Interaction between trialkyltin alkoxide and phenyl isocyanate in the formation of tin carbamate: A computational and experimental study

• Organotin catalysts are used to catalyze reactions of isocyanates and alcohols.
• Complexes between isocyanate, alcohol and catalyst are formed.
• Initial reaction between oxygen of isocyanate and tin center of catalyst.
• Insertion reaction to form O-substituted carbamate proposed as reaction mechanism.

Organotin catalysts are used to catalyze the reaction of isocyanates and alcohols in the manufacture of urethanes. Therefore it is important to understand the mechanism of the catalysis to get a greater control of the reaction to obtain specific properties of the final product. Until now the proposed mechanism related to organotin catalysis of urethane formation is based on the mechanism suggested by Bloodworth and Davies (1965) on the reaction between trialkyltin alkoxide and phenyl isocyanate. In the present work computational and experimental methods were used to investigate the interaction between trialkyltin alkoxide and phenyl isocyanate. The computational results agree with the experimental results reported by Bloodworth and Davies. The computational investigation also provided further insight into the interaction mechanism. The investigations indicate that initially the isocyanate oxygen is attracted towards the tin atom of the organotin alkoxide, which subsequently undergoes an insertion reaction to form an organotin O-carbamate (methyl tributylstannyl phenylcarbonimidate), which rearranges to form an organotin N-carbamate (methyl phenyl(tributylstannyl)carbamate). Model compound studies of the urethane formation in the presence of trialkyltin catalyst using 13C NMR and FT-IR data show that the reaction goes through a termolecular mechanism. This is also confirmed by comparing reaction rates between trialkyl and dialkyl tin as catalyst at similar tin content for the reaction between aromatic isocyanate and alcohol and comparing with computationally calculated intrinsic reaction coordinate profile of different transition states for similar interactions.

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