(616d) Mechanism for the Catalysis of Transesterification Using Homogeneous Tin

Carbonyl insertion is identified and computationally quantified as a mechanism for homogeneous organotin catalysis of transesterification. Organotin species are widely used catalysts for reactions of ester exchange, such as the transesterification of dimethyl terephthalate with ethylene glycol for polyethylene terephthalate (PET) production. The mechanism developed here applies directly to the production of PET but also has implications for analogous formation of other polyesters using tin catalysts.

The mechanism proceeds by pericyclic reactions, inserting the ester carbonyl into a tin-alkoxide bond and forming an orthoester intermediate before that decomposes to release the product ester. To propose reaction steps and reaction rates for both dibutyl tin (IV) oxide and tin (II) acetate systems, computational quantum chemistry is applied at a B3LYP/def2-TZVPD level with Grimme dispersion and implicit ethylene glycol solvent and transition-state theory. Ligand-exchange reaction rates are orders of magnitude faster than the core mechanism, justifying equilibrium assumptions for the ligand state of the tin. The multiple reaction steps fit simple Arrhenius forms well when consolidated using a quasi-steady-state simplification, giving activation energies of 65.9 and 61.4 kJ/mol for dibutyltin oxide and tin (II) acetate, respectively. Analogous reaction mechanisms would apply in esterification, ester hydrolysis, and polycondensation reaction types as well. There is reasonable agreement with experimentally based rate constants at 197°C. On the basis of tin-alkoxide bonds reacting with esters, the overall predicted rate constant is 180 cm³ /(mol·s) for dibutyltin oxide vs. 88 cm³ /(mol·s) experimental. For tin (II) acetate, the comparison is 1130 cm³ /(mol·s) predicted vs. 392 and 57 cm³ /(mol·s) inferred from experiments.