Amine catalyzed isocyanate reactions

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

A series of compounds of other members of the group V family of the periodic system has been reported to catalyze isocyanate reactions. These are tributylantimony arsenic, or bismuth oxide for polyester foams, antimony dicarboxylate, and soaps of antimony, bismuth and arsenic. These catalysts are generally claimed to be as efficient as amines, do not catalyze hydrolysis and give odourless polyurethanes. 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

Several syntheses of l,3-dioxoperhydropyrrolo[l,2-c]imidazoles have been developed using different strategies. a-Substituted bicyclic proline hydantoins were prepared by alkylation of aldimines 135 of resin-bound amino acids with a,tu-dihaloalkanes and intramolecular displacement of the halide to generate cr-substituted prolines 136 and homologs (Scheme 18). After formation of resin-bound ureas 137 by reaction of these sterically hindered secondary amines with isocyanates, base-catalyzed cyclization/cleavage yielded the desired hydantoin products <2005TL3131>. 

Hydrolysis of the ester forms adipic acid, used to manufacture nylon—6. Carbonylations of nitroaromatics are used to synthesize an array of products including amines, carbamates, isocyanates, ureas and azo compounds. These reactions are catalyzed by iron, ruthenium, rhodium and palladium complexes. For example, carhonylation of nitrobenzene in the presence of methanol produces a carbamate ... 

Initially, water can cause the hydrolysis of the anhydride or the isocyanate, Scheme 28 (reaction 1 and 2), although the isocyanate hydrolysis has been reported to occur much more rapidly [99]. The hydrolyzed isocyanate (car-bamic acid) may then react further with another isocyanate to yield a urea derivative, see Scheme 28 (reaction 3). Either hydrolysis product, carbamic acid or diacid, can then react with isocyanate to form a mixed carbamic carboxylic anhydride, see Scheme 28 (reactions 4 and 5, respectively). The mixed anhydride is believed to represent the major reaction intermediate in addition to the seven-mem bered cyclic intermediate, which upon heating lose C02 to form the desired imide. The formation of the urea derivative, Scheme 28 (reaction 3), does not constitute a molecular weight limiting side-reaction, since it too has been reported to react with anhydride to form imide [100], These reactions, as a whole, would explain the reported reactivity of isocyanates with diesters of tetracarboxylic acids and with mixtures of anhydride as well as tetracarboxylic acid and tetracarboxylic acid diesters [101, 102]. In these cases, tertiary amines are also utilized to catalyze the reaction. Based on these reports, the overall reaction schematic of diisocyanates with tetracarboxylic acid derivatives can thus be illustrated in an idealized fashion as shown in Scheme 29. 

In a closer study of the amine-catalyzed reaction of phenyl isocyanate with alcohols in toluene solution, Burkus 38) found that contrary to Baker s results observed in dibutyl ether solution, the second-order rate constant was not a linear function of the triethylamine concentration. 

In the presence of a tertiary base (B), there is a base-catalyzed reaction between the amines and isocyanate superimposed on the spontaneous and the product-catalyzed reactions. According to Baker and Bailey, the following steps are involved in the base-catalyzed reaction. First there is the reaction involving the isocyanate-base complex and the amine ... 

Iwakura and Okada 64) interpret the mechanism and kinetics of the fert-amine catalyzed reactions of isocyanates with n-butanethiol and n-dodecanethiol somewhat differently. They found that the catalyzed reaction was strictly first order with respect to the thiol, isocyanate, and... 

The prime function of tertiary amines is to catalyze the reaction of isocyanate with water to form COj gas for blowing the polymer. 

In the first reported direct A -carbonylation of nitroaromatics to isocyanates, simple Pd- or Rh-based systems were used to catalyze the reaction of aromatic mononitro compounds with carbon monoxide [11, 12]. Later, it became possible to work without the drastic reaction conditions that had been required initially, by using Lewis acid co-catalysts [13], Various catalysts and catalyst mixtures, normally based on Ru, Rh, or Pd complexes with co-catalysts, were described in numerous patents and publications [1, 3, 14—16], The careful choice of the composition of the triad consisting of metal salt, co-catalysts and ligand (preferably aromatic amines) led to efficient catalyst systems [14a-e] for the direct reductive carbonylation process. A quite active Pd-phenanthroline-H system with noncoordinating carboxylic acids such as 2,4,6-trimethlybenzoic acid as proton source is worth mentioning [14 d]. 

The reactivity of the model phenols and benzyl alcohols with phenyl isocyanate was determined in the presence of a tertiary amine (DMCHA) and a tin catalyst (DBTDL) by measurement of the reaction kinetics. The experimental results based on initial equal concentrations of phenyl isocyanate and protic reactants showed that the catalyzed reactions followed second order reaction with respect to the disappearance of isocyanate groups (see Figure 1). It was also found that a linear relationship exists between the experimental rate constant kexp, and the initial concentration of the amine catalyst (see Figure 2). In the case of the tin catalyst, the reaction with respect to catalyst concentration was found to be one-half order (see Figures 3-4). A similar relationship for the tin catalyzed urethane reaction was found by Borkent... 

Consider the alteration of the surface tension of the polymerizing POPT—TDI system with the addition of an RS substance such as L-19 the use of L-19 displays a number of features. Interaction of the TDI isocyanate groups with the L-19 amine groups occurs at a higher rate than reaction with the POPT hydroxyl groups. The amine groups of L-19 and of the product of the reaction with TDI are capable of catalyzing the reaction between TDI and POPT. 

As stated, tertiary amines catalyze both the hydroxyl/isocyanate and the water/isocyanate reactions. One-shot foams utilizing primary hydroxyl-terminated polyesters as well as all types of prepolymer foams require tertiary amine catalysis only. Polypropylene ether one-shot foam formulations based on triols, in part, because of their low viscosity (about 300 cP versus 10000-30000 cP for polyesters or prepolymers) require the use of tertiary amine-metal catalyst combinations, even if the percentage of primary hydroxyl groups in the polyether is increased by capping with ethylene oxide. This is because of the relatively low polypropylene glycol activity. 

The tertiary amine-catalyzed reactions cause branching and cross-linking and are primarily used for polyurethane foam formation. They are used to accelerate the isocyanate-hydroxyl reaction and give off carbon dioxide. Triethylenediamine is the most prevalent of the tertiary amine catalysts used for polyurethane manufacture. 

Farkas A, Strohm PF (1965) Mechanism of Amine-Catalyzed Reaction of Isocyanates with Hydroxyl Compounds. Ind. Eng. Chem. Fundam. 4 32-38. 

The reaction of isocyanates with alcohols and with water can be catalyzed by amines and by organometallic compounds. Tertiary amines, such as l,4-diazo-[2.2.2]-bicyclooctane (DABCO) or triethylamine, are particularly effective in promoting the isocyanate-water reaction, while organometallic complexes, such as dibutyltin dilaurate or stannous octoate, are very useful for catalyzing isocyanate-alcohol reactions. Numerous articles have been written on various aspects of the catalysis of isocyanate reactions and representative examples are cited in refs. 8-10. 

Di-tert-butyl dicarbonate, B0C2O Unfortunately, most of the known methods for transforming amines into isocyanates are not mild enough and furnish undefined products as a result of uncontrolled side reactions. However, 4-dimethylaminopyr-idine (DMAP)-catalyzed reaction with activated carbonates as Ci building blocks constitutes a convenient laboratory method for the phosgene-free isocyanation of amines. A procedure has been described whereby alkyl- and arylamines are converted into isocyanates in high yields by reaction with activated carbonates (for... 

A convenient method for the synthesis and derivatization of enantiopure a-iso-cyanato carboxylic acid esters starting from a-amino acid esters has been devised [314], The isocyanates are obtained in enantiomerically pure form (> 99% ee) by a DMAP-catalyzed isocyanation with B0C2O, which proceeds in 10 min at room temperature (for typical procedures employing B0C2O, see the B0C2O Section of the present chapter). In situ derivatization of the isocyanates by reaction with amines and alcohols affords the corresponding enantiopure ureas and carbamates. Methyl esters of various amino acids 472 have been carbonylated by B0C2O at ambient temperature [314]. 

The catalysis of isocyanate reactions has been extensively studied because of its critical importance in many of these processes. Noncatalyzed (or rather, self-catalyzed) reactions may sometimes be fast enough in practice isocyanate reactions with amines are so fast that only recent studies using stopped-flow methods could lead to useful data [255, 256], metallic or tertiary amine catalysts being ineffective in this case. 

For both procedures the reaction temperatures normally never exceed 90° C. Many diisocyanates can be used in these reactions. In those cases in which more than just statistical control is needed, isophorone diisocyanate (IPDI) is generally employed which has two different isocyanate groups (primary/secondary). The alcohol-isocyanate reactions are generally catalyzed with Lewis acids such as dibutyltin dilaurate (DBTDL). Tertiary amines can be used as well. 

Thermosetting acrylic binder systems utilize copolymers of functional and nonfunctional acrylic (or similar) monomers. The functional monomers are incorporated for reactivity with crosslinkers. The most common functional monomer for reactions is the hydroxyl group. The hydroxyl groups on the acrylic copolymers react with melamine and urea resins (amino resins) and with polyisocyanates. These reactions are shown in Figure 11. The reaction of hydroxy functional polymers with amino resins require acid catalysis and heat. The reaction with polyisocyanates can occur at room temperature as well as at higher temperatures. A number of materials will catalyze the hydroxyl/isocyanate reaction (organotin compounds, acids, amines, metal salts, etc.)(9). 

Because of the reactive nature of isocyanate groups with the mentioned functional groups, polyisocyanates can be formulated to react under a wide variety of cure conditions. Many substances catalyze these reactions. Common catalysts are organotin compounds, other metallic salts, amines, and acids. This breadth of catalytic species can also introduce problems, because catalysts may be unknowingly introduced from other components of a formulation or even as contaminants of other formulating materials. 

Scheme 1.14 Tertiary-amine-catalyzed thiol-isocyanate click reaction.

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