Carbon dioxide-amine reaction

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

Exxon s Flexsorb SE solvents achieve high hydrogen sulfide selectivity by virtue of their molecular structure. These solvents are sterically hindered secondary amines. A bulky molecule is used to shield the available hydrogen radical on the nitrogen atom and prevent the insertion of carbon dioxide. The reaction with hydrogen sulfide is not sensitive to the amine s structure, so the steric hinderance affords higher hydrogen sulfide selectivity. 

It should be appreciated that this example is one of the few very simple practical cases of a packed column reactor. The removal of carbon dioxide by reaction is a step in many important industrial processes, e.g. the nitrification of natural gas and of hydrogen in the manufacture of ammonia. Most of these processes0 " use as the liquid absorbent a solution which can be regenerated, for example solutions of amines or potassium carbonate0", and the design of these columns is distinctly more complicated. 

Vinyl carbamates are accessible from carbon dioxide, amines, and al-kynes. The reaction is catalyzed by Ru3(CO)l2. As a typical example, phenylacetylen, C02, and HNEt2 are converted into the isomeric phenyl-vinylcarbamates (toluene, 140°C, 50 bar, 20 hr, yield 36%, CT 18) (391,392) ... 

CARBONYL DIFLUORIDE or CARBONYL FLUORIDE (353-50-4) CFjO Noncombustible gas. Reacts with water or steam, producing hydrogen fluoride gas and carbon dioxide. Violent reaction with strong oxidizers bases including amines, amides, and inorganic hydroxides alcohols and glycols dinitrogen tetraoxie, hexafluoroisopropylideneaminolithium ethers in the presence of metal salts. If flow... 

The reaction of carbonimidoyl dichlorides with excess water results in complete hydrolysis with formation of the corresponding amine and carbon dioxide ( ). This reaction affords isocyanate as an intermediate, as evidenced by the isolation of 1,3-diphenylurea from the reaction mixture, obtained by treating phenylcarbonimidoyl dichloride with water In the reaction of tetrameric cyanogen chloride with water at 20°C 2-amino-4,6-dichloro-l,3,5-triazine (LXXXVI) is obtained, whereas at 100 C total hydrolysis to cyanuric acid (LXXXVII) occurs... 

Isocyanates also react with water. The intermediate carbamic acid is so unstable that it decomposes immediately to form amine plus carbon dioxide. This reaction is important for two reasons (1) carbon dioxide bubbles foam the polyurethane as it forms this is the leading process for making foam, and (2) the amine by-product reacts to form more urea groups, which therefore strengthen the final polymer. 

To illustrate the specific operations involved, the scheme below shows the first steps and the final detachment reaction of a peptide synthesis starting from the carboxyl terminal. N-Boc-glycine is attached to chloromethylated styrene-divinylbenzene copolymer resin. This polymer swells in organic solvents but is completely insoluble. ) Treatment with HCl in acetic acid removes the fert-butoxycarbonyl (Boc) group as isobutene and carbon dioxide. The resulting amine hydrochloride is neutralized with triethylamine in DMF. 

The reaction of a vinyl ether with carbon dioxide and a secondary amine gives a carbamic ester (246). 

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). 

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. 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

The reactions of primary amines and maleic anhydride yield amic acids that can be dehydrated to imides, polyimides (qv), or isoimides depending on the reaction conditions (35—37). However, these products require multistep processes. Pathways with favorable economics are difficult to achieve. Amines and pyridines decompose maleic anhydride, often ia a violent reaction. Carbon dioxide [124-38-9] is a typical end product for this exothermic reaction (38). 

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