Kinetics and mechanism of urethane formation catalyzed by organotin compounds. I. The reaction of phenyl isocyanate with methanol in dibutyl ether under the action of dibutyltin diacetate

The kinetics of the dibutyltin diacetate (DBTDA) catalyzed reaction of phenyl isocyanate with methanol in dibutyl ether at 25°C were studied by monitoring the rate of change in the absorbance of the reaction mixture at 281.6 nm. The rate equation was The value of k was calculated at 0.96 liter/(mole sec). In addition, it was ascertained that protons behave like extremely strong inhibitors for the catalyzed reaction. On the basis of these data a mechanism for urethane formation is proposed. The subsequent reaction steps are (1) complexation of methanol to DBTDA, (2) dissociation of the complex into a proton and an anion of composition {(n-C₄H₉)₂Sn(OCOCH₃)₂(OCH₃)}^-, (3) insertion of the isocyanate into the tin-alkoxy bond (the rate-determining step), and (4) methanolysis of the urethane precursor formed with simultaneous regeneration of the anion. With this mechanism it is possible to explain the observed kinetics as well as the deviations that occur in the rate expression if strong acids are added to the reaction mixture. The latter effect is caused by a shift in the equilibrium that describes the dissociation of the complex into ions. The retardation of the reaction by weak acids like acetic acid is caused by complexation of DBTDA by the acid.