THE DIMERIZATION OF ISOCYANATES UNDER HIGH-PRESSURE

The dimerization of phenyl isocyanate in the presence of pyridine was accelerated under high pressure to give 1,3-diphenyl-1,3-diazetidine-2,4-dione in high yield. The apparent activation volume of this reaction at 30-degrees-C was estimated to be - 23 ml/mol. Although 3-methylpyridine and 4-methylpyridine were better catalysts than pyridine, 2-methylpyridine and 2,6-dimethylpyridine had poor catalytic activity for the dimerization. It shows that the steric hindrance around nitrogen atom of a catalyst influences strongly the catalytic activity for the dimerization of phenyl isocyanate. The selectivity of the dimerization increased with an increase in pressure or amount of pyridine as a catalyst. The trimer of phenyl isocyanate was obtained predominantly if trialkylamine was used as a catalyst, and also the trimer was given as a main product when polar solvents such as acetonitrile were used. A new cyclic transition state was proposed for the dimerization mechanism of phenyl isocyanate under high pressure. Although m- and p-substituted phenyl isocyanate dimerized in good yields under high pressure, the dimers of bulky aryl isocyanates such as o-chlorophenyl and 1-naphtyl isocyanate or alkyl isocyanate such as butyl and cyclohexyl isocyanate were not obtained.