Isocyanates dimerization catalysts

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

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 ... 

Free radical catalysts, such as potassium persulfate and azo compounds, are ineffective in initiating this type of polymerization of isocyanates. Also, catalysts, such as triethylphosphine and triethylamine, which are known to catalyze dimer and triraer formation at higher temperatures, are not effective in initiating the polymerization to linear 1-nylons. 

Isocyanates can also react with other isocyanate molecules to form oligomers (Fig. 4). This polymerization is more likely to occur in the presence of basic catalysts.Isocyanate dimers, also called uretidine-diones, can only be formed by aromatic isocyanates, and uretidinedione formation is inhibited by ortho substituents. Hence, only 4,4-diphenylmethanediiso-cyanate (MDI) dimerizes at room temperature, and its rate of formation is quite low. At higher temperatures, MDI would form an insoluble polymeric mate-rial. Trimers of isocyanates are also possible these structures are called isocyanurates. Isocyanurates are formed by both aliphatic and aromatic isocyanates, and the resulting structure is highly stable to a temperature of approximately Isocyanurates give... 

Isocyanate dimerization is an equilibrium reaction. Dissociation of the dimer occurs only at elevated temperatures (42). In the absence of catalysts, temperatures as high as 175 C are required to completely dissociate the dimer of 2,4-TDI, although initial dissociation was observed to occur at 150 C (43). The preparation of aliphatic isocyanate dimers has also been reported (44). 

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... 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

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. 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Buckles and McGrew [J. Am. Chem. Soc. 88 (15), 1966] have studied the dimerization of phenyl isocyanate in liquid solution in the presence of a catalyst. 

The synthesis of the module is provided in Scheme 10.5 (Kushner et al. 2007). Double alkylation of ethyl acetoacetate followed by guanidine condensation afforded alkenyl-pyrimidone intermediate 24 (Kushner et al. 2007). Isocyanate 25 was coupled to pyrimidone 24 to yield 26. Upon dimerization in DCM, RCM effectively cyclized the two UPy units (Mohr et al. 1997 Week et al. 1999). A one-pot reduction and deprotection through hydrogenation using Pearlman s catalyst gave diol module 27. Finally, capping 27 with 2-isocyanatoethyl methacrylate at both ends provided the UPy sacrificial cross-linker 28, which was thoroughly characterized by H- and C-NMR, Fourier transform IR (FTIR), and mass spectrometry. 

Dimerization of phenyl isocyanate, catalyzed by lanthanide complexes, has been reported by Deng et al. <2003CHJ574>. A number of lanthanide complexes were tried and Sm(SPh)3(hmpa)3 was found to be the most effective catalyst. Conversion was as high as 96% with 2500 1 of substrate to catalyst ratio (Scheme 47). 

One of the major fields of isocyanate catalysis is pol3mierization. The formation of cyclic dimers and trimers from aryl isocyanates was established over 100 years ago by Hofmann (8). This early work has been reviewed by Saunders (1). Dimers are formed by aryl isocyanates at room temperature in the presence of certain amines or phosphines. Trimeriza-tion occurs in the presence of bases such as potassium acetate. Linear polymerization has been recently reported by Shashoua and co-workers at lower temperatures in polar solvents with an anionic catalyst such as metallic sodium (9, 10). 

Which polymer is formed depends upon the relative rates of subsequent reactions. If chain termination then occurs with loss of X, a cyclic dimer is produced if a third isocyanate molecule is added, followed by loss of X, a cyclic trimer occurs if chain termination is relatively slow, addition of further monomers takes place with formation of a linear polymer. Conditions such as temperature, catalyst concentration, and character contribute to the reaction pattern. The tendency to cyclize no doubt plays a specially large part in isocyanate polymerization. 

Catalysts which have been found to promote dimerization of phenyl isocyanate include pyridine (11), methylpyridine (12), triethylamine (13), X-methyl- (or ethyl-)morpholine, triethylphosphine, and other alkyl or alkyl-arylphosphines (14, 15). Alkylphosphines bring about a very violent polymerization since they act as active catalysts and the polymerization is quite exothermic. Triphenylphosphine is inactive. Alkyl-arylphosphines are not as active as alkylphosphines and permit better control of the reaction. Another convenient method (14, 16) for control of phosphine-catalyzed dimerization involves the addition of an alkylating agent such as benzyl chloride in an amount stoichioraetrically equivalent to the substituted phosphine present. Complete deactivation of the catalyst results. By this means the reaction may be mitigated or even quenched and then activated by the addition of more catalyst. 

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. 

Other aliphatic dllsocyanates that are being used commercially in urethane coatings are 3-(isocyanatomethy1)-3,5,5-trimethyIcyclohexyl isocyanate (Veba-Chemie A.G.) (28). "dimeryl" diisocyanate derived from dimerized linoleic acid (Henkel Corp.) (29). and xylylene diisocyanate (XDI) (Takeda Chemical Co.) (30). It is interesting to note that no catalysts are required for the reaction of XDI with hydroxyl compounds and that its reactivity is similar to that of TDI. 

The rate of self-polymerization of isocyanates to dimers (uretidine diones) depends upon the electronic or steric influences of ring substituents. Ortho substitution greatly retards dimerization of the NCO groups, with the ortho NCO slower to dimerize. This dimerization is catalyzed strongly by trialkyl phosphines (38, 39) and more mildly by tertiary amines, such as pyridine (40. 41). MDI dimerizes slowly on standing at room temperature even without catalysts but is stable at low or at slightly elevated temperatures (40-50 °C). 

In contrast to dimer formation, trimerization is not an equilibrium reaction since trimers are stable in the range of 150-200 °C. Ortho substitution of an aromatic isocyanate greatly reduces the ease of trimerization. Many catalysts have been reported for the trimerization of aliphatic and aromatic isocyanates (39. 45-57). 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

Like cyanic esters, isocyanic esters can dimerize and trimerize. The dimeric uretediones75,76 are formed under the influence of trialkylphosphine catalysts, but the trimeric isocyanuric esters are formed when potassium acetate76 or sodium ethoxide77 is used. 

Pro-azaphosphatranes lb [93], Ic [67a], and Id [67a] are superior catalysts for the trimerization of isocyanates at room temperature in nearly quantitative yield, with Id reacting considerably faster than lb which was much faster than Ic. By contrast, P(NMe2)3 produces only small yields of the dimer, which is an undesirable impurity in activators. 

Aryl isocyanates readily polymerize in the presence of catalysts into dimers containing 2 moles of the monomeric isocyanate. The reaction and structure is generally considered to be ... 

Finally, it is emphasized here that isocyanates are very reactive chemical species, which do not react only with alcohols. For example, under certain conditions, they can react with themselves at room temperature to generate dimers (even without catalyst) and trimers (Figure 10.5). Because of the electrophilic nature of the -N=C=0 carbon, isocyanates react with most protic compounds too (Figure 10.6). Of course, in that regard, they can also react... 

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... 

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