Aryl isocyanates trimerization reactions

In their extensive efforts to devise a new strong nonionic base, Verkade and coworkers found that a highly basic dendrimer containing a PAPT base fragment could act as an efficient catalyst for Michael addition reactions, nitroaldol (Henry) reactions and aryl isocyanate trimerization reactions [42] (Figure 6.3). In view of the characteristic nature of this dendrimer, which has sixteen catalytic sites per molecule, the attachment of other superbase functionalities might also be attractive. 

Verkade and co-workers have shown the usefulness of their phosphazanes in various stoichiometric as well as catalytic reactions <1999PS(144)101>. Compound 290 was used to promote the cyanohydration of benzaldehyde with trimethylsilyl cyanide (TMSCN). The cyanohydrin was isolated in 95% yield, but no enantioselectivity was noticed <2002JOM(646)161>. Compounds 291 and 292 were attached to dendrimers and shown to be effective in the catalysis of Michael reactions, nitroaldol reactions, and aryl isocyanate trimerizations <2004ASC1093>. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

As mentioned earlier, metal carboxylates are trimerization catalysts 23, 24). They also catalyze the reaction of aryl isocyanates and substituted ethyl carbanilates to give ethyl a,7-diarylallophanates even at room temperature (24). 

The cyclotrimerization reaction of alkyl- and aryl isocyanates to give trisubstituted isocya-nurates (hexahydro-5-triazinetriones) is a well known reaction. Many base and acid catalysts are effective and only highly sterically hindered isocyanates fail to undergo trimerization. In all cases the symmetric trimers 17 are obtained, and only in the trimerization of methyl isocyanate using a trialkylphosphine catalyst is the unsymmetrical trimer 18 isolated as a coproduct... 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

N-Substituted amides can be prepared by direct attack of isocyanates on aromatic rings.The R group may be alkyl or aryl, but if the latter, dimers and trimers are also obtained. Isothiocyanates similarly give thioamides. The reaction has been carried out intramolecularly both with aralkyl isothiocyanates and acyl isothiocyanates.In the latter case, the product is easily hydrolyzable to a dicarboxylic acid this is a way of putting a carboxyl group on a ring ortho to one already there (34 is... 

---