Phenyl isocyanate dimer trimer

A special technique of trimerization has been described by Kogon 24, 25). Phenyl isocyanate reacts with ethyl alcohol to form a urethane (ethyl carbanilate). At 125° a substantial yield of ethyl a,7-diphenyl allophanate is observed as well as a small amount of phenyl isocyanate dimer. However, when A-methyhnorpholine (NMM) is added as a catalyst, the reaction is altered and the product is triphenylisocyanurate (isocyanate trimer) in high yield. The reaction sequence is believed to be ... 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

When a toluene solution of a mixture of cyclotrisilane 34 and cyclohexyl isocyanate (or f-butyl isocyante) was heated at 70 °C, cyclic di- and trisiloxanes 37 and 38, i.e. the cyclic dimer and trimer of the silanone 36, were obtained together with the corresponding isonitrile RN=C. The formation of 37 as well as 38 was completely suppressed in the presence of hexamethylcyclotrisiloxane (19 D3) instead, quantitative conversion of 35 into 39, the formal insertion product of the silanone 36 into the Si—O bond of D3, occurred (Scheme 14). Since neither cyclodisiloxane 37 nor cyclotrisiloxane 38 reacted with D3 under the reaction conditions, the possibility that 37 or 38 is the precursor of 39 was ruled out. Whereas the oxidation of 35 with cyclohexyl and t-butyl isocyanates proceeded with exclusive formation of 37 and 38 (as the silicon-containing compounds) the reaction of 35 with phenyl isocyanate resulted in the formation of 37 in low yield. Furthermore, in this case the presence of D3 did not totally suppress the formation of 37. According to the authors, these results indicate that the oxidation of 35 with cyclohexyl and f-butyl isocyanates appears to use other reaction channels than that with phenyl isocyanate. 

The dimerization of aryl isocyanates to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. In 1993 trimerization of phenyl isocyanate catalyzed by a fluoride ion was also reported, and a small amount of l,3-diazetidin-2,4-dione was obtained at room temperature <1993JOC1932>. 

More recently, Belzner et al. reported a new type of oxygen transfer reaction from isocyanates to bis[2-(dimethylaminomethyl)phenyl]silylene (8)18 which was thermally generated from the corresponding cyclotrisilane 7, and they obtained some convincing results of the involvement of silanone 9 (Scheme 3). However, they found that silanone 9 is not stable enough to be isolated. Only cyclic di- and trisiloxanes 10 and 11 (i.e., the cyclic dimer and trimer of the silanone 9) were obtained together with the corresponding isonitrile as other main products when... 

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