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Novel and efficient bicyclic guanidine catalyst for the cyclotrimerization of isocyanates : a quantum chemical study

Polyurethane foams are widely used polymers composed of organic units joined by carbamate (urethane) linkers. Management of their physical properties represents significant challenge in the creation of new materials with desired properties. Since the cyclotrimerisation of organic isocyanates is one of the most sensitive steps in the whole process, identifying efficient catalysts for this chemical transformation represents an important and needed research goal. In this work we considered triazabicyclodecene (TBD) as a potential catalyst for the cyclotrimerization of isocyanates. We have investigated the reaction pathway involving a sequential addition of metyl-isocyanate (MIC) using well established quantum chemistry methods at the MP2/6– 311++G(2df, 2pd)//M062X/6–31+G(d) level. Our results show that the first MIC molecule is activated by the TBD catalyst through the nucleophilic attack of its imino nitrogen atom to strongly electrophilic carbon on MIC followed by the intramolecular MIC–TBD proton transfer, thus opening the possibility for the activation of another MIC molecule on the second nitrogen atom on TBD. After that, the two MIC systems combine to give a dimer, to be followed by the analogous formation of an open- chain trimer chemically bonded to TBD. The last step involves the cyclization of the trimer and the liberation of the final hexacyclic product concomitant with the regeneration of the TBD catalyst. The overall reaction pathway reveals that the investigated trimerization is thermodinamically a very favorable process (ΔrG = –34.3 kcal/mol) with reasonable reaction barrier (ΔG‡= 27.7 kcal/mol) in the THF solution. In addition, it provides a convincing insight to why dimerization is not favorable from both thermodynamic and kinetic points of view, being strongly in line with experimental observations. Compared to the uncatalyzed reaction, TBD lowers the activation barrier by 8.4 kcal/mol, thus increasing the reaction rate by the factor of 106. These significant results suggest TBD as a much efficient catalyst than some other systems proposed in the literature based on proazaphosphatranes.

isocyanates ; cyclotrimerization ; catalysis

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