Persistent Boryl Radicals as Highly Reducing Photoredox Catalysts for Debrominative Borylations

Organic free radicals are commonly perceived to be highly reactive species with short lifetimes, yet there are many examples that defy this convention by displaying remarkable stability. Although these persistent radicals can be relatively unreactive in their ground states, photoexcitation can generate highly reactive excited states. Despite this, they have found limited application as reagents or catalysts in photochemical reactions. Here we report the identification of persistent boryl-bipyridine radicals that function as highly reducing photoredox catalysts. These radicals, which are generated by simply mixing a bipyridine with a diboron reagent, were found to possess excited state reduction potentials that rival the most powerful photoreductants reported to date. We show that this class of doublet state photoredox catalyst can promote borylations of alkyl bromides and various other transformations.