Urea and Urethane Reactions

Compounds containing the O-H group such as diols will, under the appropriate conditions, react with isocyanates to give a urethane or, in pure organic literature, a carbamate. Secondary alcohols react at about one-third the rate of primary alcohols. Complex alcohols react very slowly with isocyanates and form a mixture of urethanes and olefins. 

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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). 

These not only exhibit all the structural and spectroscopic characteristics of anomeric amides 2, their reactivity patterns embrace a range of reactions at the amide nitrogen that in many respects parallel those at a saturated carbon.37 46 In addition, they are mutagenic as well as DNA damaging agents that have anticancer capability.37,38,40,46 50 To date nearly a hundred of these have been synthesised in our laboratory while, recently, a range of urea and urethane analogues have also been reported.51 53... 

The reaction may be subject to limitations in the alkene structure similar to those of the nitrile process. Besides simple amides, one can also utilize urea and urethanes in this reaction. Demercuration is best effected by using alkaline sodium borohydride in the presence of a primary amine such as Bu"NH2. 

In the majority of cases the addition product is stable, but in some special cases it is only moderately stable and may either dissociate to form the initial reactants again or decompose to other products. Secondary reactions of isocyanates that are important in the formation of urethane polymers are those with urea and urethanes. These reactions result in the formation of biuret and allophanate, respectively (Figure 2.19). The relative reaction rates of active-hydrogen-bearing compounds with isocyanate are given in Table 2.7. 

Figure 2.19 The reaction of isocyanate with urea and urethane...

Relative Rates of Reactions of Aromatic Isocyanates and Substituted Ureas and Urethanes ... 

Other hydrolyzable groups consist of biuret and allophanate crosslinks formed from the reaction of Isocyanate radicals with active hydrogen atoms of the urea and urethane linkages, respectively. 

Lactim ethers and thioethers are reported to react with imides <93H(35)1055>, amino ketones and acetals <82jhci93>, amino acids <90JHC1973>, active methylene compounds <82JHC193, 82JHC43I, 88T3309), and diethyl ethoxymethylenemalonate <84H(22)2285>. 7,8,9,10-Tetrahydro-6//-l,3,5-tri-azino[l,2-fl]azepine-2,4(3/f)-dione has been prepared from 7-amino-3,4,5,6-tetrahydro-2(//)-azepine and diphenyl imidodicarboxylate <93JHC55l>, and caprolactam acetals are reported to react with amino heterocycles <938521 > and ureas and urethanes <82KGS1553>. Most of these reactions lead to fused azepines either directly or after subsequent treatment of the isolated intermediate. 

This reaction has general application in the preparation of amine, urea, and urethane derivatives. 

The main route for the formation of NOCs is by nitrosation reactions that take place between a nitrosating agent like nitrite (NOi") in an acidic environment, nitrous acid (HNO2), nitrogen oxides (NO2 or NO,c)5 nitrosyl chloride (NOCl), and, on the other hand, a nitrosable agent like some amines, amides, guanidines, urea, and urethane compounds. 

The types of chemical reactions that are frequently used in reactive blending can be grouped into imidization, ring opening and amidation reactions, and interchange reactions between polycondensates. Other types of reactions that are less frequently used include esterification, urea and urethane link formation, ionic bonding, and concerted addition in which a comonomer such as maleic anhydride is copolymerized with a double bond of an unsaturated polymer. 

Isocyanates will react with all compounds containing hydrogen atoms attached to a nitrogen atom [2]. There are four basic reactions chemists employ to make polyurethanes. The reaction of isocyanates with hydroxyl groups to produce urethane is the primary reaction. The reaction of isocyanates with amines yields urea and the reactions of isocyanates with urea and urethane produce biurets and allophanates, respectively. 

Table 2 Relative rates of reactions of aromatic isocyanates and substituted ureas and urethanes.

Diamines or diols can be used as chain extenders in copolymerization reactions, but urea linkages formed from diamines often lead to polymer insolubility.(28) Poor solubility is avoided by using diols, but this requires a two stage preparation (Scheme 1) because of the disparity in reactivity of alcohols and amines with isocyanates. To form urea and urethane linkages separately, the reaction between isophorone diisocyanate and benzene dimethanol was carried out first in bulk at 115 °C without catalyst (Equation 1). Secondly, reaction between aminopropyl endcapped dimethylsiloxane oligomers and diisocyanate intermediates was effected in THE at room temperature (Equation 2). Completion of the polymerization reaction was established by NMR and infrared spectroscopy.(8)... 

Acetoiicetyliition Reactions. The best known and commercially most important reaction of diketene is the aceto acetylation of nucleophiles to give derivatives of acetoacetic acid (Fig. 2) (1,5,6). A wide variety of substances with acidic hydrogens can be acetoacetylated. This includes alcohols, amines, phenols, thiols, carboxyHc acids, amides, ureas, thioureas, urethanes, and sulfonamides. Where more than one functional group is present, ring closure often follows aceto acetylation, giving access to a variety of heterocycHc compounds. These reactions often require catalysts in the form of tertiary amines, acids, and mercury salts. Acetoacetate esters and acetoacetamides are the most important industrial intermediates prepared from diketene. 

In contrast to the above additions A-allyl- and substituted A-allyl-amides, -urethanes, -ureas and -thioureas undergo intramolecular cyclization only in 6(3-96% sulfuric acid to give the corresponding oxazolinium and thiazolinium salts. Treatment of these cations with base yields 2-oxazolines and 2-thiazolines in moderate to good yields. The reaction is illustrated by the conversion of A-2-phenylallylacetamide (342) into 2,5-dimethyl-5-phenyl-2-oxazoline (343) in 70% yield 70JOC3768) (see also Chapter 4.19). 

Not only are these reactions of importance in the development of the cross-linked polyurethane networks which are involved in the manufacture of most polyurethane products but many are now also being used to produce modified isocycuiates. For example, modified TDI types containing allophanate, urethane and urea groups are now being used in flexible foam manufacture. For flexible integral foams and for reaction injection moulding, modified MDIs and carbodi-imide MDI modifications cU"e employed. 

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