11C, 12C and 13C-Cyanation of Electron-Rich Arenes via Organic Photoredox Catalysis

First-in-Human Evaluation of [18F]FDOPA Produced by Organo-Photoredox Reactions

Photoredox is a powerful synthetic tool in organic chemistry and has been widely used in various fields, including nuclear medicine and molecular imaging. In particular, acridinium-based organophotoredox radiolabeling has significantly impacted the production and discovery of positron emission tomography (PET) agents. Despite their extensive use in preclinical research, no PET agents synthesized by acridinium photoredox labeling have been tested in humans. [18F]FDOPA is clinically used for tumor diagnosis and the evaluation of neuropsychiatric disorders, but its application is limited by complex synthesis methods, the need for expensive modules, and/or the high cost of consumable materials/cassettes. In this report, we integrated a photoredox labeling unit with an automated module and produced [18F]FDOPA for human study. This research not only represents the first human study of a PET agent generated by acridinium-based organophotoredox reactions but also demonstrates the safety of this novel labeling method, serving as a milestone/reference for the clinical translation of other PET agents generated by this technique in the future.

Unlocking flavin photoacid catalysis through electrophotochemistry

Molecular flavins are one of the most versatile photocatalysts. They can coordinate single and multiple electron transfer processes, gift hydrogen atoms, form reversible covalent linkages that support group transfer mechanisms, and impart photonic energy to ground state molecules, priming them for downstream reactions. But one mechanism that has not featured extensively is the ability of flavins to act as photoacids. Herein, we disclose our proof-of-concept studies showing that electrophotochemistry can transform fully oxidized flavin quinones to super-oxidized flavinium photoacids that successfully guide proton-transfer and deliver acid-catalyzed products. We also show that these species can adopt a second mechanism wherein they react with water to release hydroxyl radicals that facilitate hydrogen-atom abstraction and sp3C-H functionalization protocols. Together, this unprecedented bimodal reactivity enables electro-generated flavinium salts to affect synthetic chemistries previously unknown to flavins, greatly expanding their versatility as catalysts.

1,3-Difunctionalization of [1.1.1]propellane through iron-hydride catalyzed hydropyridylation

Current methodologies for the functionalization of [1.1.1]propellane primarily focus on achieving 1, 3-difunctionalized bicyclo[1.1.1]pentane or ring-opened cyclobutane moiety. Herein, we report an innovative approach for the 1, 3-difunctionalization of [1.1.1]propellane, enabling access to a diverse range of highly functionalized cyclobutanes via nucleophilic attack followed by ring opening and iron-hydride hydrogen atom transfer. To enable this method, we developed an efficient iron-catalyzed hydropyridylation of various alkenes for C - H alkylation of pyridines at the C4 position, eliminating the need for stoichiometric quantities of oxidants or reductants. Mechanistic investigations reveal that the resulting N-centered radical serves as an effective oxidizing agent, facilitating single-electron transfer oxidation of the reduced iron catalyst. This process efficiently sustains the catalytic cycle, offering significant advantages for substrates with oxidatively sensitive functionalities that are generally incompatible with alternative approaches. The strategy presented herein is not only mechanistically compelling but also demonstrates broad versatility, highlighting its potential for late-stage functionalization.

Photo- and Cu-Mediated 11C Cyanation of (Hetero)Aryl Thianthrenium Salts

We present a photo- and Cu-mediated 11C cyanation of bench-stable (hetero)aryl thianthrenium salts via an aryl radical addition pathway. The thianthrenium substrates can be readily accessed via C-H functionalization, and the radiocyanation protocol proceeds under mild conditions (<50 °C, 5 min) and can be automated using open-source, readily accessible augmentations to existing radiochemistry equipment.

Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C-N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Room-Temperature Copper-Mediated Radiocyanation of Aryldiazonium Salts and Aryl Iodides via Aryl Radical Intermediates

Radiocyanation is an attractive strategy for incorporating carbon-11 into radiotracer targets, particularly given the broad scope of acyl moieties accessible from nitriles. Most existing methods for aromatic radiocyanation require elevated temperatures (Cu-mediated reactions of aryl halides or organometallics) or involve expensive and toxic palladium complexes (Pd-mediated reactions of aryl halides). The current report discloses a complementary approach that leverages the capture of aryl radical intermediates by a Cu-11CN complex to achieve rapid and mild (5 min, room temperature) radiocyanation. In a first example, aryl radicals are generated via the reaction of a CuI mediator with an aryldiazonium salt (a Sandmeyer-type reaction) followed by radiocyanation with Cu-11CN. In a second example, aryl radicals are formed from aryl iodides via visible-light photocatalysis and then captured by a Cu-11CN species to achieve aryl-11CN coupling. This approach provides access to radiocyanated products that are challenging to access using other methods (e.g., ortho-disubstituted aryl nitriles).