Isocyanate, addition reactions

Another type of polyol often used in the manufacture of flexible polyurethane foams contains a dispersed soHd phase of organic chemical particles (234—236). The continuous phase is one of the polyols described above for either slab or molded foam as required. The dispersed phase reacts in the polyol using an addition reaction with styrene and acrylonitrile monomers in one type or a coupling reaction with an amine such as hydrazine and isocyanate in another. The soHds content ranges from about 21% with either system to nearly 40% in the styrene—acrylonitrile system. The dispersed soHds confer increased load bearing and in the case of flexible molded foams also act as a ceU opener. 

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). 

Poly(phenylene oxide)s undergo many substitution reactions (25). Reactions involving the aromatic rings and the methyl groups of DMPPO include bromination (26), displacement of the resultant bromine with phosphoms or amines (27), lithiation (28), and maleic anhydride grafting (29). Additional reactions at the open 3-position on the ring include nitration, alkylation (30), and amidation with isocyanates (31). 

Methylarsine, trifluoromethylarsine, and bis(trifluoromethyl)arsine [371-74-4] C2HAsF, are gases at room temperature all other primary and secondary arsines are liquids or solids. These compounds are extremely sensitive to oxygen, and ia some cases are spontaneously inflammable ia air (45). They readily undergo addition reactions with alkenes (51), alkynes (52), aldehydes (qv) (53), ketones (qv) (54), isocyanates (55), and a2o compounds (56). They also react with diborane (43) and a variety of other Lewis acids. Alkyl haUdes react with primary and secondary arsiaes to yield quaternary arsenic compounds (57). 

In some cases it may be desired to increase the cross-link density and hence the rigidity independently of the isocyanate-water reaction. Compounds such as glycerol, pentaerythritol and various amines have been employed as additional cross-linking agents. 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

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. 

As explained in Chapter 1, the urethane group is the product of the reaction of a hydroxy compound with an isocyanate group (Reaction 4.8). This reaction occurs by step kinetics, yet is usually an addition process since no small molecule is lost as the reaction proceeds. 

Having the same atomic composition as cyanates but drastically different properties are the fulminates, which contain CNO-. Many organic compounds having the formula R-N-C-O are known (the isocyanates). Cyanides undergo an addition reaction with sulfur to produce thiocyanates. 

Isocyanates are quite reactive and they react with compounds which contain active hydrogen such as hydroxy, amine, carboxylic acids, amide, urea, etc. They can also undergo addition reaction ... 

Michael addition reactions are particularly useful when linear aliphatic bis-nitramines are used because the products contain two terminal functional groups like in the diester (182). The terminal functionality of such products can be used, or modified by simple functional group conversion, to provide oligomers for the synthesis of energetic polymers such oligomers often use terminal alcohol, isocyanate or carboxy functionality for this purpose. 

Metal-Oxygen Compounds. Trialkyltin alkoxides are remarkable for the variety of addition reactions they undergo with carbonyl and thiocarbonyl compounds. Bloodworth and Davies have reported reactions of tri-w-butyltin alkoxides with isocyanates, carbon dioxide, sulfur dioxide, isothiocyanates, carbon bisulfide, chloral, and ketene. The reactions observed were as follows ... 

Several examples of l,2,4-thiadiazetidin-3-one derivatives have been prepared from the addition of Y triene iminosulfur compounds to sulfonyl isocyanates. The reaction sequence is complex and apparently involves addition, loss of f-butyl isocyanate and subsequent addition to a second molecule of iminosulfur compound. One intermediate has been trapped, and the final product structure was firmly established by X-ray determination to be the 3-one derivative shown in Scheme 136 (78AG(E)677, 80CB2434). 

Zirconium and hafnium dialkylamides are highly reactive compounds. They undergo (i) protolytic substitution reactions with reagents such as alcohols, cyclopentadiene and bisftrimethylsilyOamine 63 64 (ii) insertion reactions with C02, CS2, COS, nitriles, phenyl isocyanate, methyl isothiocyanate, carbodiimides and dimethyl acetylenedicarboxylate 69-72 and (iii) addition reactions with metal carbonyls.73 These reactions are summarized with reference to Zr(NMe2)4 in Scheme 1. 

Quinoxaline 1-oxide (209) reacts with phenyl isocyanate to give 2-anilinoquinoxaline (210) together with 1,3-diphenyl-l-(2-quinoxalinyl)-urea (211) and cyclized oxidation product of the urea 212.215 2-Quinoxalinone 4-oxide (205) and its 1-methyl derivative undergo addition reactions, e.g., with phenyl isocyanate and benzyne to give compounds 214 and 216, respectively.216 These reactions are formulated as proceeding via the intermediate cycloadducts 213 and 215. Compound 216 has also been obtained by photolysis of 3-(o-hydroxy-phenyl)quinoxaline 1-oxide.51 1,3-Dipolar cycloaddition of quinoxaline... 

The isocyanate reaction pathway seems to have no major advantage over the other methods with N-activation (vide supra), since it implies an additional reaction step and the isocyanates react directly with hydrazides giving azapeptides. Advantages may be the easier handling of the solid and stable TV-azolides in comparison to the liquid, toxic, and relatively unstable isocyanates. 

These compounds offer interesting possibilities for further elaboration as they enter into addition reactions with, for example, the 1,2-quinone (60), yielding tricyclic compounds (61) (79TL237), and their bicyclic analogues (62) combine with phenyl isocyanate to give adducts (63), which eliminate carbon dioxide to afford pyrimidine betaines (64). Similarly, dialkyl acetylenedicarboxylates produce quinolizinones (65) (Scheme 19) (79CB1585). 

The coordinated azide undergoes addition reactions with a variety of unsaturated compounds. Thus with carbon monoxide under very mild conditions the azide is converted to isocyanate with the loss of nitrogen (equation 17).293-295... 

Addition Reactions. Isocyanates undergo addition reactions with a wide variety of substrates. Preferred addition occurs across die C=N hond of the NCO moiety. 

Most reactions of this type were found to involve acyclic 1,4-dipolar intermediates which cydize to four-membered heterocydes or are intercepted by isocyanate or C=X components, such as C=N, C=S, and CR2, to form a six membered ring. This group of reactions is illustrated in Figure 1. Depending on the nature of the isocyanate and the double-bond system, any of the products shown in Figure 1 can be obtained. Variations in the component ratio or judicious choice of reagents are noted to have pronounced control of product type. Additional reaction details, as well as a description of the multiple transformations involving adjacent substituents, have been summarized (28). 

Urea derivatives are of general interest in medidnal chemistry. They may be obtained either from urea itself (barbiturates, see p. 306) or from amines and isocyanates. The latter are usually prepared from amines and phosgene under evolution of hydrogen chloride. Alkyl isocyanates are highly reactive in nucleophilic addition reactions. Even amides, e.g. sulfonamides, are nucleophilic enough to produce urea derivatives. 

On prolonged reaction with aryl isocyanates, the pyrido [ 1,2-a]pyrimidines (262) give pyrido[l,2-a]-s-triazines (266) in poor yield.328 The suggested pathway is the addition of the aryl isocyanate to form the 3,3-disubstituted product (263), which eliminates the ketene (265) the residual part of the molecule (264) reacts with an additional mol of aryl isocyanate to give 266. The ketene (265) forms the quinoline (267), by ring closure, which gives 268 by aryl isocyanate addition. The quinoline (267 R = CH2Ph, X = H) could be isolated. 

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