Isocyanates trimerization

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>. 

There are few examples of this type of synthesis. Typically, an azirine reacts with isocyanates or isothiocyanates to yield the triazine via a series of dipolar intermediates. The products may be contaminated with the isocyanate trimer the example of Scheme 114 below is typical. The novel spiro compound (188) is formed by a similar route (Scheme 115) (79HCA1429, 81LA264). 

When compound 13 is heated above the melting point, it undergoes rearrangement to give Wethyl-l,2,4-dithia-zolidine-3,5-dione 44 as the main product and triethyl isocyanurate 45, an ethyl isocyanate trimer, as the minor product (Equation 5) <1996JOC6639>. 

Benzoyl isocyanate trimerizes to form 7-benzoylimino-2,5-diphenyloxazolo[5,4-rf]-[l,3]oxazine (243). The latter in the presence of sodium methoxide undergoes a Dimroth-type rearrangement to yield the fused pyrimidine (244) after debenzoylation (72JOC2583). 

Modification of poly(carbodiimide) foams with polyols afford hybride foams containing urethane sections. However, the thermal stabilities of the poly (urethane carbodiimide) foams are lower. Using isocyanate trimerization catalysts, such as l,3,5-tris(3-dimethylaminopropyl)hexahydro-s-triazine, in combination with the phospholene oxide catalyst gives poly(isocyanurate carbodiimide) foams with improved high temperature properties. The cellular poly(carbodiimide) foams derived from PMDI incorporate six-membered ring structures in their network polymer structure. ... 

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 ... 

A comprehensive review of trimerization catalysts was prepared by Zhitankina et al (116). Isocyanate trimerization catalysts are shown in Table 11. 2-Oxazolidone catalysts and carbodiimide catalysts are shown in Tables 12 and 13 respectively. 

In the case of urethane-modification, two kinds of catalysts, i.e., urethane-formation catalysts and isocyanate-trimerization catalysts, are usually used. Major trimerization catalysts are listed in Table 11 and major urethane catalysts are listed in Tables 8 through 10. 

Another possible cross-linking reaction is isocyanurate, or isocyanate trimer, formation ... 

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. 

Because of the high cross-Knk density of polyisocyanurates, the resultant foam tends to be brittle. Consequently, there has been a move toward making polyisocyanurate-polyurethance combinations. For example, the isocyanate trimerization reaction has been carried out with isocyanate end-capped TDI-based prepolymers to make isocyanurate-containing polyurethane foams. Isocyanate trimerization in the... 

IR spectral analysis shows that isocyanate trimerization with aqueous alkali solutions takes place independently of the formation of disubstituted urea, amine, carbamates, and sodimn carbonate. The qualitative output of these reaction products varies depending on the NaOH concentration and the ratio of NCO and OH groups. 

Figure 3.29 depicts individual band intensities of the end products calculated by comparison with internal stand2ird (1600 cm ) versus NaOH concentration. The quantity of NCO groups failing to react (2275 cm band) in the end products of reactions (2)—(6) is sharply reduced at NaOH concentrations above 20%, and the output of disubstituted urea (1640 cm band) shows almost no modification with NaOH concentration change. Sodiiun carbamate (1580 cm band), amine (1620 cm band), and isocyanate trimer (1715 cm band) notably increase. The qualitative ratio of these compoimds in reactions (2)— (6) products is probably determined by hydration in the NaOH—H2O system. 

Change in the ratio of NCO and OH groups leads to redistribution of the end products. When the NCO/OH ratio is 1 3 rather than 1 1, output of isocyanate trimer and amines incresises, whereas the disubstituted luea production remains practically the same. It may be concluded that disubstituted urea formation is determined by availability of free water molecides, while alkali concentration and NCO/OH group ratio influence the reactions of isotyanate cyclotrimerization and amine and sodium carbamate formation significantly. 

Consider isocyanate trimerization as an example. This reaction in the presence of alkali and alkaline-earth metals was described in detail earlier. With crown-ether in contact with the concrete surface, it forms a crown-complex with the calcium cation of calcium oxide... 

The polymerization of the isocyanate groups to high-molecular-weight products occurs only at low temperatures and with specific initiators. At somewhat higher temperatures, reactive isocyanates are dimerized to uret-diones (labile), while at even higher temperatures, less reactive isocyanates trimerize to isocyanurates (stable) ... 

Polyisocyanurate (PIR) n. A polymer containing isocyanurate rings, i.e., isocyanate trimer, and forming foams that have better fire resistance than rigid polyurethanes, but are more brittle, so are... 

Aliphatic isocyanurates (isocyanate trimers) have a strong C=0 band at 1700-1680 cm with a weak shoulder near 1755 cm" Aromatic isocya-nurates have a higher C—0 frequency at 1715-1710 cm with a weak shoulder near 1780 cm" because of the electron withdrawing group on the nitrogen. 

In the reaction of the phenyl phosphoramidate anion 53 (R = Ph) with carbon dioxide only triphenyl isocyanurate is obtained because the generated phenyl isocyanate trimerizes under the reaction conditions. Also, from sodium diethyl A -alkoxy-phosphoramidate (R = OR) and carbon dioxide only 1,3,5-trialkoxyisocyanurates are obtained . 

Isocyanates undergo dimerization reactions by a [2+2] cycloaddition across their C=N bonds to give diazetidinediones 3. The isomeric unsymmetrical dimers have never been isolated but they are postulated to be intermediates in the formation of carbodiimides from isocyanates. The isocyanate dimers usually dissociate back to the monomers on heating. Therefore, they are considered to be masked isocyanates. The dimerization of isocyanates requires the use of a base or a Lewis acid as a catalyst, and often isocyanate trimers are formed as coproducts. 

Aliphatic isocyanate dimers are not common, and usually low yields are obtained in their dimerization reactions. An exception is the use of 1,2-dimethylimidazol as a catalyst for the dimerization of benzyl isocyanates, which provides the cyclodimers in good yields (see Table 3.1). In the benzyl isocyanate dimerization benzyl isocyanate trimers are also formed as coproducts, and when the reaction is conducted for more than 16 h at room temperature the dimers are slowly converted to trimers. The structure of the catalyst is of importance, as shown in Table 3.1. A slight change in the substituents of the heterocyclic carbene catalyst affords either a 64 % yield of cyclohexyl isocyanate dimer or a 100 % yield... 

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