Construction of axial chirality via asymmetric radical trapping by cobalt under visible light

Abstract

The 3d metals have been identified as economic and sustainable alternatives to palladium, the frequently used metal in transition-metal-catalysed cross-couplings. However, cobalt has long stood behind its neighbouring elements, nickel and copper, in asymmetric radical couplings owing to its high catalytic activity in the absence of ligands resulting in unfavourable un-asymmetric background reactions. Here we disclose an asymmetric metallaphotoredox catalysis (AMPC) strategy for the dynamic kinetic asymmetric transformation of racemic heterobiaryls, which represents a visible-light-induced, asymmetric radical coupling for the construction of axial chirality. This success can also be extended to the reductive cross-coupling variant featuring the use of more easily available organic halide feedstocks. The keys to these achievements are the rational design of a sustainable AMPC system that merges asymmetric cobalt catalysis with organic photoredox catalysis in combination with the identification of an efficient chiral polydentate ligand. Controlling the enantioselectivity in metallaphotoredox-catalysed radical cross-couplings using cobalt has proven challenging. Now, the identification of a chiral polydentate ligand enables cobalt-catalysed enantioselective couplings with a broad scope of radicals affording chiral heterobiaryl products.