Rationally designed acidic deep eutectic solvent induced catalysis and spontaneous catalyst recycling of Pd-catalyzed ethylene alkoxycarbonylation

Abstract

Pd-catalyzed alkoxycarbonylation is an atom-efficient and green chemistry compliant protocol for ester synthesis. However, for such noble metal containing homogeneous catalytic systems, the circumvention of thermal stress deactivation of subtly designed ligands during the separation process still lacks rational countermeasures, restricting their widespread application. In this work, the coupling of efficient homogeneous catalysis and spontaneous heterogeneous separation of alkoxycarbonylation was accomplished with a rationally designed natural metabolite L-carnitine based acidic deep eutectic solvent (DES) as an acid co-catalyst. Mild and versatile alkoxycarbonylation between ethylene and long-chain aliphatic alcohols employing DES sustained exceptionally high yield (>99%) without any apparent depression in activity after long-term operation, enabling over four times the catalyst lifespan of conventional homogeneous catalysts. Catalyst recycling at the reaction temperature was implemented by the formed DES, marking it an essential milestone in improving the operational efficiency of alkoxycarbonylation. The mechanism of DES formation through synergistic interactions of proton transfer along with hydrogen bonding formation and catalyst recycling induced by ligand protonation was proposed after the molecular insight was elucidated using DFT calculation.