Organotin-mediated synthesis of macrocyclic polyesters: mechanism and selectivity in the reaction of dioxastannolanes with diacyl dichlorides

Abstract

The reaction of 2,2-di-n-butyl-1,3,2-dioxastannolane with diacyl dichlorides for the synthesis of macrocyclic tetraesters has been investigated to gain an insight into its mechanistic pathway and to ascertain the occurrence of genuine template effects. Experimental results show unambiguously that the reaction is a clean cyclo-oligomerization involving a fast transesterification equilibrium that affords a thermodynamic distribution of oligomeric esters. No evidence to support a structural or ‘covalent’ template effect by tin was found. Instead, dioxastannolane is not only the reagent, but also behaves as an efficient transesterification catalyst that equilibrates the macrocyclic ester mixture. In this context, the reaction can hardly be considered selective.