Isocyanate replacement

In a 3.0-mL conical vial containing a boiling stone and equipped with an air condenser protected by a calcium chloride drying tube place 15 mg of an anhydrous alcohol or phenol. Remove the air condenser from the vial and add 2 drops of phenyl isocyanate or a-naphthyl isocyanate. Replace the air condenser immediately. If the unknown is a phenol, add 1 drop of pyridine in a similar manner. 

Urethane All ds. Uralkyds are alkyds with a part or even all of the dibasic acids replaced by diisocyanates. The isocyanate group, —N=C=0,... 

AH of the amine hydrogens are replaced when MDA or PMDA reacts with epoxides to form amine based polyols. These polyols can be used in reactions with isocyanates to form urethanes or with additional epoxide to form cross-linked thermo set resins. 

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 Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

Substitution If intensification is not possible, then an alternative is to consider using a safer material in place of a hazardous one. Thus it may be possible to replace flammaole solvents, refrigerants, and heat-transfer media by nonflammable or less flammable (high-boiling) ones, hazardous products by safer ones, and processes which use hazardous raw materials or intermediates by processes which do not. As an example of the latter, the product manufactured at Bhopal (carbatyl) was made from three raw materials. Methyl isocyanate is formed as an intermediate. It is possible to react the same raw materials in a different order so that a different and less hazardous intermediate is formed. 

In one projected commercial modification of the process the phosgenation stage is replaced by one in which the nitro compounds are reacted with CO and an alcohol to form a urethane. This is then split to form an isocyanate in the second step. 

Safety No year goes by without some widely used chemical being declared suspect on toxicity grounds. The paint industry has responded rapidly to eliminate toxic chemicals from coatings or to show how they can be used safely in an industrial environment. Examples are the elimination of specific ether-alcohol solvents and the introduction of air-fed hoods for spraying isocyanates. Of particular interest in corrosion prevention is the current pressure to eliminate chromate pigments. Currently there are no equally effective alternatives and the emphasis has had to be on safe usage. The search for replacements continues. 

Biodegradable poly(phosphoester-urethanes) containing bisglycophosphite as the chain extender were synthesized. Methylene bis-4-phenyl isocyanate (MDI) and toluene diisocyanate (TDI) were initially used as diisocyanates. Since there was a concern that the degradation products could be toxic, the ethyl 2,6-diisocyanatohexanoate (LDI) was synthesized and replaced the MDI (or TDI). The hydrolytic stability and solubility of these polymers were tested. Preliminary release studies of 5-fluorouracil from MDI based poly(phosphoester-urethane) and methotrexate from LDI based poly(phosphoester-urethane) are also reported. 

Among heteroaromatic compounds able to react with nitrile oxides as dipo-larophiles, furan, probably, is the best known. Recently, a novel nitrile oxide was generated from a sulfoximine and converted in situ to a cycloadduct with furan (Scheme 1.25) (287). The starting racemic N-methyl-S-nitromethyl-S-phenylsul-foximine 124 was prepared in 87% yield via nitration of N,S-dimethyl-S-phenyl-sulfoximine. Reaction of 124 with p-chlorophenyl isocyanate and a catalytic quantity of triethylamine, in the presence of furan, afforded dihydrofuroisoxazole 125, the product of nitrile oxide cycloaddition, in 42% yield (65 35 diastereomer ratio). The reaction of 125 with phenyllithium and methyllithium afforded compounds 126, which are products formed by replacement of the sulfoximine group by Ph and Me, respectively. 

Isocyanates can be replaced with a phthalimide and a succinimide to give hemiaminals 122 and 123 in 68% and 65% yield, respectively (Scheme 53).185... 

N-(2-Hydroxypropyl)carbamates (8.139, Fig. 8.13,b) are prodrugs that resemble the A-(2-hydroxyphenyl)carbamates discussed above. Here, activation yielded the tranquilizer mephenoxalone (8.140, Fig. 8.13,b) and an alcohol or a phenol such as paracetamol. Other active oxazolidinones could be obtained by replacing the MeO group in 8.139 (Fig. 8.13, b) with another substituent. For this series, the mechanism of activation is not an intramolecular nucleophilic attack, but, rather, decomposition of the deprotonated carbamate group as shown in Fig. 8.7,b, Reaction b, with the intermediate isocyanate being trapped to form the oxazolidinone ring. 

Finally, a third formulation was devised which excluded the use of Freon 11 In the polyol component. Freon 11 was placed In the Isocyanate component and both Isocyanate and polyol components were changed to meet viscosity considerations. Subsequent aging studies showed the Isocyanate to age similarly to the previously aged (Formulation 2) Isocyanate. The polyol showed virtually no Increase In acid number at any aging temperature over 13 months. Thus, at ambient temperature we would expect a 6-8 year system lifetime on the Isocyanate before a 10% change In analytical properties would dictate a material change-out. The polyol appears to have a greater lifetime, but would probably be replaced at the same time. 

Finally, the Lessen rearrangement provides a practical procedure for replacing the hydroxamic group of a hydroxamic acid (12) by an amino group (14) (equation 2). The initial rearrangement product is an isocyanate (13) which readily reacts with nucleophiles, for example with OH and NH functionalities to give amines (14) and ureas (15). 

Isocyanates 345 react with phenanthrenequinone 346 and triphenylarsine oxide to give photochromic oxazines 347 (Equation 48) <1993PS(81)37>. The isocyanate can be replaced by a phosphinimine and the phenanthrene structure can also be replaced by the corresponding phenanthroline (Equation 49) <2003WO42195>. The /ra r-fused tetrahydrooxazine 349 was prepared from epoxide 348 and 2-aminoethyl sulfate (ethanolamine 0-sulfonic acid) (Equation 50) <1987AP625>. 

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