Catalysis isocyanate reaction

I. S. Bechara, The Mechanism of Tin-Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols, in Urethane Chemistry and Applications, ACS Symposium Series 172, K. N. Edwards, (Ed.), American Chemical Society, Washington, DC, 1981. 

Chloroperoxidase catalysis by, 58, 302 in chlorination of pyrazoles, 57, 337 Chlorophyll, thioaldehyde synthetic intermediate to, 55, 3 Chlorosulfonyl isocyanate, reaction with 2-arylhydrazono-3-oxobutanoate, 59, 148 Chromatography, of [l,2,4]triazolo[l,5-a]-pyrimidines, 57, 106 Chrom-3-enes, see 2//-l-Benzopyrans Chromium tricarbonyl complexes of 3,5-diphenyl-l-(alkyl- or oxido-)-thiabenzenes, 59, 206, 227 indoles, lithiation of, 56, 181, 184 of pyridine, 58, 160 pyridines, lithiation of, 56, 230, 239 of 2f/-thiopyrans, 59, 227 Chromones, see l-Benzopyran-4-ones Cinnamonitrile, a-cyano-, condensations with thio-, seleno-amides, 59, 184, 186 Cinnoline, nitration, MO calculation, 59, 302... 

Although reactions of aromatic isocyanates with alcohols generally follow second-order kinetics or modified second-order kinetics as outlined by Baker and Holdsworth [19], Sato [126] reported that aliphatic and edkenyl isocyanates were much more subject to alcohol and urethane catalysis. The reactions of these compounds were found to ee with the expression... 

In contrast to the general references given above, this chapter is concerned specifically with catalysis of isocyanate reactions. Reactions of isocyanates provide an example of classical catalysis in that a catalyst-reactant complex is first formed which is then able to react with a second reactant molecule with an over-all high reaction velocity and specificity. Factors affecting rate and amount of complex formation, provision of paths of low activation energy, as well as steric and electronic effects, are all important. 

The catalytic order was 1.0 for the acetonylacetonates of Cr, Cu, Ck), V, and Fe. Only for the Mn compound was n = 1.75. This result indicates that in the concentration range studied more than one manganese atom is necessary to provide the intermediate complex necessary for the catalysis of the isocyanate reaction. While no specific picture is made as to the nature of the metal catalysis, it is suggested that this type of catalysis may be associated with the formation of the triplet state and related to the paramagnetic properties of the metal. 

Catalysis of Isocyanate Reactions with Protonic Substrates... 

Entelis assumes the formation of an activated alcohol-isocyanate binary complex during the catalysis of the methanol-phenyl isocyanate reaction by dibutyltin dilaurate (DBTDL) (3, 5) Activated alcohol-isocyanate-catalyst ternary complexes have also been proposed by others. However, significant differences can be noted in the structures of either the postulated one (2, 4, 6, 7) or two (8) coordination positions of the isocyanate to the metal. To explain the synergistic effects observed when tertiary amine and organometallic compounds are combined, several authors suggest the formation of an activated quaternary complex I, II or III (2, 6, 9, 10, 11, 27). 

Thus, several mechanistic pathways based on polarization effects have been proposed to explain the catalysis of the alcohol-isocyanate reaction. These propositions appear to be often unsatisfactory and cannot explain even the majority of the experimental results reported in the literature. For an example, why is the polyurethane formation catalyzed by potassium acetate (JJ and not at all by MgC03 nor CsCl (14) The role played by the nature of the metal remains also unexplained. Robins reports the incremental temperature rise noted 1 minute after the mixing of reagents and catalyst (7, 16). This parameter is related to the catalytic activity and is an effective way to show the role played by the organometallic compound in its interaction with the alcohol. A similar conclusion can be drawn from Table I (15) where the Sn+ derivative is much more active than the Sn+2 oxidation state or Pb+. ... 

The Mechanism of Tin—Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols... 

Figure 6. Effect of Ph,P on the catalysis of isocyanate reactions by DBTDL. Kev solvent — dioxane 30°C catalyst concentration = 0.0014M [PhNCO] — fOH] = 0.07M. PhNCO 4- H.O X, DBTDL DBTDL + Ph3P. PhNCO + 2-BuOH 6, DBDTL A, DBTDL + Ph J>.

It is well established that under certain reaction conditions isocyanates can form cyclic trimers or linear polymers. Shashoua et al. (1) have found that catalysis and reaction temperature are key factors which determine the composition of the resulting reaction products. They observed that the formation of linear polymers proceeded only at low temperatures (< -20°C) and the formation of cyclic trimers at ambient or higher temperatures. Under certain steric conditions formation of polycyclic structures were observed by Butler et al. 02,3) and Iwakura (4). 

A great number of both tertiary amines and metal catalysts are available, and a detailed discussion of these as well as their mechanisms is beyond the scope of this paper. The reader is referred to several reviews on the subject of catalysis in isocyanate reactions (118-120). 

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 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. 

Huynh-Ba, G., and Jerome, R. (1981). Catalysis of isocyanate reactions with protonic substrates a new concept for the catalysis of polyurethane formation via tertiary amines and organometaUic compounds. In Urethane Chemistry and Applications (Edwards, D. N., ed.), ACS, Washington D.C. 

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. 

As often happens with polymerization reactions, simple rate laws can seldom describe the whole course of reaction because of catalysis or inhibition by the urethane groups formed or by the initial reagents. Self-association of metallic catalysts, or their loose complexation by products or reagents, also prevents correlation of rates of reaction by simple proportionality or even power-law relations. Catalysis of isocyanate reaction with hydroxyls [250, 251] is by far the best understood. 

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). 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

Phenyl isocyanates are generally more reactive than alkyl isocyanates in their reactions with alcohols, but with CuCl catalysis even alkyl isocyanates will react readily with primary, secondary, or tertiary alcohols (45-95% yield). ... 

Carbodiimide functionality can be produced by reacting isocyanates at elevated temperature with proper catalysis (Scheme 4.15). Although carbodiimides undergo a variety of reactions,23 most commonly as dehydrating agents, in the presence of excess isocyanate they will form uretone imines. This not only increases the average functionality of the isocyanate product but also lowers its freezing point. For example, a liquefied (or modified) version of 4,4,-MDI can... 

Verkade and co-workers have shown the usefulness of their phosphazanes in various stoichiometric as well as catalytic reactions <1999PS(144)101>. Compound 290 was used to promote the cyanohydration of benzaldehyde with trimethylsilyl cyanide (TMSCN). The cyanohydrin was isolated in 95% yield, but no enantioselectivity was noticed <2002JOM(646)161>. Compounds 291 and 292 were attached to dendrimers and shown to be effective in the catalysis of Michael reactions, nitroaldol reactions, and aryl isocyanate trimerizations <2004ASC1093>. 

A related cyclization of 2-(alkynyl)phenylisocyanates with terminal alkynes to oxindoles was also reported by the same group (Equation (115)).472 (E)-exo-olefinic oxoindoles are selectively obtained. It was proposed that a palladium acetylide generated by the C-H activation of terminal alkynes regioselectively inserts to the alkyne moiety and the resulting vinylpalladium intermediate adds to the C=0 part of the isocyanate to give a (Z)-oxindole. This (Z)-isomer is isomerized to the ( )-isomer under the reaction conditions through catalysis of the phosphine. 

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