A catalytic platform for direct isotopic fluorination of C-H bonds in organic molecules for PET imaging

Background

The field of radiochemical synthesis, particularly for the development of positron emission tomography (PET) tracers, is crucial for advancing diagnostic imaging in medicine. PET imaging relies on the incorporation of positron-emitting isotopes, such as fluorine-18 ($^{18}$F), into biologically relevant molecules. The ability to efficiently and selectively introduce $^{18}$F into complex organic substrates is essential for producing high-specific-activity tracers that can be used to visualize physiological processes in real time. As the demand for personalized medicine and molecular imaging grows, so does the need for robust, versatile, and mild methods to label a wide range of pharmaceuticals and biomolecules with $^{18}$F, enabling researchers and clinicians to track disease progression, monitor therapeutic response, and accelerate drug development.

Current approaches to $^{18}$F-labeling, however, suffer from significant limitations that hinder their broader application. Traditional methods often require the pre-installation of reactive functional groups, such as leaving groups or activated aromatic rings, which restricts the types of molecules that can be labeled and complicates late-stage modification of drug candidates. These processes typically involve harsh reaction conditions—such as high temperatures, strong bases, or phase transfer catalysts—that can degrade sensitive functional groups and limit compatibility with complex, bioactive molecules. Additionally, the substrate scope is often narrow, and the radiochemical yields can be low, making it challenging to produce sufficient quantities of labeled tracers for clinical or research use. These obstacles underscore the urgent need for new technologies that enable direct, efficient, and mild fluorination of C-H bonds in diverse organic molecules, thereby expanding the toolkit for PET tracer synthesis and biomedical imaging.

Technology

This technology offers a groundbreaking method for the direct fluorination of C-H bonds in organic molecules, specifically enabling the incorporation of fluorine isotopes such as the positron-emitting $^{18}$F used in PET imaging. Utilizing non-heme manganese catalysts coordinated with bidentate or polydentate nitrogen-donor ligands, the system efficiently converts sp³ C-H bonds—including both aliphatic and benzylic positions—into C-F bonds. The process employs a mild, room-temperature protocol that combines the substrate, catalyst, a fluorinating agent (such as no-carrier-added [$^{18}$F]fluoride), and an oxidant in solvents like acetone or acetonitrile. Notably, the reaction proceeds rapidly (within 10 minutes) and achieves high radiochemical conversions (30–90%) without the need for phase transfer catalysts. The method demonstrates broad substrate compatibility, tolerating a wide range of functional groups, and has been successfully applied to synthesize diverse $^{18}$F-labeled pharmaceuticals and bioactive molecules.

What differentiates this technology is its unique catalytic mechanism and operational simplicity. Unlike traditional fluorination methods that require pre-functionalized substrates, harsh conditions, or limited substrate scope, this approach leverages a high-valent oxo-Mn(IV) intermediate to abstract hydrogen and facilitate equatorial fluorine transfer directly to the substrate radical. This mechanistically distinct pathway, supported by both computational and experimental evidence, enables efficient late-stage fluorination of complex molecules under exceptionally mild conditions. The platform’s versatility and efficiency make it a significant advancement for radiochemical synthesis, particularly in the preparation of high-specific-activity PET tracers, thereby opening new avenues for biomedical imaging and drug development.

Benefits

• Enables direct fluorination of sp³ C-H bonds under mild, room-temperature conditions
• Uses non-heme manganese catalysts with broad substrate and functional group tolerance
• Facilitates incorporation of positron-emitting isotope $^{18}$F for PET radiotracer synthesis
• Eliminates need for pre-installed reactive groups and harsh reaction conditions
• Achieves high radiochemical conversions (30%–90%) rapidly (around 10 minutes)
• Compatible with complex pharmaceuticals and bioactive molecules for late-stage fluorination
• Provides a mechanistically distinct and efficient fluorine transfer from equatorial ligand positions
• Supports production of high-specific-activity PET tracers from simple precursors, advancing biomedical imaging and drug development

Applications

• PET radiotracer synthesis
• Late-stage drug labeling
• Radiolabeled biomolecule production
• Pharmaceutical metabolism studies

Additional info

This catalytic system utilizes non-heme manganese complexes with polydentate nitrogen ligands for direct sp³ C-H fluorination. A high-valent oxo-manganese(IV) intermediate abstracts hydrogen, followed by equatorial fluorine transfer to the resulting carbon radical. Operating at room temperature with m-CPBA, this method enables efficient $^{18}$F-labeling of complex pharmaceuticals for PET imaging, providing high radiochemical yields and broad functional group tolerance without requiring pre-installed reactive groups.