Silver Chloride/Triphenylphosphine-Promoted Carboxylation of Arylboronic Esters with Carbon Dioxide at Atmospheric Pressure

Aim and Objective: The carboxylation of organoboronic esters with CO2 is an attractive way of preparing the functionalized carboxylic acid derivatives. For the silver-catalyzed carboxylation of arylboronic esters with CO2, sensibility of catalytic system to CO2 pressure resulted in a high CO2 pressure required in the reaction. This work focuses on solving this problem and promoting this transformation under atmospheric CO2 pressure. Material and Method: Arylboronic esters were synthesized according to the published procedure. Arylorganic esters, silver compound, base, PPh3 and solvent were introduced into a 25 mL Schlenk flask. Then, the gasexchanging operation via a “freeze-pump-thaw” method was conducted. After being enclosed with a CO2 balloon, the resulting mixture was stirred at 70oC for 12 h. Then, the reaction was quenched by 1 M HCl and extracted with ethyl acetate. The yields of the corresponding carboxylic acids were determined by GC. The structures of products were confirmed by NMR and GC-MS. Results: The carboxylation of arylboronic esters with ambient CO2 was achieved on the basis of the silver chloride/triphenylphosphine system by using cesium carbonate as the base and dimethyl sulfoxide as the solvent. Through this protocol, various functionalized carboxylic acids were synthesized in 85-99% yields. Conclusion: The silver-catalyzed carboxylation of arylboronic esters with ambient CO2 was established. This approach represents an alternative way to prepare carboxylic acids derivatives from the organoboron compounds with CO2 under mild reaction conditions.