Mechanistic Insights into Selective Indium-Catalyzed Coupling of Epoxides and Lactones

Spiro-orthoesters, produced from the coupling of lactones and epoxides, are presequenced monomers capable of generating alternating poly(ether-alt-esters) after ring-opening polymerization. Recently, we reported the selective synthesis of spiro-orthoesters through the coupling of epoxides and lactones catalyzed by a cationic alkyl indium catalyst. Herein, we investigate the mechanism of this reaction using structure–function relationships and computational studies. We show that this reaction is governed by Michaelis–Menten-type saturation kinetics, which, along with the low Lewis acidity of these indium catalysts, explains their selectivity.