A Mechanochemical Kolbe-Schmitt Reaction: Catechol Carboxylation Provides Building Blocks for Renewable Plasticizers

Catechol, an important aromatic platform molecule which can be derived from biomass, was carboxylated by mechanochemical Kolbe-Schmitt reaction of disodium catecholate with CO₂, providing a mixture of mono- and dicarboxylated catechol derivatives. While classical protocols require harsh reaction conditions, involving a high temperature and/or CO₂ pressure, a mild ball milling method was developed. This represents the first mechanochemical Kolbe-Schmitt reaction featuring a low CO₂ pressure and reactivity at room temperature. From the individual catechol-based mono- and dicarboxylic acid reaction products, a library of novel renewable plasticizers was synthesized through esterification of the carboxylic acid functionalities and O-acylation of the phenolic hydroxy groups. The resulting esters were evaluated in poly(vinylchloride) (PVC) and poly(lactic acid) (PLA), revealing plasticizing efficiencies competitive to benchmark commercial plasticizers. These efficiencies were maintained when the best performing ester substitution pattern was installed on the ball mill-derived mixture of mono- and dicarboxylated catechols, making resource intensive separation (e.g. chromatographic separation) of these ortho-dihydroxybenzene(di)carboxylic acids redundant.