Synthesis of perbromobenzoic acids and perbromobenzenes from aromatic carboxylic acids by permercuration and bromodemercuration

Abstract

Permercuration of the benzoic acids XC6H4CO2H (X = o-Me, F, Cl, or Br; m- Me, F, Cl, Br, CF3, NO2 or OMe; or p-Me, F, Cl, Br, CF3 or NO2) and 2,6- X2C6H3CO2H (X = Me, Cl, or Br) with molten mercuric trifluoroacetate at c. 180-245° followed by bromodemercuration gave the corresponding perbromobenzoic acids XC6Br4CO2H or 2,6-X2C6Br3CO2H, together with the corresponding perbromobenzenes C6Br5X or m-X2C6Br4 which were formed owing to decarboxylation under permercuration conditions. Similar treatment of the acids XC6H4CO2H (X = o-NO2, CF3, or OMe; or p-OMe) and 2,6-F2C6H3CO2H gave only the appropriate perbromobenzenes. Possible mechanisms for permercuration induced decarboxylation are proposed on the basis of the effect of substituents on yields of perbromobenzoic acids and perbromobenzenes. Decarboxylation occurs more widely under permercuration conditions than on pyrolysis of mercuric carboxylates. Regiospecific mercuration meta to the carboxyl group and not decarboxylation occurred on thermal decomposition of mercuric p-methoxybenzoate.