Rhodium(ii)-catalyzed C–H carboxylation of ferrocenes with CO2

Carbon dioxide (CO2) is considered as a green and renewable C1 feedstock. However, due to its thermodynamic and kinetic stability, transition metal-catalyzed carboxylation of unactivated C–H bonds with CO2 still remains challenging. Herein, a protocol for Rh(ii)-catalyzed and CO2-involved carboxylation of inert sp2 C–H bonds from the cyclopentadienyl (Cp) ring of ferrocene derivatives was reported, which produces a series of ferrocene-embedded lactones that are important molecular skeletons widely used in pharmaceuticals or ligands. Moreover, moderate enantioselectivity was achieved with a chiral NHC ligand, which can be regarded as the first example for asymmetric C–H carboxylation with CO2. Preliminary mechanistic experiments supported bivalent rhodium as the real active catalyst, and a reasonable catalytic cycle was proposed accordingly.