Mechanistic insight into epoxide methoxycarbonylation catalyzed by Co complexes

The epoxide methoxycarbonylation is essential for the synthesis of advanced aldehydes and polyols, but the reaction mechanism has not been well resolved yet. In this work, we have established more detailed mechanism pathways for ethylene oxide methoxycarbonylation over Co-based complexes, the reaction pathway of the main by-product acetaldehyde, and examined the effect of nitrogen-containing heterocycles on the pathways using DFT calculation and experimental study. The conversion of ethylene oxide and product selectivity is sensitive to the structure of nitrogen-containing heterocycles. The addition of pyrazole as a ligand generated the highest conversion (98.3%) and dramatically inhibited by-product (e.g., acetaldehyde, 2-methoxyethanol) formation with the highest selectivity of methyl 3-hydroxypropionate (84.3%). The calculations reveal that the epoxide ring opening is the rate-determining step and the homonuclear ion pair mechanism is the most energetically preferred pathway. The ligand with promoting effect on the methyl 3-hydroxypropionate formation rate can lower the energy barrier of the ring-opening step with a proportional correlation established.