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

Abstract

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 Rh2(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. Catalytic arene carboxylation with CO2 is a challenging reaction, with limited substrate scope and selectivity. Now, a redox neutral method based on a ligand-assisted Rh-catalysed C–H activation is shown, which features broad scope and provides access to important and biologically relevant structural motifs