Utilization of CO2, an ideal one-carbon (C1) feedstock that is abundant and renewable, to produce fine chemicals has attracted tremendous attention of chemists for a long time. Compared with traditional methods for C–C bond formation via CO2 fixation, transition-metal-catalyzed direct C–H bond carboxylation with CO2 is a promising research area, because it obviates extra steps to preactivate/prefunctionalize the substrate.
Therefore, such C–H bond carboxylation is considerably more step- and atom-economical. However, due to the high thermodynamic stability and kinetic inertness of CO2, transition-metal-catalyzed carboxylation of unactivated aryl C–H bond with CO2 under mild conditions is extremely rare and remains a formidable challenge for modern catalysis research.
Recently, in a study published in Nature Catalysis, the research group led by Prof. LI Gang at Fujian Institute of Research on the Structure of Matter (FJIRSM) of Chinese Academy of Sciences reported an efficient and versatile ligand-enabled, site-selective carboxylation of 2-arylphenols with CO2 through a Rh(II)-catalyzed C–H bond activation under redox neutral conditions, which represents an unprecedented transformation in the field of C–H bond carboxylation via CO2 utilization.
The highlight of this C–H carboxylation with CO2 was its unconventional site-selectivity. Assisted by the chelation of the phenolic hydroxyl group and the promotion of a phosphine ligand, the carboxylation occurred selectively on the less nucleophilic aryl ring of 2-phenylphenols with Rh(OAc)2 as the catalyst, overriding the site selectivity dictated by the well-known Kolbe–Schmitt type reaction.
Notably, the reaction did not rely on the use of an external reductant or oxidant and rendered a redox-neutral and relatively mild catalytic system for the direct carboxylation of arenes under an atmospheric pressure of CO2.
The substrate scope of the method was broad. Besides various 2-phenylphenols with different substitution patterns, this carboxylation was also compatible with a range of heterocycles, which included 2-furanylphenols, 2-thiophenylphenols, 2-pyrrolylphenols and 2-pyridinylphenols.
Importantly, products of the reaction are key structural motifs in natural products, pharmaceutically interesting compounds and functional molecules.
In addition, the author identified a potential active catalyst through mechanistic studies, which helped to discover a new set of more effective reaction conditions with a lower catalyst loading.
Based on the structure of the isolated active complex and a series deuterium experiments as well as several control experiments, the author proposed a possible catalytic cycle for this C–H carboxylation protocol.
Figure: Phosphine-ligand promoted, Rh2(OAc)4 catalyzed C–H carboxylation of 2-arylphenols. (Image by Prof. LI’s group)
Contact:
Prof. LI Gang
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Email: gangli@fjirsm.ac.cn
