¹⁸F-Radiofluorination of Aryl C-H Bonds using a Tandem Ir C-H Borylation/Cu Radiofluorination Strategy

Objectives: Our group and others have previously described Cu mediated radiofluorination of aryl organoboron compounds with [¹⁸F]fluoride.(1,2) Using this in conjunction with iridium catalysed C-H borylation,(3,4) we have developed a tandem borylation/radiofluorination for the conversion of aromatic C-H bonds to C-¹⁸F bonds. We anticipate that this methodology will be amenable to the chemoselective and regioselective late-stage radiofluorination of densely functionalised radiotracer precursor scaffolds.

Methods: Organoboronates were synthesised using Ir catalysed C-H borylation. A solution of Iridium catalyst, pinacolborane and substrate were heated and monitored with NMR spectroscopy. Fluorine-18 was produced in a cyclotron via the ¹⁸O(p, n)¹⁸F nuclear reaction and azeotropically dried using standard methods. To the dried [¹⁸F]fluoride was added solvent, which was added to a solution of crude aryl organoboronate, Cu complex, and additives (where applicable). The reaction mixture was heated for 20 min, quenched, and analysed by radio-TLC and -HPLC.

Results: The efficiency and selectivities of the aromatic C-H borylation have been well documented, with electron-rich and electron-deficient arenes displaying high reactivities. A model substrate 1,3-dimethoxybenzene can be C-H functionalised in high (>90%) NMR yield. The corresponding crude organoboronate undergoes radiofluorination in excellent (>70%) RCY. For reference, the RCY using pure, independently synthesised organoboronate is >90%, demonstrating that the radiofluorination is highly tolerant of the C-H borylation reagents. Other electron rich and electron-deficient arenes are also amenable in moderate to excellent RCYs.

Conclusions: We successfully developed a method for the tandem borylation/radiofluorination of aromatic C-H bonds. This approach offers greater operational simplicity than previously reported methods, and can be employed for the synthesis of fluorine-18 radiolabelled aromatics in excellent RCCs. The application of this method to the synthesis of clinically relevant radiotracers is currently underway. Acknowledgements: This work was supported by NIH (R01EB021155). References: [1] Tredwell M, Preshlock SM, Taylor NJ, Gruber S, Huiban M, Passchier J, et al. A General Copper-Mediated Nucleophilic ¹⁸F Fluorination ofArenes. Angew Chem Int Ed. 2014;53(30):7751-5. [2] Mossine A V, Brooks AF, Makaravage KJ, Miller JM, Ichiishi N, Sanford MS, et al. Synthesis of [¹⁸F]Arenes via the Copper-Mediated [¹⁸F]Fluorination of Boronic Acids. Org Lett, 2015;17(23):5780-3. [3] Ishiyama T, Takagi J, Ishida K, Miyaura N, Anastasi NR, Hartwig JF. Mild iridium-catalyzed borylation of arenes. High turnover numbers, room temperature reactions, and isolation of a potential intermediate. J Am Chem Soc. 2002;124(3):390-1. [4] Cho J-Y, Tse MK, Holmes D, Maleczka RE, Smith MR. Remarkably Selective Iridium Catalysts for the Elaboration of Aromatic C-H Bonds. Science,2002;295:305-8.

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