Insights into the Organocatalyzed Synthesis of Urethanes in Supercritical Carbon Dioxide: An In Situ FTIR Spectroscopic Kinetic Study
Résumé
The kinetics of urethane (carbamate) formation in supercritical CO2 (scCO(2)) has been studied by insitu FTIR spectroscopy by using the reaction between n-butanol and toluene-2,4-diisocyanate as a model for the synthesis of polyurethanes. We have evaluated the efficiency of different organocatalysts for this transformation to eliminate the need for potentially toxic organotin compounds. Bicyclic guanidines and amidines (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene and 1,8-diazabicyclo[5.4.0]undec-7-ene) lead to a significant acceleration of the reaction rate, but we found that their activity in scCO(2) is reduced compared to reactions in conventional media because of the formation of guanidinium or amidinium alkylcarbonate salts. In scCO(2), these catalysts are also sensitive to the presence of water and can lead to the formation of allophanate or isocyanurate byproducts. On the other hand, we have shown that catalysis with amine N-oxides (N-methylmorpholine-N-oxide) is suited to the scCO(2) medium.