Visible-Light Photocatalytic Radical Alkenylation of -Carbonyl Alkyl Bromides and Benzyl Bromides

Through the use of [Ru(bpy)3Cl2] (bpy=2,2-bipyridine) and [Ir(ppy)3] (ppy=phenylpyridine) as photocatalysts, we have achieved the first example of visible-light photocatalytic radical alkenylation of various -carbonyl alkyl bromides and benzyl bromides to furnish -vinyl carbonyls and allylbenzene derivatives, prominent structural elements of many bioactive molecules. Specifically, this transformation is regiospecific and can tolerate primary, secondary, and even tertiary alkyl halides that bear -hydrides, which can be challenging with traditional palladium-catalyzed approaches. The key initiation step of this transformation is visible-light-induced single-electron reduction of CBr bonds to generate alkyl radical species promoted by photocatalysts. The following carboncarbon bond-forming step involves a radical addition step rather than a metal-mediated process, thereby avoiding the undesired -hydride elimination side reaction. Moreover, we propose that the Ru and Ir photocatalysts play a dual role in the catalytic system: they absorb energy from the visible light to facilitate the reaction process and act as a medium of electron transfer to activate the alkyl halides more effectively. Overall, this photoredox catalysis method opens new synthetic opportunities for the efficient alkenylation of alkyl halides that contain -hydrides under mild conditions.