Carbaminic acids

An isocyanate 2 formed by a Curtius rearrangement can undergo various subsequent reactions, depending on the reaction conditions. In aqueous solution the isocyanate reacts with water to give a carbaminic acid 6, which immediately decarboxylates to yield an amine 3. When alcohol is used as solvent, the isocyanate reacts to a carbamate 7 ... 

In aqueous alkaline solution, the isocyanate is unstable it reacts by addition of water to give the intermediate carbaminic acid 5, which subsequently decarboxy-lates to yield the amine 3. 

The fert-butyloxycarbonyl (BOC) group is cleaved using TFA in an aprotic solvent like CH2C12. The cleavage proceeds via protonation of the carbonyl group of carbamate 16, subsequent elimination of carbenium ion 58 and liberation of unstable free carbaminic acid 60 which breaks down yielding the unprotected indole 61 with loss of C02. Cation 58 is deprotonated giving 2-methyl propene 59. 

When primary aliphatic isocyanates that show the lowest reactivity compared to secondary or aromatic isocyanates are used in combination with hydrophilic (pre-) polymers, crosslinking may be performed in aqueous solution without the use of additional crosslinkers. At neutral pH, hydrolysis of isocyanates to carbaminic acid with subsequent decarboxylation yields amines. These amines react much more rapidly than water with isocyanates, resulting in crosslinking if the functionality per macromolecule is more than two [43], This crosslinking reaction can be quenched by adjustment of the pH value. At pH values above 10, carbamate formation is faster than decarboxylation, whereas at pH values below 3 an almost quantitative protonation of the formed amino groups results in the formation of ammonium. In both cases, chemical crosslinking is prevented. 

Another general approach to acid anhydrides from carboxylic acids was developed with esters of cyanuric acid as reagents (equation 37). If the reactions are carried out in solvents like carbon tetrachloride or benzene, the resulting aryl esters of carbaminic acid are insoluble and can be removed by filtration. 

The reaction of amide chlorides with IV-unsubstituted carbaminic acid esters or lV,, -trisubstituted ureas or thioureas affords the corresponding amidinium salts in good yields. 

In this step, in contrast to that of (18-29), the ring of the growing macromolecule (here the dimer) is activated toward the monomer anion by an additional CO group ( ). Therefore, the monomer anion reacts preferentially with the polymer, not with the monomer, which still has to be converted to anionic form. This reaction step includes the regeneration of the monomer anion by the formation of the carbamine acid group [(18-31)], etc. 

Caffeine, 131 ( arbamide, 126 Carbamine reaction, 71 Carbolic acid, 179 Carbon, qualitative analysis t Carbon, quantitative analysis 4 Carlus method, 22, 28 Chaiitiway s Xi QX o 174 Chloracetic acid, S7 Chloral, 99 Chloral hydrade, 99 ( hlorbydrin, tii / Chlorobenzoic acid, 166 < hlorofonTi, 70 / Chlorutulueiie, 16 ... 

Amino acids (as well as ammonia and amines) react with carbon dioxide in a reversible reaction to give carbaminates according to the following scheme ... 

The position of the equilibrium is pH dependent because of the protolytic properties of the molecules involved, the amino acid, the carbon dioxide carbonic acid hydrogen carbonate carbonate system, and the car-baminic acid. At low and at very high pH values only negligible amounts of carbaminates will be present in equilibrium with amino acids and the carbonate system. The C-nmr investigations have shown that the highest fraction of carbaminate present in equilibrium with glycine and the carbonate system... 

The formation and decomposition of carbaminates is not instantaneous at physiological temperatures. Information on rate constants can be found in the literature (Jensen and Faurholt, 1952 Caplow, 1968). As a conclusion it can be stated that, in the presence of the carbonate system at weakly basic pH, a substantial fraction of an amino acid may be transformed to the carbaminate. This may give rise to changed reaction possibilities for the amino acid, the actual amino acid concentration being lower than in the absence of the carbonate system. Furthermore, if the amino acid is removed in another reaction, it may be slowly regenerated by decomposition of the carbaminate. Carbaminates may also be responsible for double-spot formation of amino acids in paper chromatography and especially in paper electrophoresis with carbonate buffer systems (Frahn and Mills, 1%4). 

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