Phenylisocyanate, trimerization

Attempts to initiate a desirable second insertion of PhN=C=X into the remaining silicon-hydrogen bond in compounds 6a or 6b failed. However, if a 10-fold excess of PhN=C=X (X = S, O) is used, trimers of phenylisothiocyanate (8a) or phenylisocyanate (8b) are formed. The latter can also be obtained in much better yields by direct reaction of 1 with a 10-fold excess of PhN=C=X. 

Hofmann [14] in 1858 was the first to report the trimerization of isocyanates to isocyanurates. Hofinann used triethylphosphine as a catalyst to trimerize phenylisocyanate to triphenylisocyanurate. 

Researchers (39,41) have investigated the addition of various amines to the carbanilation reaction mixtures to decrease the reaction time needed for derivatization of cellulose, especially the reaction time required for a sample with high molecular weight. In DMSO and DMF, the amines catalyzed the conversion of the phenylisocyanate to its trimer phenylisocyanurate. In addition, several amines actually retarded the carbanilation reaction. Most significant was that the presence of several amines in the DMSO-phenylisocyanate reaction mixture caused depolymerization of the cellulose, especially high-molecular-weight cellulose. In some cases, the depolymerization was severe. All three components (amine, phenylisocyanate, and DMSO) were required for depolymerization to take place. 

Aminometallation reactions have been shown to be useful in synthesis (152,197), not only with Sn—N compounds, but also with amine derivatives of B, Si, P, As, S, Zn, and Hg (127, 205). Because the organolead grouping can generally be cleaved easily, this reaction should be important for the introduction of amino groups. We confirmed this conclusion when we reacted BusPbH with an excess of phenylisocyanate. The adduct initially formed catalyzes trimerization (188, 194). 

In a similar fashion, alcohols undergo addition to isocyanates (R-N=C=0), which are nitrogen analogues of carbenes, to produce esters of carbamic acids (called urethanes). Thus, as shown in Table 8.6 (item 19) and in Scheme 8.59, the addition of cyclohexanol (CeHnOH) to phenylisocyanate (C6H5N=C=0) produces the cyclohexyl ester of phenylcarbamic acid. Interestingly, as shown in Scheme 8.59, it appears that the proton found on the nitrogen of the urethane is transferred directly to nitrogen from an alcohol dimer (or trimer). 

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