Carbamate 0- isocyanate

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 ... 

Carbamates have mainly been used in solid-phase synthesis as linkers and protective groups for amines (see Sections 3.6.2 and 10.1.10.1). Carbamates are generally prepared by treating amines with aryl carbonates or chloroformates, which can be prepared from alcohols and phosgene or synthetic equivalents thereof. The alternative route, in which carbamates, isocyanates, or carbamoyl chlorides are reacted with alcohols, is less widely used, but can also lead to satisfactory results on insoluble supports (Tables 14.7 and 14.8). 

The hydrosilation reaction is highly compatible with a number of functional groups including ether, epoxide, acetal, ketone, ester, nitrile, amine, amide, nitro, carbamate, isocyanate, phosphate, sulfide, sulfone, and others." As a consequence, it is a very attractive reaction for the construction of complex organic molecules. Sometimes allyUc displacement is observed as a side reaction, especially in the reaction of allyl halides. ... 

Building block for organic carbamates-isocyanates-carbonates, supplanting... 

The carbonylation of amines and of nitrocompounds is of interest because it provides a non phosgene route to the synthesis of carbamates, isocyanates and ureas which are products of remarkable commercial value. 

Carbamates Isocyanates Hydroxynitriles Aminoalcohols Amides (Carboxamides)... 

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... 

Subsequent work by Pirkle and co-workers demonstrated that highly chlorinated silanes are even more effective at inducing this transformation [202-204]. Recently, a systematic study of the reaction of carbamates with various chlorosdanes has been reported [205]. It was found that, in general, the silane reactivity increases in the order MesSiCl < Me2SiCl2 < MeSiCla < HSiCh. Moreover, steric hindrance at the reaction center slows down the reaction, and, with N-Boc carbamates, isocyanate production is quite sluggish. In the cases involving O-alkyl carbamates, the reactions normally need to be conducted between room temperature and 70 °C. 

The structure-property behavior of novel tertiary alcohol-chain extended segmented polyurethane-urea copolymers was investigated. These copolymers were synthesized by utilizing the carbamate-isocyanate... 

Ethyl carbamate in wine is formed (mostly at the end of fermentation) from urea. The intermediates of its degradation are probably cyanates and cyanic acid (HO-C=N), also known as hydrogen cyanate, which may isomerise to isocyanic acid (H-N=C=0). Iso-cyanic acid can also arise by protonation of the cyanate anion and nucleophilic addition of ethanol to isocyanic acid yields ethyl carbamate. Isocyanic acid also reacts with other nucleophilic reagents, such as water (with formation of ammonia and carbon dioxide), thiols and amino groups of proteins. By catalysis with ornithinecar-bamoyl transferase, citrulline is transformed into ornithine and carbamoyl phosphate, the ethanolysis of which yields ethyl carbamate (Figure 12.39). 

Phenyl- and a-naphthyl-urethanes (Phenyl- and a-naphthyl-carbamates). Both phenyl isocyanate and a-naphthyl isocyanate react with alcohols to yield phenyl-urethanes and a-naphthyl-urethanes respectively ... 

Alcohols (but not ethers) also react with phenyl isocyanate or with the corre-sponding crystalline a-naphttiyl isocyanate to give carbamates or urethanes (see Section 111,27, ), but these substances are hardly suitable as class reagents. 

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-... 

Rea.ctlons, As with other tertiary alcohols, esterification with carboxyUc acids is difficult and esters are prepared with anhydrides (181), acid chlorides (182), or ketene (183). Carbamic esters may be prepared by treatment with an isocyanate (184) or with phosgene followed by ammonia or an amine (185). 

A simpler nonphosgene process for the manufacture of isocyanates consists of the reaction of amines with carbon dioxide in the presence of an aprotic organic solvent and a nitrogeneous base. The corresponding ammonium carbamate is treated with a dehydrating agent. This concept has been apphed to the synthesis of aromatic and aUphatic isocyanates. The process rehes on the facile formation of amine—carbon dioxide salts using acid haUdes such as phosphoryl chloride [10025-87-3] and thionyl chloride [7719-09-7] (30). 

The reaction of isocyanates with alcohols to form carbamates is catalyzed by amines and a variety of organometaHic compounds. 

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

Industrially, polyurethane flexible foam manufacturers combine a version of the carbamate-forming reaction and the amine—isocyanate reaction to provide both density reduction and elastic modulus increases. The overall scheme involves the reaction of one mole of water with one mole of isocyanate to produce a carbamic acid intermediate. The carbamic acid intermediate spontaneously loses carbon dioxide to yield a primary amine which reacts with a second mole of isocyanate to yield a substituted urea. 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

This approach is compHcated by the fact that the isocyanate is produced via the thermolytic cleavage of the methyl carbamate. Reactions with the unconverted carbamate caimot be prevented. Much effort has been focused on improving the selectivity of the latter step. 

An alternative approach involves the reaction of an alkyl carbamate with a tertiary olefin (89,90). The resultant carbamates are thermally cracked at temperatures of 150—350°C to yield the isocyanate. The isocyanate is generally purified via distillation. 

Aliphatic Isocyanates. Aflphatic diisocyanates have traditionally commanded a premium price because the aflphatic amine precursors ate mote expensive than aromatic diamines. They ate most commonly used in appHcafions which support the added cost or where the long-term performance of aromatic isocyanates is unacceptable. Monofuncfional aflphatic isocyanates, such as methyl and -butyl isocyanate, ate used as intermediates in the production of carbamate-based and urea-based insecticides and fungicides (see Fungicides, agricultural Insectcontroltechnology). 

Naphthol is mainly used in the manufacture of the insecticide carbaryl (59), l-naphthyl A/-methyicarbamate/ iJ-2j5 - (Sevin) (22), which is produced by the reaction of 1-naphthol with methyl isocyanate. Methyl isocyanate is usually prepared by treating methylamine with phosgene. Methyl isocyanate is a very toxic Hquid, boiling at 38°C, and should not be stored for long periods of time (Bhopal accident, India). India has developed a process for the preparation of aryl esters of A/-alkyl carbamic acids. Thus l-naphthyl methylcarbamate is prepared by refluxing 1-naphthol with ethyl methylcarbamate and POCl in toluene (60). In 1992, carbaryl production totaled > 11.4 x 10 t(35). Rhc ne-Poulenc, at its Institute, W. Va., facihty is the only carbaryl producer in United States. 

Reaction between poly(vinyl alcohol) and isocyanates yields substituted carbamate esters (127—134) ... 

Carbamic acid [463-77-4] NH2COOH, is the hydrated form of isocyanic acid [75-13-8] H—N=C=0. It is not known in the free state hydrolysis rapidly gives ammonia and carbon dioxide. 

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