Isocyanates with alcohols

The reaction of isocyanates with alcohols to form carbamates is catalyzed by amines and a variety of organometaHic compounds. 

The reaction of isocyanates with alcohol is strongly exothermic. A portion of this heat can be removed and better quality control achieved by forming a prepolymer with half of the alcohol and the diisocyanate. The new prepolymer has isocyanate terminals and can be used with the remaining quantities of the difunctional and/or the trifunctional alcohol in the propellant mix. In recent years isocyanate-terminated polymers... 

For the most part, the preparation of monomeric and polymeric carbamates (urethanes), semicarbazides, and ureas consists of condensation reactions of isocyanates with alcohols, hydrazines, or amines. The synthesis of ureas and semicarbazides are described in Chapters 6 and 8, respectively. 

The reaction of a-naphthyl isocyanate with alcohols has been reported to be a convenient analytical method for the preparation of solid derivatives [16-18]. In addition, the by-product dinaphthylurea is very insoluble in hot ligroin (b.p. 100°-120°C). The urethanes are readily soluble in hot ligroin, and on cooling the solution they recrystallize to sharp-melting solids. It is recommended that two recrystallizations be performed to obtain substances for analysis. Primary alcohols react well without the need for heating the reaction mixture. Secondary alcohols require additional heat, and the yields of urethane oft are smaller than when primary alcohols are used. Tertiary alcohols other than /-butyl [17] or /-amyl [17] were not able to react under the conditions used. Table V lists some representative alcohols and their a-naphthylurethane derivatives. 

Amines can be linked to polymeric alcohols as carbamates. Carbamate attachment of amines can be achieved by reaction of isocyanates with alcohol linkers, or by treatment of alcohol linkers with phosgene [339,427,428] or a synthetic equivalent thereof, followed by exposure to the amine (Figure 3.26). The reagents most commonly used for the activation of alcohol linkers are 4-nitrophenyl chloroformate [69,429-436] and carbonyl diimidazole [427,437-440], The preparation of support-bound carbamates is discussed in Section 14.6. 

Brock [22] showed that the reaction of aryl isocyanates with alcohols could be correlated by the Hammett [14] linear free energy relationship, log fe/feo = p2o. Using data from published sources, with reaction temperatures in the range of 20—39.69°C and using several solvents, the value for p was calculated to be 1.69. Substituent constants (a) were given by Brock and also by Kaplan [123]. 

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... 

The discovery of the catalytic effect of metal compounds on the reaction of isocyanates with alcohols came relatively recently (37) and led eventually to finding of the most potent catalysts of this kind in certain organometallic derivatives of tin (SS). 

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. 

The rate of the triethylamine-catalyzed reaction of phenyl isocyanate with n-butanethiol depends on the solvent used and increases in the order benzene, toluene, dibutyl ether, dioxane, methyl ethyl ketone, and nitrobenzene. This solvent effect increased with the ionizing power of the solvent and was the reverse of that observed in the reaction of isocyanates with alcohol. 

The presently proposed mechanism of synergism implies that ligands other than amine which can coordinate with the tin ion should also synergize its catalytic activity toward alcohol-isocyanate reactions. This apparently is the case for triphenyl phosphine-DBTDL combination. When triphenyl phosphine was added to DBTDL, it accelerated the rate of reaction of isocyanate with alcohol and water (Figure 6). Although triphenyl phosphine is known to dimerize isocyanates, (12), the dimerization... 

The molten carbamate is first mixed with a paraffin fraction. A stream of nitrogen removes the methanol as it is formed, to prevent the recombination of isocyanate with alcohol. The effluent from the final reactor is purified in a series of three distillation columns. The first separates methanol at the top, the second the paraffin fraction, and the third, operating under vacuum, serves to collect the TDI at the bottom. 

It is possible to generalize the reaction of aryl isocyanates With alcohols, amines, and water. ... 

PURs include a wide range of materials ranging from thermoplastic elastomers to flexible as well as rigid foams. Polyurethanes are commonly formed upon reaction of isocyanates with alcohols (see Figure 8), in order to form polymeric structures both reaction partners have to contain at least two reactive functions (diisocyanates or diols). 

He, like other organic chemist, was aware of the reactions of organic isocyanates with alcohols and amines for the quantitative, exothermic, room temperature production of urethanes and ureas, respectively. Actually, Bayer had characterized a large number of new amines by the production of ureas from the action of isocyanates. 

Baker WB, Gaunt J (1949) The mechanism of the aryl isocyanates with alcohols and amines. Part III. The base-catalysed reaction of phenyl isocyanate with alcohols. Journal of the Chemical Society 9-18. 

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. 

Isocyanates react with alcohols and phenols to form urethanes. In general, rates of urethane formation decrease in the following order primary alcohols > secondary alcohols > 2-alkoxyethanols > l-alkoxy-2-propanols. Isocyanates can react with urethanes to form allophanates. This reaction is much slower than the reaction of isocyanate with alcohol. Isocyanates react rapidly with primary and secondary amines to form ureas. The reaction is much faster than the reaction of isocyanates with alcohols. Isocyanates can react with ureas to form biurets. Biuret formation is slower than urethane formation, but faster than allophanate formation. Isocyanates react with water to form imstable carbamic acids, which dissociate into carbon dioxide and an amine. The amine is so much more reactive that it reacts with another isocyanate (in preference to water) to form mea. The reactivity of water with isocyanates is somewhat slower than that of secondary alcohols, but much more rapid than that of imcatalyzed reaction with methanes or ureas. 

Reactions of isocyanates with alcohols are catalyzed by a variety of compounds, including bases (tertiary amines, alkoxides, and carboxylates), metal salts and chelates, organometallic compounds, acids, and urethanes. The most widely used catalysts in coatings are organotin (IV) compounds, most commonly dibutyltin dilaurate (DBTDL) (dibutylbis[(l-oxododecyl)oxy]stannate) [77-58-7] and tertiary amines, commonly diazabicyclo[2.2.2]octane (DABCO). Combinations of DABCO and DBTDL often act synergistically. 

This work demonstrated that the way in which the alcohol product is irreversibly removed from the reaction solution, in the form of an alkyl catecholborate 359, is pertinent to the significance of this method compared with those which employ the thermal decomposition of carbamate esters, in which recombination of the resulting isocyanate with alcohol is possible. 

Carbamates are usually prepared by the addition of an alcohol to an isocyanate. The reaction of isocyanates with alcohols is fast and quantitative (indeed, it is used to characterize alcohols). The reaction with phenols is slower, but can be catalyzed by tertiary amines. 

The reaction of isocyanate with alcohols is strongly exothermic (170-190 kJ/mol). One of the basic reactions in the urethane foam technology is the reaction of isocyanate with water with evolution of carbon dioxide and amine formation ... 

Baker, J. W., and Gaunt, J. (1949). The mechanism of the reaction of aryl isocyanates with alcohols. Part HI. The spontaneous reaction of phenyl isocyanate with various alcohols. Further evidence relating to the anomalous effect of dialkyanilines in the base-catalysed reaction. J. Chem. Soc., 9 19. 

Reaction of isocyanates with different active H-compounds that form the fundamental basis for polyurethane coating systems are shown in Figure 2.60. The order of relative rates for these reactions is -NH -OH > H O > -COOH. Many types of catalysts are available that are used to control these reactions. It should be noted that while urethanes are produced only from reaction of isocyanates with alcohols, other reactions are also effectively controlled and used in different polyurethane coating systems. 

The insertion reaction of isocyanates into Ti-0 bonds in Ti(0-i-Pr)4 has been used to catalyze the reaction of sterically hindered isocyanates with alcohols. The reaction occurs at room temperature and the yields are very high " . ... 

Figure 1 Reaction of isocyanates with alcohols R1 and R2 stand for an aromatic or aryl group.

The use of ionic liquids as solvents for nucleophilic substitutions has been reviewed. The high solubility of triamino-trinitrobenzene in fluoride-containing ionic liquids has been the subject of molecular dynamics simulations, and is likely to be due to the formation of a Zundel complex, where a proton is shared between an amino group and a fluoride ion. The solvent effects on the rate of reaction of phenyl isocyanate with alcohols have been described by multiparameter linear equations with increased solvent polarity favouring reaction. ... 

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