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

Abstract

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.

Rethinking the Lewis Acidity of Cationic Gallium and Indium Catalysts

“…The significantly slower reactivity of the methylindium ( 2a ) and the methylgallium ( 1a ) complexes compared to the isobutyl- and neosilyl derivatives can be attributed to the reduced steric bulk of methyl ligands which discourages ligand exchange between THF and ECH required to initiate polymerization. Recently, we observed the same alkyl effect on displacing coordinated moieties with another set of cationic indium catalysts used for the coupling of epoxides and lactones …”