Isocyanates from 2 molecules

Example 12.10. Polyurethane (PUR) foams are widely used in the building industry. The name refers to the fact that its monomer contains urethane ( 0-CO-NH ) as a functional group. The polymer is usually made from diisocyanates. Isocyanates are molecules which have a N=C=0 group attached. Upon addition of water the reaction shown in Fig. 12.16 starts. 

The alignment of the isocyanic acid molecule in the oriented solid (35) has been determined from an analysis of the e.s.r. spectrom of the irradiated solidthe NCO axis of the molecules is perpendicular to that of the adjacent acid molecule, through a hydrogen bond. 

Pyrolysis of pyridine derivatives is a method for allylic and benzylic deamination the preparation of nitriles from aldehydes, and the preparation of isocyanates from acid chlorides or hydrazides Flash vacuum pyrolysis continues to yield interesting reactions and products. Recently, it has been reported that quite sensitive acetylene derivatives can be obtained by this method from 4-alkylideneisoxazol-5(4H)-ones by ring degradation. On the other hand, 2 carbamyl azide molecules cyclize under these conditions to form l,2,4-triazolidine-3,5-dione 1,2-ylids . 2H-Cyclohepta[b]furan-2-ones have been obtained by ring expansion of phenyl propiolates... 

Mass spectral fragmentation patterns of alkyl and phenyl hydantoins have been investigated by means of labeling techniques (28—30), and similar studies have also been carried out for thiohydantoins (31,32). In all cases, breakdown of the hydantoin ring occurs by a-ftssion at C-4 with concomitant loss of carbon monoxide and an isocyanate molecule. In the case of aryl derivatives, the ease of formation of Ar—NCO is related to the electronic properties of the aryl ring substituents (33). Mass spectrometry has been used for identification of the phenylthiohydantoin derivatives formed from amino acids during peptide sequence determination by the Edman method (34). 

Alkyl uracyls have been known for some time to act as diuretic agents in experimental animals. The toxicity of these agents precluded their use in the clinic. Appropriate modification of the molecule did, however, yield diuretic agents with application in man. Reaction of allylamine with ethyl isocyanate affords the urea, 89 (the same product can of course be obtained from the same reagents with reversed functionality). Condensation with ethyl cyanoacetate affords aminotetradine (90). In... 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and poly ethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different diisocyanates are used commercially typical examples are shown in Table 1.2. 

It is of interest to note that the concentration of active chains is highly sensitive to the extent of reaction. Such results because, as a first approximation, three inactive chains result from each isocyanate group that remains unreacted in the gel. Two molecules of monohydroxy PPO are required, again as a first approximation, to produce three inactive chains. 

Some routes of chemical transformations of nitrile oxides connected with the problem of their stability were briefly discussed in Section 1.2. Here only two types of such reactions, proceeding in the absence of other reagents, viz., dimerization to furoxans and isomerization to isocyanates, will be considered. All other reactions of nitrile oxides demand a second reagent (in some cases the component is present in the same molecule, and the reaction takes place intramolecularly) namely, deoxygenation, addition of nucleophiles, and 1,3-dipolar cycloaddition reactions. Also, some other reactions are presented, which differ from those mentioned above. 

The hydroxamate anion 566 is invoked as intermediate in the formation of urea 568 from the iV-tosylated /3-lactam 564 by a Lossen rearrangement in the presence of a secondary amine 565 (equation 252). Apparently, attack of amine in an S/v2 fashion at the C(4) carbon of the /3-lactam 564 opened the ring to form hydroxamate anion 566, which then initiated a facile Lossen rearrangement to isocyanate 567 trapped with another molecule of base to give 568. 

Valine.—This amino acid is contained mixed with leucine in the fractions of the esters which boil between 6o° and 90° C. Its isolation and separation from leucine is of extreme difficulty, since these compounds, as well as their copper salts into which they are converted by boiling with freshly precipitated cupric oxide, tend to form mixed crystals. Its isolation was only effected by these means in certain cases, and its amount is really much more than the figures represent from its yield. It is best characterised by conversion into its phenylhy-dantoine derivative by treatment with phenyl isocyanate in alkaline solution. The phenylureido acid is first formed, and this loses a molecule of water, as shown by Mouneyrat, and is changed into its anhydride or phenylhydantoine by treatment with hydrochloric acid. The following reactions occur —... 

The one-pot prqraration of 99 through the reactions of various isocyanates with amide anions 98, which were generated in situ from 97, proved to be a usefiil method for the syntheses of these potentially biologically useful molecules <99BCJ1071>. 

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). 

The nature of a hydrophilic prepolymer permits the addition of large amounts of water. The isocyanate reacts with the water to abstract COt. The amine that also results from the reaction then reacts with an isocyanate group to produce a urea linkage. The reaction continues until the water or the isocyanates are consumed. If provisions are made to trap the CO2 in the mass, a foam is produced. If such provisions are not made, the CO, will bubble away, leaving behind a low gel-strength hydrogel. Careful examination of the resultant molecule might cause one to rename it a polyurethane/polyurea. 

According to this mechanism, one molecule of isocyanate undergoes addition to 0=0, and the other addition to C=N. Evidence is that l80 labeling experiments have shown that each molecule of CO2 produced contains one oxygen atom derived from the isocyanate and one from 71,751 precisely what is predicted by this mechanism. Certain other catalysts are also effective.752... 

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