Pushing the equilibrium of regio-complementary carboxylation of phenols and hydroxystyrene derivatives

• Significantly enhanced conversions of ortho-carboxylation in presence of water-miscible organic co-solvents.
• Broad operating window in terms of reaction temperature, substrate concentration and bicarbonate source.
• Reusability of the biocatalyst over several cycles without loss of activity.

The enzymatic carboxylation of electron-rich aromatics, which represents a promising ‘green’ equivalent to the chemical Kolbe–Schmitt reaction, is thermodynamically disfavored and is therefore impeded by incomplete conversions. Optimization of the reaction conditions, such as pH, temperature, substrate concentration and the use of organic co-solvents and/or ionic liquids allowed to push the conversion in favor of carboxylation by a factor of up to 50%. Careful selection of the type of bicarbonate salt used as CO2 source was crucial to ensure optimal activities. Among two types of carboxylases tested with their natural substrates, benzoic acid decarboxylase from Rhizobium sp. proved to be significantly more stable than phenolic acid decarboxylase from Mycobacterium colombiense; it tolerated reaction temperatures of up to 50 °C and substrate concentrations of up to 100 mM and allowed efficient biocatalyst recycling.