Carbon dioxide urethans

One-part urethane sealants (Table 3) are more compHcated to formulate on account of an undesirable side reaction between the prepolymer s isocyanate end and water vapor which generates carbon dioxide. If this occurs, the sealant may develop voids or bubbles. One way to avoid this reaction is to block the isocyanate end with phenol and use a diketamine to initiate cure. Once exposed to moisture, the diketamine forms a diamine and a ketone. The diamine reacts with the isocyanate end on the prepolymer, creating a cross-link (10). Other blocking agents, such as ethyl malonate, are also used (11). Catalysts commonly used in urethane formulations are tin carboxylates and bismuth salts. Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

The acid chloiide (chloioformamide [463-72-9] "uiea chloiide"), NH2COCI, and its salts have been prepared. Ammonium carbamate [1111 -78-OJ can be obtained as a white crystalline sobd by reaction of dry carbon dioxide and ammonia. It is an impurity in commercial ammonium carbonate [506-87-6] (see Ammonium compounds). Esters of carbamic acid are quite stable. The best known is the ethyl ester usually called urethane [51 -79-6],... 

Other Derivatives. Ethylene carbonate, made from the reaction of ethylene oxide and carbon dioxide, is used as a solvent. Acrylonitrile (qv) can be made from ethylene oxide via ethylene cyanohydrin however, this route has been entirely supplanted by more economic processes. Urethane intermediates can be produced using both ethylene oxide and propylene oxide in their stmctures (281) (see Urethane polymers). 

The water reaction evolves carbon dioxide and is to be avoided with solid elastomers but is important in the manufacture of foams. These reactions cause chain extension and by the formation of urea and urethane linkages they provide sites for cross-linking, since these groups can react with free isocyanate or terminal isocyanate groups to form biuret or allophanate linkages respectively (Figure 27.5). 

In an interesting reaction, pyrrolysis of the urethane leads to extrusion of carbon dioxide and formation of 23, propiomazine Although this agent contains the ethylenediamine side chain, its main use is as a sedative. 

Apart from the Important reaction leading to the formation of urethane groups, carbon dioxide can be released during curing by hydrolysis of the Isocyanate group, leading to the formation of urea groups (10). 

Urethane hydrolyzes into an amine, an alcohol, and carbon dioxide. So the possible degradation products of a poly(phosphoester-urethane) are diamines, diols, phosphates, carbon dioxide, and even ureas. Urea is possible because the isocyanate is extremely sensitive to moisture, which would convert the isocyanate to an amino group. One is therefore bound to have traces of diamine in the polymerization that leads to a urea bond in the backbone. We think the cytotoxicity seen in the macrophage functional assay comes from the TDI structure. 

The foams can be obtained by the action of a diiscyanate on a polyol and water. The reaction with water forms carbon dioxide and the reaction with polyol forms a urethane polymer. Catalysts play a crucial role in the process. Tin octeate and dibutyl tin dilaurate are preferred catalysts along with tertiary amines. 

Triethylamine (7) is a strong, hindered base originally employed for mixed-anhydride reactions. However, it reduces the rate of anhydride formation if the solvent is dichloromethane or chloroform (see Section 7.3) and promotes disproportionation of the anhydride under conditions in which /V-mcthylmorpholinc does not (see Section 7.5). It causes enantiomerization of urethane-protected amino-acid /V-carboxyanhydrides and reaction between two molecules with release of carbon dioxide (see Section 7.14). It is also used in the synthesis of Atosiban (see Section 8.9). There is no reaction for which it is recommended as superior to other tertiary amines, except possibly for coupling employing BOP-C1 (see Section 8.14). 

DBU = l,8-diazabicyclo[5.4.0]undec-7-ene (15) is a nonnucleophilic base employed in conjunction with piperidine in dimethylformamide (1 1 48) for removal of fluorenylmethyl-based protectors. The piperidine is necessary as a nucleophile to trap the expelled moiety that does not react with DBU. DBU has no effect on phthalimido [Pth-NH of -Lys(Pht)-], dialky-lphosphoryl [-Tyr(P03R2)-], or Dde-NH [-Lys(Dde)- see Section 6.4], but it promotes aspartimide formation at the pertinent residues of susceptible sequences (see Section 6.13). In dichloromethane, it promotes a reaction between two molecules of urethane-protected amino acid /V-carboxyanhy-dride with release of carbon dioxide (see Section 7.14). 

It is also known that once the tributyltin radical adds to the sulfur of N-hydroxypyridine-2-thione, breakage of the N-0 bond and decarboxylation are very fast and probably concerted. They are driven energetically by formation of the very stable carbon dioxide molecule. It comes as no surprise that if atoms other titan carbon are attached to the carboxyl group, then they would also end up as free radicals after decarboxylation. This is shown for a urethane analog as a source of nitrogen-centered free radicals. A wide variety of other free-radical species can be produced by this strategy, and it is thus quite useful. 

The chemistry is slightly different when diols are used. The bonds formed are urethane bonds. This means that the final product formed is pure polyurethane. No carbon dioxide is liberated during the reaction. The general reaction is shown in Figure 2.4. 

Insertion complexes of lanthanide isopropoxides with isocyanate can act as carbon dioxide carrier for the carboxylation of active methylene compounds , as exemplified in Scheme 25 [241], The catalytic formation of urethane is based on insertion of isocyanate into the La-OtBu bond of La3(OfBu)9(THF)2 [242]. 

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