Cyclotrimerization of isocyanates

Louie and co-workers have shown the utility of NHCs in the cyclotrimerization of isocyanates [172], Isocyanurates were obtained in excellent yield with catalyst loading as low as 0.001 mol% (Eq. 37). 

According to Shashoua et al (1), the linear polymerization preferentially proceeds at temperatures below -30°C. At room temperature and at elevated temperatures, no formation of linear polymers was observed, but cyclotrimerization of isocyanates occurred. 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

The cyclotrimerization of isocyanates is initiated by anionic type of catalysts and proceeds via propagation, transfer and termination steps ( ). It was found that in the case where the cyclotrimerization reaction proceeds with a long kinetic chain length, the kinetics of the reaction followed second order with respect to the isocyanate as measured by the disappearance of the isocyanate groups and was first order with respect to the initial concentration of the catalyst ( ,5). ... 

It was found that the cyclic sulfonium zwitterions of the general formula IV are very active catalysts for cyclotrimerization of isocyanates (2). 

Similar effects of relative permitivity D on the cyclotrimerization were also observed in the case of substituted ammonium carboxylate catalysts. It was also observed that, besides relative permitivity, the specific solvation of reactants by aprotic dipolar solvents had a considerable effect on the rate constant of cyclotrimerization of isocyanates (see Table II). 

Duong HA, Cross MJ, Louie J (2004) TV-Heterocyclic carbenes as highly efficient catalysts for the cyclotrimerization of isocyanates. Org Lett 6 4679-4681... 

The isocyanurate linkage is obtained by the cyclotrimerization of isocyanate groups, as shown in the following model reaction. 

The solvents used in the cyclotrimerization of isocyanate had a very strong effect on the reaction rate with the increase of the relative permitivity of the solvent system, the cyclotrimerization rate constants increased. The experimental data... 

Triazine-2,4,6(l//,3//,5//)-triones are best prepared by the cyclotrimerization of isocyanates with CsF or tbaf catalysts. The electrochemical approach of Carelli et al. may also prove useful (see Section 6.12.9.5.2) <85SC249>. 

Poly(isocyanurates) are produced by the cyclotrimerization of isocyanates, for example, p-diphenyl diisocyanates (see also Section 28.4) ... 

Reactions involving the [4 + 1 + 1] principle, an example of which is shown in equation (136), are rather uncommon and of strictly limited utility [3 + 2 + 1] and [2 + 2 + 2] processes, on th,e other hand, are well known. Representative [3 + 2+1] three-bond formation processes are given in equations (137)—(141), from which it can be seen that the common situation is where ammonia, a substituted amine or formamide constitutes the one-atom fragment. Many [2 + 2 + 2] atom fragment syntheses are known and some are familiar reactions. Thus, the cobalt(I)-catalyzed condensation of nitriles and isocyanates with alkynes gives pyridines and 2-pyridones, often in excellent yield (e.g. equation 142), while the cyclotrimerizations of nitriles, imidates, isocyanates, etc., are well established procedures for the synthesis of 1,3,5-triazine derivatives (e.g. equation 143). Further representative examples are given in equations (144)-(147), and the reader is referred to the monograph chapters for full discussion of these and other [2 + 2 + 2] processes. Examination of the... 

During the initial stages of cyclotrimerization of difunctional isocyanates, the formation of polyfunc-tional NCO-terminated oligomers takes place ... 

The deactivation reaction transfers an active catalyst into the inert (non-reactive) polymer. This phenomenon, when cyclic sulfonium zwitterions act as anionic initiators, can be utilized for the control of the cyclotrimerization of difunctional isocyanates. Therefore the degree of oligomerization of difunctional isocyanates can be controlled by the concentration of the initiator, rate of addition of the initiator, as well as by the temperature of the reaction system. 

Related co-cyclotrimerizations of two alkyne molecules with limited isocyanates have also been achieved using cobalt and nickel catalysts. With respect to intramolecular versions, two examples of the cobalt(I)-catalyzed cycloaddition of a,m-diynes with isocyanates have been reported to afford bicyclic pyri-dones only in low yields, although 2,3-dihydro-5(lff)-indolizinones were successfully obtained from isocyanatoalkynes and several silylalkynes with the same cobalt catalysis [19]. On the other hand, the ruthenium catalysis using Cp RuCl(cod) as a precatalyst effectively catalyzed the cycloaddition of 1,6-diynes 21 with 4 equiv. of isocyanates in refluxing 1,2-dichloroethane to afford bicyclic pyridones 25 in 58-93% yield (Eq. 12) [20]. In this case,both aryl and aliphatic isocyanates can be widely employed. 

Multi]de insertion reactions of isocyanates have been observed in the presence of Ni catalysts. Pyri-midinediones are obtained in low yield from reaction of diphenylacetylene with excess alkyl isocyanates in the presence of Ni(COD). Similarly, alkyl and aryl isocyanates undergo simple cyclotrimerization to form symmetrical triazinetriones in the presence of both low-valent Ni and Ti catalysts. 

The spraying process is carried out at ambient temperatures. The spraying of urethane-modified isocyanurate foam is not as easy as urethane-foam spraying because the cyclotrimerization reaction of isocyanate groups requires relatively higher temperatures than for urethane foams. An example of the spraying of urethane-modified isocyanurate foams was reported (198). The spraying was conducted with formulations at a low-NCO/OH equivalent ratio. 

The complexes 222-226 exhibited a high catalytic activity on the cyclotrimeriza-tion of aromatic isocyanates to produce triaryl isocyanurates (Scheme 83). They are the first reported Cp-free rare-earth metal complexes showing high activity and selectivity on the cyclotrimerization of aryl isocyanates. For comparison, the starting trisamide complex [Yb N(SiMe3)2 3( T-Cl)Li(THF)3] was studied for the catalysis and showed a catalytic activity comparable with those of the new complexes 222-226. All the complexes showed no catalytic activity on the cyclotrimerization of 4-nitrophenylisocyanate and exhibited a relatively low catalytic activity on the cyclotrimerization of aliphatic isocyanates [167]. 

The cyclotrimerization of monofunctional isocyanates, preparation and properties of polycocyclotrimers based on isocyanates were investigated. 

It was found that the reactivity of isocyanates in the cyclotrimerization reactions increased with the presence of the electron withdrawing groups in the vicinity of the isocyanate group, with the increased nucleophili-city of a catalyst and relative permitivity of the solvent system. 

The isocyanurate rings are thermally stable (decomposition started at temperatures above A00°C (5)) and therefore cyclotrimerization of difunctional isocyanates can be utilized for preparation of thermally stable polymers. (2 J3)... 

Figure 1. The Hammett correlation for cyclotrimerization of substituted phenyl isocyanates. Key , p-MePhNCO O, PhNCO , p-CIPhNCO T, 40°C catalyst, sodium ethoxide solvent, DMF.

Figure 2. Effect of relative permitivity D of the solvent system on the cyclotrimerization of phenyl isocyanate. Conditions solvent system, acetonitrile-ethyl acetate catalyst, TMHAE and temperature, 25°C.

Cyclotrimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 1,3,5-triazines. New catalysts have been introduced in some cases to carry out these transformations with increased yields and under relatively milder reaction conditions. The cyclotrimerization of monocyanamides has been reviewed <89RCR879>. 

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. 

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