Isocyanates ureas

Preparation from Nitrene Intermediates. A convenient, small-scale method for the conversion of carboxyhc acid derivatives into isocyanates involves electron sextet rearrangements, such as the ones described by Hofmann and Curtius (12). For example, treatment of ben2amide [55-21-0] with halogens leads to an A/-haloamide (2) which, in the presence of base, forms a nitrene intermediate (3). The nitrene intermediate undergoes rapid rearrangement to yield an isocyanate. Ureas can also be formed in the process if water is present (18,19). 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

Since chiral amines are much more available than chiral isocyanates, ureas have often been obtained from reaction of an amine used as the chiral source with a stoichiometric amount of a non-chiral isocyanate [41]. Similarly, thioureas are obtained by reacting isothiocyanates with amines. The corresponding ureas and thioureas (examples in the following sections) are... 

Conversion of Carbamates into Urethanes, Isocyanates, Ureas, and Carbodiimides... 

Hydrolysis of the ester forms adipic acid, used to manufacture nylon—6. Carbonylations of nitroaromatics are used to synthesize an array of products including amines, carbamates, isocyanates, ureas and azo compounds. These reactions are catalyzed by iron, ruthenium, rhodium and palladium complexes. For example, carhonylation of nitrobenzene in the presence of methanol produces a carbamate ... 

Luda, M.P. Costa, L. Bracco, P. Levchik, S.V. Relevant factors in scorch generation in fire retarded flexible polyurethane foams II Reactivity of isocyanate, urea and urethane groups. Polym. Degrad. Stab. 2004, 86, 43-50. 

Phosgene is widely used in organic chemistry as a building block providing the C = O such as in carbamates, carbonates, isocyanates, ureas, heterocycles etc., or as a reagent for chlorination, dehydration, alkylation, de-alkylation, protection and activation etc. 

Isocyanate-urethane and isocyanate-urea reactions then yield biurets and allophanates, which produce crosslinking and branching ... 

Du Bois and coworkers have extended such a strategy for intramolecular C—H aminations to the synthesis of other 5-membered rings. Whereas amides would have led to rearrangement to form the corresponding isocyanate, urea and guanidine derivatives produce imidazoUdin-2-ones and 2-aminoimidazolines, respectively (Eqs. (5.8) and (5.9)) [46]. Rh2(esp)2, a tethered dicarboxylate-derived rhodium complex is used, as this catalyst displays better reactivity and selectivity [45]. 

The foregoing examples of phosgene-free methods for synthesis of carbamates, isocyanates, ureas, polycarbonates, dyes, polymers, and other pharmaceutical or agrochemical intermediates and products all illustrate ways that green chemistry reduces the environmental impact... 

Benomyl (Benlate) ra-Butyl isocyanate Urea fungidde... 

Figure 5.9 [ CJCarbonylation of primary and secondary amines an alternative route to carbon-14-labeled isocyanates, ureas, and thiocarbamates...

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