Protodefluorinated Selectfluor^® aggregatively activates Selectfluor^® for efficient radical C(sp³)−H fluorination reactions

Efficient fluorination reactions are key in the late-stage functionalization of complex molecules in medicinal chemistry, in upgrading chemical feedstocks, and in materials science. Radical C(sp³)−H fluorinations using Selectfluor^® – one of the most popular fluorination agents – allow to directly engage unactivated precursors under mild photochemical or thermal catalytic conditions. However, H−TEDA(BF₄)₂ to date is overlooked and discarded as waste, despite comprising 95% of the molecular weight of Selectfluor^®. We demonstrate that the addition of H−TEDA(BF₄)₂ at the start of fluorination reactions markedly promotes their rates and accesses higher overall yields of fluorinated products (~3.3 × higher on average across the cases studied) than unpromoted reactions. Several case studies showcase generality of the promotor, for photochemical, photocatalytic and thermal radical fluorination reactions. Detailed mechanistic investigations reveal the key importance of aggregation changes in Selectfluor^® and H−TEDA(BF₄)₂ to fill gaps of understanding in how radical C(sp³)−H fluorination reactions work. This study exemplifies an overlooked reaction waste product being upcycled for a useful application.