precursors or the use of strong stoichiometric oxidants.

Stress degradation study on entrectinib and characterization of its degradation products using HRMS and NMR

Entrectinib is a potent inhibitor of receptor tyrosine kinases and anaplastic lymphoma kinase. It is designated as an orphan drug. There exists no report of comprehensive degradation profiling of the drug in the literature. Therefore, the present study focused on establishment of its stress degradation chemistry under hydrolytic (acidic, alkaline, neutral), oxidative (H₂O₂), photolytic and thermal conditions. For the purpose, the stressed solutions were subjected to HPLC studies on a C8 column by employing a gradient elution method, in which acetonitrile and 10 mM ammonium acetate were used as the mobile phase components. The results showed that entrectinib was labile to alkaline, H₂O_2, and photoneutral conditions in the solution state. The drug proved to be stable under acidic, solid-state photolytic, and thermal conditions. A total of sixteen degradation products were formed, which were characterized with the help of high resolution mass spectrometry, and in one case additional help was taken of 1D and -2D NMR data. The knowledge of the structures of the degradation products helped in establishment of degradation pathway of the drug and the involved mechanisms. Also, the toxicity profile of the drug and its degradation products was predicted using ADMET Predictor™ software, which indicated mutagenic potential of atleast five degradation products.

Functionalisation of ethereal-based saturated heterocycles with concomitant aerobic C–H activation and C–C bond formation

Herein we disclose a novel method for the aerobic C–H activation of ethereal-based heterocycles to generate various α-functionalised building blocks.

Metal-free visible-light-promoted C(sp3)-H functionalization of aliphatic cyclic ethers using trace O2

Presented is a light-promoted C-C bond forming reaction yielding sulfone and phosphate derivatives at room temperature in the absence of metals or photoredox catalyst. This transformation proceeds in neat conditions through an auto-oxidation mechanism which is maintained through the leaching of trace amounts of O2 as sole green oxidant.

Copper-catalyzed synthesis of pyrido-fused quinazolinones from 2-aminoarylmethanols and isoquinolines or tetrahydroisoquinolines

Pyrido-fused quinazolinones were synthesized via copper-catalyzed cascade C(sp2)-H amination and annulation of 2-aminoarylmethanols with isoquinolines or pyridines. The transformation proceeded readily in the presence of a commercially available CuCl2 catalyst with molecular oxygen as a green oxidant. Moreover, the dehydrogenative cross-coupling of 2-aminoarylmethanols with tetrahydroisoquinolines was explored, in which CuBr exhibited higher catalytic activity than CuCl2. Broad substrate scope with good tolerance of functionalities was observed under the optimized reaction conditions. The bioactive naturally occurring alkaloid rutaecarpine could be obtained by this strategy. The remarkable feature of this protocol is that complicated heterocyclic structures are readily achieved in a single synthetic step from easily accessible reactants and catalysts. This pathway to pyrido-fused quinazolinones would be complementary to existing protocols.

Metal-free, regioselective, visible light activation of 4CzIPN for the arylation of 2H-indazole derivatives

Highly regioselective organo photocatalysis of 4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) for the arylation of 2H-indazole is demonstrated. The present synthetic route provides a highly safe and easily accessible aniline precursor as an arylation reagent. The photoactivated 4CzIPN organocatalyst is found to be more efficient for single electron transfer without any organic base for the radical reaction. The carbazole-based photocatalyst (4CzIPN) with wide redox potential is stable and recyclable for further reaction transformations. Many indazole and aniline derivatives were used in the reaction and provided the arylated indazole derivatives in good to excellent yield.

Highly regioselective organo photocatalysis of 4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene) for the arylation of 2H-indazole is demonstrated.

1,5,7-Triazabicyclo[4.4.0]dec-5-ene Enhances Activity of Peroxide Intermediates in Phosphine-Free α-Hydroxylation of Ketones

The critical role of double hydrogen bonds was addressed for the aerobic α-hydroxylation of ketones catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), in the absence of either a metal catalyst or phosphine reductant. Experimental and theoretical investigations were performed to study the mechanism. In addition to initiating the reaction by proton abstraction, a more important role of TBD was revealed, that is, to enhance the oxidizing ability of peroxide intermediates, allowing DMSO to be used rather than commonly used phosphine reductants. Further characterizations with nuclear Overhauser effect spectroscopy (NOESY) confirmed the presence of double hydrogen bonds between TBD and the ketone, and kinetic studies suggested the attack of dioxygen on the TBD-enol adduct to be the rate-determining step. This work should encourage the application of TBD as a catalyst for oxidations.

Visible-Light-Promoted Metal-Free Synthesis of (Hetero)Aromatic Nitriles from C(sp3 )-H Bonds*

The metal-free activation of C(sp3 )-H bonds to value-added products is of paramount importance in organic synthesis. We report the use of the commercially available organic dye 2,4,6-triphenylpyrylium tetrafluoroborate (TPP) for the conversion of methylarenes to the corresponding aryl nitriles via a photocatalytic process. Applying this methodology, a variety of cyanobenzenes have been synthesized in good to excellent yield under metal- and cyanide-free conditions. We demonstrate the scope of the method with over 50 examples including late-stage functionalization of drug molecules (celecoxib) and complex structures such as l-menthol, amino acids, and cholesterol derivatives. Furthermore, the presented synthetic protocol is applicable for gram-scale reactions. In addition to methylarenes, selected examples for the cyanation of aldehydes, alcohols and oximes are demonstrated as well. Detailed mechanistic investigations have been carried out using time-resolved luminescence quenching studies, control experiments, and NMR spectroscopy as well as kinetic studies, all supporting the proposed catalytic cycle.

Selective oxygenation of unactivated C-H bonds by dioxygen via the autocatalytic formation of oxoiron(v) species

Selective catalytic oxygenation of unactivated C-H bonds for a series of substrates by dioxygen using iron complexes was performed without the use of a co-reductant. Mechanistic studies indicate that the reaction proceeded via the autocatalytic formation of an oxoiron(v) intermediate, which brings high regioselectivity and stereoretention.

Aerobically-initiated C(sp3)–H bond amination through the use of activated azodicarboxylates

In this article we report a procedure for α-C(sp³)–H amination of ethereal compounds through use of azodicarboxylates as the nitrogen source and freely-available atmospheric oxygen to access ethereal radical intermediates via aerobic C–H activation.