Isocyanates, addition alcohols

Heptamethylbiguanide catalyses the addition of phenyl isocyanate to alcohols more powerfully than do other amines, a property that is ascribed to its high basic strength (211). 

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. 

Brady et al. have reported an efficient method for catalytic phosphityla-tion of alcohols (14) with phosphoramidites (15) and isocyanate additives (Scheme 6). ... 

The primary degradation reaction was dissociation of allophanate into phenyl isocyanate and alkyl carbanUate, followed by dissociation of the alkyl carbanUate into phenyl isocyanate and alcohol. Decarboxylation of the ethyl carbanUate fragment also took place slowly. A small amount of diphenyl carbodimide was observed at the pyrolysis temperature of 450°C. In addition, decarboxylation of the isopropyl carbanUate fragment took place at 550°C. A small amount of diphenyl carbodimide was observed from 350°C to 550°C. 

An acyl azide can rearrange donation ofthe nitrogen lone pair, 1,2-alkyl shift and loss of nitrogen gas occurs to give a highly reactive isocyanate, addition of benzyl alcohol to the carbonyl then occurs. 

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. 

Without additional reagents Urethans from isocyanates and alcohols... 

It is to be noted that not all polymers made by the condensation method form a condensate during the reaction. Polyurethanes which are formed by a reaction of isocyanates and alcohols are such an example. Also, ring opening polymerization reactions are considered to be of the addition type even though they form polymers which can also be formed by a condensation reaction, e.g., the polymerization of caprolactam to form nylon 6,6 (see Painter and Coleman, (1994)). As a result, most modem texts do not use the polymerization descriptions, condensation and addition. Rather, the terms step growth and chain are used in place of condensation and addition respectively. 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

Isocyanates are Hquids or soHds which are highly reactive and undergo addition reactions across the C=N double bond of the NCO group. Reactions with alcohols, carboxyUc acids, and amines have been widely exploited ia developiag a variety of commercial products. Cycloaddition reactions involving both the C=N and the C=0 double bond of the NCO group have been extensively studied and used for product development (1 9). 

The third approaeh to synthetic polymers is of somewhat less commereial importance. There is in fact no universally accepted deseription for the route but the terms rearrangement polymerisation and polyaddition are commonly used. In many respects this process is intermediate between addition and condensation polymerisations. As with the former teehnique there is no moleeule split out but the kinetics are akin to the latter. A typical example is the preparation of polyurethanes by interaction of diols (di-alcohols, glycols) with di-isocyanates Figure 2.7). 

Another major use of organotin compounds is as curing agents for the room temperature vulcanization of silicones the 3 most commonly used compounds are Bu2SnX2, where X is acetate, 2-ethylhexanoate or laurate. The same compounds are also used to catalyse the addition of alcohols to isocyanates to produce polyurethanes. 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

Problem 31.9 Show the mechanism of the nucleophilic addition reaction of an alcohol with an isocyanate to yield a urethane. 

Carbamates (substituted urethanes) are prepared when isocyanates are treated with alcohols. This is an excellent reaction, of wide scope, and gives good yields. Isocyanic acid HNCO gives unsubstituted carbamates. Addition of a second mole of HNCO gives allophanates. 

The details of the mechanism are poorly understood, though the oxygen of the alcohol is certainly attacking the carbon of the isocyanate. Hydrogen bonding complicates the kinetic picture. The addition of ROH to isocyanates can also be catalyzed by metallic compounds, by light, or, for tertiary ROH, by lithium alkoxides ° or n-butyllithium. ° ... 

There are actually three reactions called by the name Schmidt reaction, involving the addition of hydrazoic acid to carboxylic acids, aldehydes and ketones, and alcohols and alkenes. The most common is the reaction with carboxylic acids, illustrated above.Sulfuric acid is the most common catalyst, but Lewis acids have also been used. Good results are obtained for aliphatic R, especially for long chains. When R is aryl, the yields are variable, being best for sterically hindered compounds like mesi-toic acid. This method has the advantage over 18-13 and 18-14 that it is just one laboratory step from the acid to the amine, but conditions are more drastic. Under the acid conditions employed, the isocyanate is virtually never isolated. 

Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

Transesterification from thioesters to esters188 or amides189,190 (lactams) are conducted by Cu1- or CuII-based Lewis acids (Scheme 45). Addition of alcohols to isocyanates is accelerated by CuCl.191... 

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