Urethans isocyanates, synthesis

The preparation of acyl azides has received considerable attention due to the value of these compounds as synthetic intermediates. In the Gurtius rearrangement for example, acyl azides arc converted into isocyanates, urethans, ureas and amines and this aspect of the chemistry of acyl azides is considered in detail in a later chapter. The use of acyl azides in peptide synthesis has increased the scope of general... 

Carbonyl azides are usually obtained from the hydrazides, although in principle they can also be prepared directly from the carbonyl chlorides and sodium azide. They are frequently used as intermediates for synthesis of isocyanates, urethanes, ureas, amines, etc., and are often used in such work without isolation. 

Many of the commercial reactants used in urethane synthesis are impure materials, for example MDI may be used as pure MDI, a low melting-point (38°C) solid polymeric or crude MDI, which is a dark-coloured low viscosity liquid. Alternatively, MDI and TDI prepolymers may be blended and perhaps have various additives incorporated, all of which may affect the final quantity of free NCO available for reaction. Also, active isocyanate content of a prepolymer decreases with storage, a decrease of about 0-05% per month being normal. For these reasons it is necessary to measure and specify the amount of isocyanate available for reaction at any specific time, and this is done by calculation of the isocyanate equivalent weight of the mixture using the following procedure ... 

Several modified forms of this synthesis are available. For example, treatment of either isocyanate (28) or urethane (29) derivatives with phosphoryl chloride followed by stannic chloride has been reported to give the substituted isoquinoline [80388-01-8] (158). 

The basis of most one-component, moisture-curing urethanes is the synthesis of an isocyanate-terminated prepolymer, as shown ... 

The waterborne prepolymer process is similar to the prepolymer synthesis described earlier, except that most of the waterborne prepolymers are based on aliphatic isocyanates and contain an internal emulsifier. There are several types of internal emulsifiers, both anionic and cationic. A good summary of these stabilizers is found elsewhere [56], The majority of the waterborne urethanes are anionic dispersions. An internal surfactant, such as dimethylolpropionic acid, is often incorporated into the prepolymer ... 

The chain extension step may then take place in the water phase. Hydrazine and ethylene diamine are commonly used chain extenders for waterborne urethane dispersions. The isocyanates react with the diamine chain extenders much faster than with the water, thus forming polyurea linkages and building a high molecular weight polymer. More detailed information regarding the synthesis and process of making waterborne polyurethane dispersions is found in Dieterich s review article [58]. 

Polyols and polyamines are the most important coreactants for isocyanates. As briefly outlined in Section 4.2.2, the two most common classes of urethane-grade polyols are the polyethers and polyesters. In this section their synthesis and structure are discussed. Other polyol types, such as acrylic resins and polycarbonates, are of more limited applicability and are not presented here. 

Waste PETP was depolymerised by glycolysis to give hydroxyl-terminated oligomers(DPET), which were used in the synthesis of urethane oils. The effect of depolymerisation temps., the type of glycol and the amount of catalyst on the yield and composition of the depolymerisation products was studied. The physical properties of the urethane oils were compared with those of a commercially-available product. The reaction of DPET with isocyanates produced random linkage between different molecules with or without terephthaloyl groups. 15 refs. 

The anionic method of polymerization is most useful for the synthesis of low molecular weight hydroxy-terminated oligomers and polymers that are to be further processed. For example, the treatment of hydroxy-terminated oligomers with isocyanates has been used to obtain polyester-urethanes (9,20), while triblock copolymers (PCL-PEG-PCL) are prepared by initiating the polymerization of e-caprolactone with the disodium alcoholate from polyethylene glycol (26). 

Many of the common condensation polymers are listed in Table 1-1. In all instances the polymerization reactions shown are those proceeding by the step polymerization mechanism. This chapter will consider the characteristics of step polymerization in detail. The synthesis of condensation polymers by ring-opening polymerization will be subsequently treated in Chap. 7. A number of different chemical reactions may be used to synthesize polymeric materials by step polymerization. These include esterification, amidation, the formation of urethanes, aromatic substitution, and others. Polymerization usually proceeds by the reactions between two different functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups. 

The synthesis of polyurethanes is usually presented as proceeding via the formation of carbamate (urethane) linkages by the reaction of isocyanates and alcohols ... 

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. 

Resolution of alcohols (cf., l-(naphthyl)cthyl isocyanate, 8, 356 357). A practical synthesis of the methyl ester of (S)-5-HF,TF. (1) from arachidonic acid involves chromatographic separation of the diastereomeric urethanes preparecT from the Isocyanate derived from dehydroabietylamine (hydrogen chloride and phosgene).1 Urethanes from other chiral amines are less useful. The urethanes are cleaved with triethylamine and trichlorosilanc to give the corresponding pure enantiomeric esters, which can be hydrolyzed by base. 

Cyclization of side chain nitriles has found extensive use in the synthesis of benzocyclobutenes (70 n = 2),104 the versatile synthons which open on mild thermolysis to give o-quinodimethanes for inter- and intra-molecular [4 + 2] trapping.108 The nitrile group in (70) can be manipulated into a variety of functionalities for appending the dienophile portion. For example, in the synthesis of chelidonine, the nitrile (71) was converted, by hydrolysis followed by Curtius degradation and reaction of the formed isocyanate with benzyl alcohol, to a urethane (72). The latter was then condensed with a benzyl bromide to get the compound (73), which was elaborated further as shown in Scheme 14.109... 

But it is important that multistriatin be made in enantiomerically pure form as well as one diastereomer. Looking back over the synthesis, the first chiral intermediate is 42 and, after some failures, reaction with the isocyanate (+)-(/ )-46 gave a mixture of the urethanes 47 that could be separated by crystallisation. Removal of the urethane by reduction with LiAlH4 gave enantiomerically pure alcohol 42 from which enantiomerically pure (>99%) multistriatin 3 could be made by the methods above. 

Light stable PUR 111 was created by incorporation of 4-(2,3-dihydroxyprop-oxy)-2-hydroxybenzophenone during PUR synthesis [139]. Similarly, urethane based coatings may be stabilized by immobilization of approx. 5% of 4-hydroxy-5-tert-butyl-3-[2-(5-chlorobenzotriazolyl)]methyl-isocyanate, added into the reaction mixture during PUR synthesis [140]. 

The isocyanates (9) were transformed into amino derivatives by standard reactions. Acid-sensitive aminocyclopropanes were obtained preferentially from benzyr ) -naphthyl or trifluorethyl-urethanes which were cleaved by hydrogen, EtSNa/DMF or alkaline hydrolysis. The hydrazinolysis of phthalimido compounds obtained from the urethane and phthalic anhydride also was useful Trimethylsilylethanol proved to be advantageous for the isocyanate-amine conversion the resulting urethanes gave amines upon addition of Bu4N p- 161 (e.g. synthesis of amine (21) from truns-crysanthemic acid via isocyanate (20) S equation 3). Contrarily to... 

Koch and coworkers reported an efficient synthesis of alkoxy aminocyclopropane derivatives by the photolysis of alkoxypyrrolinones (320) . The primarily formed isocyanates (321/322) were converted into urea or urethane derivatives (323/324) (equation 76). Pyrrolinone photoreaction was shown to be a very convenient basis for the... 

Another interesting bromide ion-catalyzed reaction is the anodic oxidation (undivided cell) of secondary formamides in methanol leading to urethanes [232]. This reaction proceeds via the intermediate A-bromo amide, which under elimination of HBr forms the isocyanate, which is attacked by methanol. Thus, a phosgene-free technical synthesis of urethanes is made possible. Urethanes can be used as stable isocyanate equivalents [Eq. (42)]. 

The synthesis and properties of 2-oxazolidones and 2-oxazolidone-containing polymers have been reviewed by Pankroatov et al (115). It is interesting to note that some isocyanate cyclotrimerization catalysts also act as catalysts for urethane, oxazolidone and/or carbodiimide linkages. 

Although employed, at one time, primarily as a war gas, phosgene is now an important chemical intermediate for the synthesis of a large number of commercial materials. Worldwide, it is used mainly in the manufacture of isocyanates (for urethane polymers and organic intermediates), polycarbonates (for speciality polymers), and monomeric carbonates and chloroformates (largely for the synthesis of pharmaceuticals and pest control chemicals). The established large-scale, commercial applications of phosgene are summarized in Fig. 4.7. 

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