Direct carboxylation of arenes and halobenzenes with CO₂ by the combined use of AlBr₃ and R₃SiCl

The Lewis acid-mediated direct carboxylation of aromatic compounds with CO₂ is efficiently promoted by the addition of silyl chlorides bearing three alkyl and/or aryl substituents in total on the silicon atom. Thus, toluene, xylenes, mesitylene, and some other alkylbenzenes are treated with a 1:1 mixture of AlBr₃ and Ph₃SiCl in neat substrates under CO₂ pressure (3.0 MPa) at room temperature, to give the corresponding carboxylic acids in 60-97% yields, based on AlBr₃. Polycyclic arenes, including naphthalene, phenanthrene, and biphenyl, are regioselectively carboxylated in 91-98% yields with the aid of 1 molar equiv of AlBr₃ and Ph₃SiCl in an appropriate solvent, chosen from benzene, chlorobenzene, and fluorobenzene. These solvents, as well as bromobenzene, resist carboxylation; however, they are also carboxylated in moderate yields when treated with a 1:5 mixture of AlBr₃ and ⁱPrSiCl at elevated temperatures. The FT-IR spectrum of a mixture prepared by exposing a suspension of AlBr₃ and Ph₃SiCl in cyclohexane to CO ₂ exhibits an absorption band around 1650 cm^-1, assigned to the C=O stretching vibration of a species consisting of CO₂, AlBr₃, and Ph₃SiCl, which suggests that the silyl chlorides activate CO₂ in cooperation with AlBr₃. ¹H NMR analysis of unworked-up reaction mixtures reveals that the products merge as aluminum carboxylates. The mass balance concerning silicon indicates that the silyl chlorides are recycled during the reaction sequence. On the basis of these observations, a feasible mechanism is proposed for the present carboxylation.