Efficient chemoselective carboxylation of aromatics to arylcarboxylic acids with a superelectrophilically activated carbon dioxide-Al < inf> 2 Cl < inf> 6 /Al system

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Aromatic carboxylic acids are obtained in good to excellent yield essentially free of diaryl ketones by carboxylation of aromatics with a carbon dioxide-Al2Cl6/Al system at moderate temperatures (20-80°C). To optimize reaction conditions and study the reaction mechanism, experimental variables including temperature, amount of Al2Cl6/Al, various Lewis acids, role of metal additive, carbon dioxide pressure, etc. were studied. The carboxylation reaction was found to be stoichiometric rather than catalytic, with aluminum chloride forming a dichloroaluminate of carboxylic acids. Although the carboxylation takes place using AlCl3 itself, the presence of metal additives, especially Al, increased the yield and selectivity of carboxylic acids. Because it was not possible to distinguish between two possible mechanistic pathways of the reaction on the basis of the experimental results, theoretical calculations using density functional theory (DFT) were also carried out. One possible pathway involves an initial complex between benzene and Al2Cl6, with subsequent formation of organoaluminum intermediates (PhAlCl2 and PhAl2Cl5). The other proceeds through the formation of various complexes of CO2 with aluminum chloride (AlCl3)n, n = 1-4. The calculations have shown that the organometallic pathway, leading eventually through the formation of phenylaluminum dichloride, is endothermic by 33 kcal/mol. In contrast, the preferred CO2-AlCl3 complex forms in an exothermic reaction (-6.0 kcal/mol) as does CO2AlCl2+. On the basis of both experimental and calculational findings, the most feasible reaction mechanism proposed involves superelectrophilic aluminum chloride activated carbon dioxide reacting with the aromatics in a typical electrophilic substitution.

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Journal of the American Chemical Society