Radical C–H 18F-Difluoromethylation of Heteroarenes with [18F]Difluoromethyl Heteroaryl-Sulfones by Visible Light Photoredox Catalysis

Abstract

The 18F-labeling of CF2H groups has been recently studied in radiopharmaceutical chemistry owing to the favorable nuclear and physical characteristics of the radioisotope 18F for positron emission tomography (PET). Following up on the reported efficiency of the [18F]difluoromethyl benzothiazolyl-sulfone ([18F]1) as a 18F-difluoromethylating reagent, we investigated the influence of structurally-related [18F]difluoromethyl heteroaryl-sulfones in the reactivity toward the photoredox C–H 18F-difluoromethylation of heteroarenes under continuous-flow conditions. In the present work, six new [18F]difluoromethyl heteroaryl-sulfones [18F]5a–[18F]5f were prepared and, based on the overall radiochemical yields (RCYs), three of these reagents ([18F]5a, [18F]5c, and [18F]5f) were selected for the fully automated radiosynthesis on a FASTlabTM synthesizer (GE Healthcare) at high level of starting radioactivity. Subsequently, their efficiency as 18F-difluoromethylating reagents was evaluated using the antiherpetic drug acyclovir as a model substrate. Our results showed that the introduction of molecular modifications in the structure of [18F]1 influenced the amount of fac-IrIII(ppy)3 and the residence time needed to ensure a complete C–H 18F-difluoromethylation process. The photocatalytic C–H 18F-difluoromethylation reaction with the reagents [18F]5a, [18F]5c, and [18F]5f was extended to other heteroarenes. Radical-trapping experiments demonstrated the likely involvement of radical species in the C–H 18F-difluoromethylation process.