Production isocyanates

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

In contrast to isocyanates, isothiocyanates are relatively unreactive toward hydroxyl-containing compounds thus they are not even affected by aqueous media [31]. Their reaction with hydroxyalkyl thioureas has been reported [32]. With free 2-mercaptoethylamine, 1 mole of phenyl isothiocyanate gave the N-substituted product, l-(2-mercaptoethyl)-3-phenyl-2-thiourea, and 2 moles gave the N,S-disubstituted product. Isocyanates, regardless of the relative amounts, reacted with 2-mercaptoethylamine to give the N,S-disubsti-tuted products. Cleavage with silver nitrate affords the l-(2-mercaptoethyl)-3-alkylureas [33]. 

More recently a new approach employing BCI3 as reactant (Scheme 1) has been proposed as a substitute for the industrial thermal cracking process [27]. In most cases, quantitative conversion to the product isocyanates was achieved under mild conditions but with the disadvantage of a high BCI3 consumption. 

Proprietary bonding agents are used almost exclusively in modern manufacturing processes for metal-rubber bonded products. Isocyanates were proved to be of value in the immediate postwar period, but their moisture sensitivity and tendency to be wiped off the metal during processing have proved as disadvantages. The majority of commercially available adhesives in use today are a complex mixture of undisclosed composition, and in many instances are specifically used for particular rubbers and substrates. 

Furthermore, the isocyanate can react with water to form an anune and carbon dioxide. Isocyanates are also added to phenol/methanal reactions to produce a physical modification of the product. Isocyanates react with water contained in or added to the reaction, to produce carbon dioxide and an amine. The CO gas foams the network product into a porous media [97]. 

The growing importance of isocyanates, especially TDI and MDI, in industry means that more and more people work with these highly reactive products. Isocyanate manufacturers consider it important to carry out further research into the biological activity of this group of substances, and to inform the users of the possible health risks if they are not handled properly. 

Despite its evident advantages, the direct synthesis of isocyanates is plagued by insufficient selectivity, mainly due to self-condensation reactions of the product isocyanates, and by the not yet satisfactory turnovers exhibited even by the most promising catalysts. These problems can be partially circumvented by running the reaction in the presence of an alcohol or an amine, so that the stable carbamates or ureas are obtained as products. This approach has found a widespread acceptance and most of the catalytic systems for the earbonylation of nitro compounds developed in the last years affords carbamates (RNHCOOR ) or ureas (RNHC(O)NHR) as products (eqs. 6 and 7) ... 

Biuret formation reaction proceeds to a considerable measure above 100 °C. Both reactions (8) and (9) are utilized to introduce crosslinks with the excess of isocyanate. The previously given reactions are the most frequent ones in the polyurethanes chemistry. There are other important reactions such as the reaction of isocyanate with itself, which may occur during storage or are intentionally carried out to obtain new products. Isocyanates (particularly the reactive aromatic ones) easily form dimers (uretdiones) ... 

Ammonium polymethacrylate shows unique degradation behaviour in this group of materials. It undergoes a two-stage decomposition, in the first stage of which NH3 and H2O are eliminated in a cyclization. At higher temperatures cold ring fraction products, isocyanic acid, HCN, CO2, CO and CH4 are formed. 

Anilides and a-Naphthalides. The Grignard reagents prepared from alkyl halides react with phenyl isocyanate (CgHjN=C=0) or with a-uaphthy l isocyanate (C,oH, N=C=0) to yield addition products that are converted by hydrolysis into anihdes and a-naphthalides respectively RX + Mg —> RMgX... 

The cyclic carbamate (oxazoIidin-2-one) 313 is formed by the reaction of phenyl isocyanate (312) with vinyloxirane[I92]. Nitrogen serves as a nucleophile and attacks the carbon vicinal to the oxygen exclusively. The thermodynamically less stable Z-isomer 315 was obtained as a major product (10 I) by the reaction of 2-methoxy-l-naphthyI isocyanate (314) with a vinyloxir-... 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

Chlorosulfonyl isocyanate has been used to introduce 3-carboxamide groups. The initial product, an A -chlorosulfonylcarboxamide, is treated with tri-n-butylstannanc to form the primary carboxamide[15], 3-Cyano groups can also be introduced using chlorosulfonyl isocyanate. The intermediate N-chlorosulfonylindole-3-carboxamide is converted to 3-cyanoindole on reaction with triethylamine[16] or DMF[17],... 

Treatment of 2-imino-3-phenyl-4-amino-(5-amido)-4-thiazoline with isocyanates or isothiocyanates yields the expected product (139) resulting from attack of the exocyclic nitrogen on the electrophilic center (276). Since 139 may be acetylated to thiazolo[4,5-d]pyrimidine-7-ones or 7-thiones (140). this reaction provides a route to condensed he erocycles (Scheme 92). 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Hydroxy-THISs add to the C-C bond of diphenylcyclopropenethione (181. Inner salts without substituents in 5-posnion react similarly with diphenylcyclopropenone (Scheme 10) (4, 18). Pwolysis of the stable adducts (9) leads to rupture of the R-C-CY bond. Subsequent ring closure yields 10. When Y = O. 10 eliminates COS. producing 2-pyridone. When Y = S. 10 is isolated together with its isocyanate extrusion product, a thiopyran-2-thione (18). 

Adducts from various quaternary salts have been isolated, in reactions with aldehydes, a-ketoaldehydes, dialkylacylphosphonates and dialkyl-phosphonates, isocyanates, isothiocyanates, and so forth (Scheme 15) (36). The ylid (11) resulting from removal of a Cj proton from 3.4-dimethyl-S-p-hydroxyethylthiazolium iodide by NEtj in DMF gives with phenylisothiocyanate the stable dipolar adduct (12) that has been identified by its NMR spectrum and reactional product, such as acid addition and thiazolidine obtention via NaBH4 reduction (Scheme 16) (35). It must be mentioned that the adduct issued from di-p-tolylcarbodiimide is separated in its halohydrogenated form. An alkaline treatment occasions an easy ring expansion into a 1,4-thiazine derivative (Scheme 17) (35). 

Another product is made with strands produced by flakers, having dimensions of about 0.8 x 13 x 300 mm (0.030 x 0.5 x 12 in.). The strands are dried and then coated with isocyanate adhesives. The strands are formed into mats with unidirectional orientation and pressed in a steam-through press. The large panels are then sawn into dimension lumber sizes. 

Health and safety information is available from the manufacturer of every adhesive sold in the United States. The toxicology of a particular adhesive is dependent upon its components, which mn the gamut of polymeric materials from natural products which often exhibit low toxicity to isocyanates which can cause severe allergic reactions. Toxicological information may be found in articles discussing the manufacture of the specific chemical compounds that comprise the adhesives. 

Methylenebis(4-phenyl isocyanate). This compound is also known as methyl diisocyanate [101-68-8] (MDI) and is produced by the condensation of aniline and formaldehyde with subsequent phosgenation. Its principal end use is rigid urethane foams other end uses include elastic fibers and elastomers. Total formaldehyde use is 5% of production (115). 

The preparation and properties of these tertiary aminimides, as weU as suggested uses as adhesives (qv), antistatic agents (qv), photographic products, surface coatings, and pharmaceuticals, have been reviewed (76). Thermolysis of aminimides causes N—N bond mpture foUowed by a Curtius rearrangement of the transient nitrene (17) intermediate to the corresponding isocyanate ... 

Estimated HCl production during isocyanate manufacture represents net HCl. This value excludes HCl consumed ia the reaction process to make methylene diphenylene diamine (MDA) and polymethylene polyamine (PMPPA) iatermediates ia the production of 4,4 -methylenebis(phenyHsocyanate) (MDl) and polymethylene polyphenyHsocyanate (PMPPl). 

Isocyanates are Hquids or soHds which are highly reactive and undergo addition reactions across the C=N double bond of the NCO group. Reactions with alcohols, carboxyUc acids, and amines have been widely exploited ia developiag a variety of commercial products. Cycloaddition reactions involving both the C=N and the C=0 double bond of the NCO group have been extensively studied and used for product development (1 9). 

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