Isocyanates cyclotrimerization reactions

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). ... 

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 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 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. 

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 properties of polycyclotrimers depend on the crosslink density, on the flexibility of the isocyanate chain, and on the degree of the completion of the cyclotrimerization reaction. 

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. 

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. 

The cyclotrimerization of carbon cumulenes is usually initiated by heat or catalysis. Especially, the use of a catalyst assures that trimerization can be accomplished in quantitative yields. The base catalyzed cyclotrimerization reaction seems to be limited to ketenes, isocyanates, isothiocyanates and carbodiimides. In the trialkylphosphine catalyzed trimerization of methyl isocyanate an asymmetric trimer is obtained. 

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... 

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... 

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. 

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... 

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 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. 

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 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... 

Although the reaction mechanism is not yet fully understood, cyanic acid or isocyanic acid seems to be the first decomposition product of urea. Trimerization of cyanic acid leads to cyanuric acid (see section on Cyanuric Acid, p 685). In the presence of catalysts, cyanic acid disproporlionates into carbon dioxide and cyanamide, which then trimerizes to melamine. Thus, decomposition products, and nol urea itself, cyclotrimerize to provide the 1,3,5-triazine rings. These reactions form the basis for the industrial production of cyanuric acid286-301 and melamine.302-315... 

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>. 

Isocyanates have several more reactions that are important in some more specialized applications (Fig. 3.2). Cyclotrimerization produces the isocyanurate ring, which is extremely stable, and can be used to build more heat resistance into polyurethanes. Excess isocyanate can react with the N-H group in polyurethanes to produce allophanate crosslinks, which add to the cure of the polyurethane. And excess isocyanate can similarly react with the N-H groups in polyureas to produce biuret cross-links, which add to the cure of the polyurea. 

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). 

This approach was then extended to the immobilization of diynes, for example, 64, and their reaction with a soluble nitrile. This immobilization strategy effectively suppressed undesired reactions, including the formation of benzenes via trimeiization of the alkynes, and does not require the typically high-dilution and syringe pump conditions required in solution phase. " The reaction of 64 with a variety of nitriles afforded fused pyridines 65 in excellent yields after cleavage from the resin to 66 (Scheme 6.16). In contrast, a solution-phase reaction of trityl-protected dipropargylamine under otherwise identical conditions resulted in only a 46% yield due to both pyridine formation between two diynes and the nitrile, and benzene formation from the reaction of two or three diynes. Similar results were observed in the formation of pyridones 67 (via cyclotrimerization with isocyanates, R NCO) and iminopyridines 68 (via cyclotrimerization with carbodiimides, R NCNR ). 

The first book on the reactions of carbon cumulenes, treating the cycloaddition reactions of ketenes in depth, was written by Staudinger in 1912 Staudinger already realized that cycloaddition reactions of ketenes are common, and often ketenes were only isolated as cyclodimers. The cyclodimers of isocyanates became prominent in the development of polyurethanes in the IG Farben Laboratory in Leverkusen, Germany in the early 1930s and the cyclotrimerization of diisocyanates led to the development of polyisocyanurate foams, with thermal stability superior to rigid polyurethane foams in the 1960s. Today, polyisocyanurate foams are used in the insulation of the fuel tank of the space shuttle. Also, carbodiimide derived cellular plastics with improved thermal stability are of interest. In recent years, cumulene derived polymers became of interest as one-dimensional molecular wires. 

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