Isocyanates cyclotrimerization

PhCH2=Ru(PCy3)2Cl2 catalyses the cyclooligomerization of trienes (143) to benzene derivatives (144) via a cascade of four metathesis reactions (Scheme 54).273 Isocyanate cyclotrimerization catalysed by dimethylbenzylamme-phenyl glycidyl ether-phenol has been studied by IR and PMR spectroscopy.274... 

The synthesis and properties of 2-oxazolidones and 2-oxazolidone-containing polymers have been reviewed by Pankroatov et al (115). It is interesting to note that some isocyanate cyclotrimerization catalysts also act as catalysts for urethane, oxazolidone and/or carbodiimide linkages. 

Complex formation of crown type with oxyethylene fragments of MGF-9 resin as the catalysts of ion phase transfer and isocyanate cyclotrimerization. 

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 increases, whereas the disubstituted urea production remains practically the same. It may be concluded that disubstituted urea formation is determined by availability of free water molecules, while alkah concentration and NCO/OH group ratio influence the reactions of isocyanate cyclotrimerization and amine and sodium carbamate formation significantly. 

Pankratov, VA., Frenkel, T.M., Shvorak, A.E., Lindeman, S.V and Struchkov, Y.T., The preparation and X-ray structural study of conplexes that catalyze isocyanate cyclotrimerization, Russ. Chem. Bull. 42 (1), 81-87 (1993). 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

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). 

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... 

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 kinetics of catalysis of cyclotrimerization was studied on the model system phenyl isocyanate/ace-tonitrile (solvent). Acetonitrile (AN, 99.64%, from Vinstron Corp.) was purified by refluxing with phosphorus pentoxide (5 g/1), then with calcium hydride (2 g/1) followed by distillation under nitrogen. Phenyl isocyanate was obtained from the Upjohn Company with a purity of 99.5%, and was purified by distillation. Tolylene diisocyanates (2,4 and 80/20 2,4/2,6 isomers) were obtained from the Mobay Chemical Co., and were purified by distillation. Cyclic sulfonium zwitterions (SZ) were obtained from the Dow Chemical Co. 

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). ... 

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

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). 

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. 

Trimerization of imidates is a valuable route to 1,3,5-triazines. Imidates can be considered as activated nitriles and cyclotrimerize more readily. Most symmetrical 2,4,6-trialkyl-1,3,5-triazines are easily formed, although large alkyl substituents may give rise to steric hindrance (61JOC2778). Symmetrical isocyanurates (525) are readily available from isocyanates, RNCO catalysts include tertiary amines, phosphines and sodium methoxide. Aldehydes RCHO and ammonia give hexahydro-1,3,5-triazines (526), known as aldehyde ammonias (73JOC3288). 

Keywords alkyl isocyanate, microwave irradiation, cyclotrimerization, isocyanu-... 

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. 

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

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 polyisocyanate-based polymer-forming reactions can be classified into three types of reactions addition reactions, condensation reactions, and cyclotrimerization reactions. Among the isocyanate reactions shown in Table 2, the addition reaction is the major isocyanate reaction in polyurethane foam preparation. A model addition reaction is shown below ... 

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

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. 

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