Redox-neutral Photocatalytic C-H Carboxylation of Arenes and Styrenes with CO2

Abstract

Carbon dioxide (CO<sub>2</sub>) is an attractive one-carbon (C1) building block in terms of sustainability and abundance. However, its low reactivity limits applications in organic synthesis as typically high-energy reagents are required to drive transformations. Here, we present a redox-neutral C−H carboxylation of arenes and styrenes using a photocatalytic approach. Upon blue-light excitation, the anthrolate anion photocatalyst is able to reduce many aromatic compounds to their corresponding radical anions, which react with CO<sub>2</sub> to afford carboxylic acids. High-throughput screening and computational analysis suggest that a correct balance between electron affinity and nucleophilicity of substrates is essential. This novel methodology enables the carboxylation of numerous aromatic compounds, including many that are not tolerated in classical carboxylation chemistry. Over 50 examples of C−H functionalizations using CO<sub>2</sub> or ketones illustrate a broad applicability. The method opens new opportunities for late-stage C−H carboxylation and valorization of common arenes.

How Radical are ‘Radical’ Photocatalysts? A Closed-Shell Meisenheimer Complex is Identified as a Super-reducing Photoreagent

“…12,17,37 Such closed-shell anthrolate anions are known to be competent photocatalysts for very similar photoreductions of chloroarenes. [38][39][40] In summary, the ultrafast relaxation resulting from internal conversion of doublet excited states to a doublet ground states suggests that synthetically useful photochemically active radical species would be extraordinary, as opposed to their reaction to produce closed shell species in which photoredox processes are driven from the more common singlet/triplet lowest energy excited state manifold.…”

Pd(II)-catalyzed carboxylation of aromatic C─H bonds with CO 2

“…Although no stoichiometric amounts of organometallic additives are needed in these reactions, they are applicable only to a specific class of substrates and yield acid derivatives as products. Other attempts to achieve the transformation include reactions occurring via transition metal migration from reactive directing groups ( 39 , 40 ) or photocatalytic carboxylation of naphthalenes and heteroarenes ( 41 ).…”