Isocyanates production from aniline

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]. 

Polymeric polyisocyanate n. A generic term for a family of isocyanates derived from aniline-formaldehyde condensation products, used as reactants in the production of polyurethane foams. 

The production of aniline is a major international business, carried on in the US, Europe and Asia, mainly for the conversion, by reaction with formaldehyde under acid-catalyzed conditions, into diaminodiphenylmethanes 9a, 9b and 9c, and then into isocyanates, mainly 4,4/-methylenebis(phenylisocyanate) (MDI, also known as 4,4 -methylene-di-paraphenylene isocyanate, 4,4 -diphenylmethane diisocyanate, methylene diphenylene diisocyanate and diisocyanato diphenyl methane) (9d), from which polyurethanes are produced. This accounts for well over 60% of total demand (Figure 1). Aniline is also used in bulk for the production of antioxidants and vulcanization accelerators for rubber. Some 15.5 million lbs. of cyclohexylamine are made each year mainly by catalytic hydrogenation of aniline. Half the demand is for use as a boiler water additive. Other major uses include in the manufacture of herbicides, plasticizers, emulsifying agents, dyes, dry-cleaning soaps, acid gas absorbents and, in Asia, cyclamate sweeteners. Apart from India, the use of aniline for dyestuff manufacture represents about 10% of demand. 

Isocyanates are formed by reacting phosgene with an appropriate hydrocarbon substrate. Many isocyanates are possible depending upon the hydrocarbon starting material. The commercially important polyurethanes are manufactured from toluene diisocyanate, based on toluene, and methylene diphenyl isocyanate, based on aniline. Both toluene diisocyanate (TDI) and methylene diphenylene isocyanate (MDI) can be used to manufacture foamed products, but only MDI is used as the primary feedstock for elastomeric polyurethanes. 

The toxicity of phosgene has spawned a lot of research into alternates for both MDI and TDI, as well as polycarbonates. In addition to safety, there are economic incentives for developing alternate routes. In the conventional MDI process, methylene diphenylmethane diamine (MDA) is formed by reacting aniline with formaldehyde. Separating excess aniline from crude MDA is an expensive operation. Also, by-product HCl formed in the conversion of MDA to MDI is an environmental issue. The final isocyanate product contains hydrolyzable chloride compounds that are difficult to separate and dispose of. The reactants must be kept bone dry to prevent corrosion, and the introduction of water can cause a runaway reaction. Similar concerns influence the search for nonphosgene routes for TDl. Conventional routes to polycarbonates also employ phosgene, which produces chlorine waste products, primarily sodium chloride, that present disposal problems. The elimination of chlorine from the polycarbonate process would constitute a major improvement. 

Dicobalt octacabonyl does in -fact catalyse the reaction o-f aldehydes with Isocyanates to give the corresponding Schi-f-f bases [ill]. However control experiments with PhN02 and C2H5OH in place o-f benzal dehyde, in the same experimental conditions, yielded but small amounts o-f the urethane << lOX and urea << 5X) derivatives. The major product was aniline [Pg.129]

Chemical/Physical. Diuron decomposes at 180 to 190 °C releasing dimethylamine and 3,4-dichlorophenyl isocyanate. Dimethylamine and 3,4-dichloroaniline are produced when hydrolyzed or when acids or bases are added at elevated temperatures (Sittig, 1985). The hydrolysis half-life of diuron in a 0.5 N NaOH solution at 20 °C is 150 d (El-Dib and Aly, 1976). When diuron was pyrolyzed in a helium atmosphere between 400 and 1,000 °C, the following products were identified dimethylamine, chlorobenzene, 1,2-dichlorobenzene, benzonitrile, a trichlorobenzene, aniline, 4-chloroaniline, 3,4-dichlorophenyl isocyanate, bis(l,3-(3,4-dichlorophenyl)urea), 3,4-dichloroaniline, and monuron [3-(4-chlorophenyl)-l,l-dimethylurea] (Gomez et al., 1982). Products reported from the combustion of diuron at 900 °C include carbon monoxide, carbon dioxide, chlorine, nitrogen oxides, and HCl (Kennedy et al., 1972a). 

A somewhat different mechanism was deduced by Baker and Bailey 4f) from their own studies of the systems phenyl isocyanate and ethyl p-aminobenzoate (benzocaine) cyclohexyl isocyanate and aniline phenyl isocyanate and aniline p-methoxyphenyl isocyanate and aniline. While Craven postulates one complex. Baker and Bailey assume two complexes of the isocyanate one with the amine, and one with the product urea. 

New processes include synthesis of /V-alkylated anilines from olefins and aniline in an inert solvent with at least one catalyst from a range that includes alkali metal alcoholates, alkaline earth metal alcoholates, alkali metal amides and alkaline earth amides36. The uses for /V,/V-dimethylaniline (11) include in the manufacture of polyester resins, sulfur recovery (in copper refining), insecticides and fungicides, dyes, pharmaceuticals, explosives, rubber products, specialty isocyanates and petroleum additives. The /V-ethylaniline (26) is a dye intermediate and rubber additive, and is used for bum control in explosives, while /V,/V-diclhylaniline is used in production of polyester resins, pharmaceuticals, diazo prints (lithographic), and dyes, and as a petroleum additive37. 

However, treatment of aniline with COFj at 150 C, in a pressure vessel in the presence of sodium fluoride (to absorb the HF), results in the production of the bis-fluoroformyl derivative (13.8) [632]. This compound may arise as a result of the formation of phenyl isocyanate from the dehydrofluorination of PhNHCOF, followed by subsequent addition of COFj (see Section 13.14.5.5) [632]. 

The complex, C, is converted subsequently to the urea by transfer of an amine hydrogen to the 0 or N of the isocyanate. Similar complexes have been postulated to account for amine catalysis of urethane formation. Hydrogen transfer appears to be a mechanistically diflacult step, as judged by the complex kinetics of the isocyanate-amine reaction. The reaction of phenylisocyanate with aniline, for example, is self-catalyzed by the amine and autocatalyzed by the product urea. This complicated kinetic behavior may result from formation of a six-membered cyclic complex which facilitates hydrogen transfer. 

The resulting carbamate salts 235 were then reacted with a freshly prepared solution of the activated betaine of TPP or TBP and DIAD to provide the isocyanates 236. After completion of the reaction, as monitored by the IR stretch of the isocyanate, the desired products were obtained by fractional distillation or flash chromatography. Aniline, benzylamine and 2,6-diisopropylaniline gave no or very poor yields of the desired isocyanates under these reaction conditions. The poor yields in these reactions are due to formation of nonreactive intermediate carbamoylhydrazines or competing triazolinone formation the latter arises from reaction of an activated arylisocyanate and the Mitsunobu betaine. 

It is extremely interesting that the same product, o-phenylbenzohydryl-aniline, was obtained also from phenyl isocyanate, from phenyl isothiocyanate and—as shown by Campbell in 1932— from the oxime of benzophenone (5). 

To prepare model compounds representing the polymers structures two model compounds were prepared by reacting imidazole-blocked phenyl isocyanate (T STC) with PMDA and BPDA (coded PMDA-M and BPDA-M, respectively) under the same conditions as polymerization reaction. However, from the same reaction with NTDA we could not obtain pure compound NTDA-M and the yield was low(27 %). Tlie product obtained always revealed some incomplete cyclization, and it needed be additionally cyclized by reflux in acetic anhydj ide for prolonged time. More conveniently and in higher yield, it could be prepared by direct condensation with aniline in cyclohexane/DMAc mixture at reflux temperature. 

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