Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

Monoalkyltin(iv) complexes are well-known catalysts for esterification reactions and polyester formation, yet the mode of operation of these Lewis acidic complexes is still unknown. Here, we report on mechanistic studies of n-butylstannoic acid in stoichiometric and catalytic reactions, analyzed by NMR, IR and MS techniques. While the chemistry of n-butyltin(iv) carboxylates is dominated by formation of multinuclear tin assemblies, we found that under catalytically relevant conditions only monomeric n-BuSn(OAc)₃ and dimeric (n-BuSnOAc₂OEt)₂ are present. Density functional theory (DFT) calculations provide support for a mononuclear mechanism, where n-BuSn(OAc)₃ and dimeric (n-BuSnOAc₂OEt)₂ are regarded as off-cycle species, and suggest that carbon-oxygen bond breaking is the rate-determining step.