THE ALCOHOLYSIS REACTION OF ISOCYANATES GIVING URETHANES - EVIDENCE FOR A MULTIMOLECULAR MECHANISM

A kinetic and mechanistic investigation of the catalyzed alcoholysis o f isocyanates was undertaken. Both experimental and theoretical result s showed that the alcoholysis should be understood by a multimolecular intervention of the alcohols. The alcoholysis of isocyanate was exami ned experimentally for 2-propanol and cyclohexanol in low and high con centrations. It is suggested that either two or three molecules of the alcohol are implicated from the kinetic study, while the reaction wit h trimers becomes dominant at high alcohol concentrations. In accordan ce with these results, theoretical study suggests an active participat ion of at least three alcohol molecules in a reacting supersystem, giv ing rise to a genuine effect. The detailed reaction mechanism for the alcoholysis reaction by methanol and methanol clusters (HN=C=O + n(CH3 OH), n = 1-3) was modeled by ab initio methods, both in the gas phase and in solution. The nucleophilic addition occurs in a concerted way a cross the N=C bond of the isocyanate rather than across the C=O, simil ar to the isocyanate hydrolysis. The bulk solvent effect, which is tre ated by a polarizable continuum model (PCM), does not affect the prefe rence of the alcohol to attack across the N=C bond as pointed out by t he gas-phase values.