Effect of polar aprotic solvents on the trimerization of phenyl isocyanate by organometallic catalysts

Studies of the trimerization of phenyl isocyanate by organometallic catalysts in the presence of various solvents have shown that dipolar aprotic solvents, such as DMSO and DMF, even in small amounts enhance greatly the rate of reaction. In accordance with their mode of action and of the effect of DMSO or DMF, the catalysts could be divided into three groups. Group I comprises tributyltin oxide, Ti(OBu)₄ and Zr(OBu)₄, which give a fast addition to the isocyanate. Maximum increase in rate was observed at DMSO:PhNCO = 1:1 due to the formation of a 1:1 charge transfer complex between them. Group 2 comprises naphthenates of Pb.Zr and Co which form complexes with the isocyanate, the reaction being much faster with the C.T. complex of DMSO and PhNCO: maximum increase in rate was observed at low DMSO concentrations, about the same as that of the catalyst. Group 3 comprises nucleophiles such as the amine catalysts, where the enhancement in rate was not great, due to the same mode of nucleophilic interaction of the catalyst and DMSO or DMF with the isocyanate.