Sulfur-Containing Salen-Type Chromium Complexes for Selective Carbonylation of Epoxides: A Pathway to Polyesters and Cyclic Lactones.

Abstract

Carbonylation, a pivotal process in efficient C1 conversion, facilitates the direct incorporation of carbon monoxide into high-value-added chemicals. This study investigates the carbonylation of epoxides using salen-type complexes with varying metal centers and tetradentate dianionic ligands, in conjunction with Co₂(CO)₈, to optimize selectivity and yield in the production of polyesters and cyclic lactones. The [ONSO]-type Cr(III) complexes exhibited a pronounced preference for polyester formation, achieving a selectivity of 73 %. This preference is attributed to the electron-donating sulfur donors, which stabilize the growing polymer chain. In contrast, [ONNO]-type complexes with Cr(III) and Al(III) predominantly yielded cyclic lactones (99 % β-butyrolactone selectivity), owing to their enhanced electrophilicity that favors the backbiting process. Detailed analyses of ethylene oxide carbonylation underscored the crucial role of catalyst structure in determining reaction pathways and product distribution. Notably, [ONNO]-type complexes with Cr(III) and Al(III) exhibited over 99 % propiolactone selectivity during the carbonylation of ethylene oxide, highlighting the significance of catalyst design in optimizing chemical reactions. These results provide valuable insights into the role of ligand design in controlling the selectivity and efficiency of epoxide carbonylation, paving the way for the development of more effective catalytic systems.