Ligand-enabled site-selectivity in a versatile rhodium(II)-catalysed aryl C-H carboxylation with CO2

Although carbon dioxide (CO2) is an attractive renewable carbon source, its utilization to produce fine chemicals through the catalytic carboxylation of unactivated carbon-hydrogen (C-H) bonds is still very limited and remains a challenge, largely because CO2 is thermodynamically and kinetically stable. In particular, the generation of (hetero)aromatic carboxylic acids via a transition-metal-catalysed C-H carboxylation of arenes with CO2 is extremely rare. Here we report a ligand-enabled site-selective carboxylation of 2-arylphenols under atmospheric pressure of CO2 through a Rh-2(OAc)(4)-catalysed and chelation-assisted C-H bond activation. Remarkably, the reaction occurs selectively on the less nucleophilic phenyl group with the promotion of a phosphine ligand, which overrides the site selectivity dictated by the well-known Kolbe-Schmitt type reaction. The non-acidic C-H bonds within several important classes of heterocycles were also efficiently carboxylated with this method. A mechanistic investigation revealed complexes of active catalysts and that this reaction proceeds under redox-neutral reaction conditions.