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. 

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... 

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>. 

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 ... 

Thus, as shown in detailed studies of the trimer, two isocyanate molecules are supplied by the dimer, and the third by the isocyanate portion of the allophanate. This mechanism applies specifically for the trimerization reaction of an isocyanate carried out in the presence of a tertiary amine catalyst and either an alcohol or a urethane. 

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

Due to the lability of the alkyl-oxygen bond in alkyl cyanates, the direct trimerization reaction to 2,4.6-trialkoxy-l,3,5-triazines runs into difficulties. Often side reactions, such as isomerization to isocyanates (Lewis acid catalysis)242 or alkylation reactions with nucleophilic catalysts or anions,243 are observed. Steric hindrance or strongly electron-accepting substituents enhance the stability of alkyl cyanates in these cases selective trimerization to the corresponding... 

Carelli et al. devised another procedure for the preparation of isocyanurates from isocyanates. The trimerization reaction was promoted by an anionic species obtained in situ by electrochemical reduction of catalytic quantities of a-bromoesters in dipolar aprotic solvents. Phenyl and 1-naphthyl isocyanate were trimerized in 90% and 85% yield respectively <85SC249>. 

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... 

Isocyanates will also undergo dimerization and trimerization reactions, such as those shown in Eqs. (4) and (5). The dimers (7), which can only be formed from aromatic isocyanates, are termed uretidinediones or uretidiones and the trimers (8) are known as isocyanurates. A catalyst is generally needed for these reactions to take place, although certain non-sterically hindered aromatic isocyanates, such as 4,4 -diphenylmethane diisocyanate (MDI) will slowly dimerize at room temperature without a... 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. The trimerization reaction proceeds best at 90-100°C. These temperatures can be achieved using a heated conveyor or a RIM machine. The key to the formation of PUIR foams is catalysis. Strong bases, such as potassium acetate, potassium 2-ethylhexoate, and tertiary amine combinations, are the most useful trimerization catalyst. A review on the trimerization of isocyanates is available (104). 

Polymer-supported, carbon dioxide-protected NHCs were also prepared for catalysis reactions [150]. These supports were prepared using DMN-H6 with 3-(bicyclo[2.2.1]hept-2-ene-5-ylmethyl)-l-(2-propyl)-3,4,5,6-tetrahydropyrimidin-l-ium-2-carboxylate and a Schrock catalyst. They were later used for the trimerization reaction of isocyanates and for the cyanosilylation of carbonyl-containing compounds. Various metals could be immobilized through the NHC hgands including rhodium(I), iridium(l), and palladium(II). The resulting monoUths polymerized phenylacetylene and were successful catalysts for Heck-type couplings [150]. 

Heman-Gomez A, Bradley TD, Kennedy AR, Livingstone Z, Robertson SD, Hevia E. Developing catalytic applications of cooperative bimetallics competitive hydroamination/trimerization reactions of isocyanates catalysed by sodium magnesiates. Chem Commun. 2013 49(77) 8659—8661. 

We at NASA reopened the question of learning to chain extend these fluoroether systems In consort with scientists at PCR Inc. The objective was to prepare long-chain difunctional polyperfluoroethers and investigate chain extension mechanisms, as well as to convert these materials to stable cross-linked polymers for sealant applications. The nitrile, acetylene, and isocyanate groups were considered. Each of these is capable of both trimerization reactions and dipolar cycloadditions. From this base line, one could then evoke both chain extension and cross-linking with a variety of reaction schemes. 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

Interesting is the participation of sulfonyl isocyanates and sulfonyl carbodiimides in mixed trimerization reactions although these monomers do not undergo cyclotrimerization reactions themselves. For example, dicyclohexylcarbodiimide reacts with two equivalents of A -p-toluenesulfonyl-Af -cyclohexylcarbodiimide to give the six membered ring [2+2+2] cycloadduct 14 in 93 % yield... 

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... 

The high selectivity for dimer formation observed with trialkylphosphines can be explained by a stepwise mechanismThe general base catalysis of oligomerization of isocyanates involves the dimeric species as intermediates in the formation of the more stable isocyanate trimers (isocyanurates). Steric effects also play a role in the dimerization of isocyanates because o-tolyl isocyanate does not dimerize. This fact is utilized in the selective dimerization of 2,4-TDI 7, using a polymeric imidazole catalyst. In this reaction only the non-hindered isocyanate group participates to form the [2-1-2] cyclodimer 8. 

The reaction of alkyl bromides or alkyl iodides with potassium cyanate in DMF is a very useful method to synthesize many aliphatic isocyanate trimers. The reaction of aromatic isocyanates with ethanol in the presence of A-methylmorpholine is another useful synthetic method to produce aromatic isocyanate trimers. The initially formed ethyl carbamate is an intermediate in the trimerization reaction. 

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