Bifunctional Photocatalysts Display Proximity-enhanced Catalytic Activ-ity in Metallaphotoredox C–O Coupling

Dual catalytic reactions may be made more effective through an improved integration of the catalytic cycles achieved using bifunctional catalysts. Herein we describe two bifunctional photocatalysts consisting of a photoactive donor-acceptor cyanoarene unit linked to a bipyridine ligand moiety that can bind transition metals. The bifunctional photocata-lysts were synthesized in 3-5 steps from commercially available compounds and fully characterized in terms of photophys-ical properties, which are strongly affected by the type of linkage used (C vs. O) to connect the cyanoarene core to the ligand. Catalytic tests carried out in the Ni-catalyzed C-O cross-coupling of alcohols to aryl bromides promoted by visible light have shown that the bifunctional systems are more active than the corresponding ‘dual catalytic systems’ (i.e., not covalently bound), taking advantage of the proximity between the two catalytic moieties (Ni-complex and photocatalyst). The best bifunctional photocatalysts were tested with several alcohols and aryl halides, giving good yields at low catalytic loading (0.5-2 mol%).