Molecular Carbonyl Insertion As The Homogeneous Catalysis Mechanism For Transesterification Of Dimethyl Terephthalate With Ethylene Glycol

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 that is at the center of polyethylene terephthalate production. 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 (DBTO) 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 degrees C. On the basis of tin-alkoxide bonds reacting with esters, the overall predicted rate constant is 180 cm(3)/(mol s) for DBTO versus 88 cm(3)/(mol s) obtained experimentally. For tin(II) acetate, the comparison is 1130 cm(3)/(mol s) predicted versus 392 and 57 cm(3)/(mol s) inferred from experiments.