Online ATR-IR investigations and mechanistic understanding of the carbonylation of epoxides - the selective synthesis of lactones or polyesters from epoxides and CO

In situ ATR-IR spectroscopy is applied as a powerful tool to study the factors that control the reaction of epoxides with carbon monoxide in the presence of [Lewis acid]⁺[Co(CO)₄]⁻ salts. Based on these investigations, a consistent mechanistic scheme is presented, comprising the main lactone and polyester products and minor components, e.g., acetone and crotonic acid derivatives. β-Alkoxy-acyl-cobalttetracarbonyl species are shown to be key intermediates from which two reaction routes start in dependence of the applied Lewis acid (LA). Labile LA-alkoxy combinations favor a backbiting process of the oxygen function on the Co-acyl bond, primarily producing lactone products. More stable LA-alkoxy entities are unreactive toward PO conversion and afford a polymerization reaction after the addition of a nucleophile. In that case, the Lewis acid remains bonded to the chain end.