Regioselective Carboxylation of Phenols with Carbon Dioxide

A few novel methods were developed for the regioselective preparation of p-hydroxybenzoic acid (pHBA) and its amino derivative by means of the Kolbe-Schmitt reaction. Thus, the carboxylation of tetraalkylammonium phenoxide at 125 degreesC under the CO2 pressure of 5.0 MPa in the presence of K2CO3 gave pHBA in a maximum yield of 56% with the regioselectivity of 97-100%. The carboxylation of potassium phenoxide (PhOK) at 230 degreesC under the CO2 pressure of 0.5 MPa. also gave pHBA regioselectively in a 39% yield, together with unaltered phenol (61%). Under such conditions, the potassium salt of salicylic acid (SA) once formed was transformed into pHBA. Heat treatment of the dipotassium salt of C-13 labeled SA indicated that the transformation occurs via two pathways, i.e., the intramolecular rearrangement of the salicylate (66%) and the decarboxylation of the salicylate followed by the recarboxylation of the resulting PhOK (34%). Furthermore, the carboxylation of cesium m-aminophenoxide and 5-amino-1-naphthoxide with CO2 gave regioselectively 4-hydroxyanthranilic and 8-amino-4-hydroxy-1-naphthoic acids, respectively, in good yields. This is a simple one-pot reaction giving these industrially useful acids with good yields.