Photochemical CF 3 Radical Generation: A Spin Trapping/Electron Paramagnetic Resonance and In Situ 19 F Nuclear Magnetic Resonance Study

Trifluoromethyl (CF 3 ) radicals, generated under photoredox catalysis, are valuable tools for introducing CF 3 groups into organic molecules, significantly altering their physical, chemical, and biological properties. Although reagents, like the Umemoto and Togni compounds, are widely used to introduce CF 3 groups, the details of how they generate CF 3 radicals are not well understood. In this article, we study the reactivity of 18 different CF 3 ‐donating reagents using two methods: spin trapping‐electron paramagnetic resonance (ST/EPR) spectroscopy and in situ 19 F nuclear magnetic resonance (NMR). ST/EPR measures how quickly CF 3 radicals are released by detecting their reaction with the spin trap 2‐methyl‐2‐nitrosopropane (MNP), forming MNP‐CF 3 adducts. In parallel, 19 F NMR monitors how the reagents are consumed and which products are formed during trifluoromethylation of alkenes. Notably, the observed reactivity trends differ between the two techniques, likely due to differences in reaction environments and side reactions such as CF 3 I formation. The comparison between each reagent must therefore be limited to the values determined by each analytical technique, EPR or NMR. This study provides nevertheless new insights into how CF 3 radicals are generated and how the structure of the reagent influences its reactivity.