Carbamic acid, protonated

The data indicate that in strong acidic solution H2C03 may be in equilibrium with protonated carbonic acid. The structures of carbamic acid and its O- and N-proto-nated forms 111 and 112 were calculated at the MP2/6-31G(d) level.140... 

Although these protecting groups may seem bizarre, their value lies in the fact that they can be removed easily by acid-catalyzed hydrolysis under very mild conditions. The sequence of steps is shown in Equation 23-10 and involves proton transfer to the carbonyl oxygen and cleavage of the carbon-oxygen bond by an SN1 process (R = tert-butyl) or SN2 process (R = phenyl-methyl). The product of this step is a carbamic acid. Acids of this type are unstable and readily eliminate carbon dioxide, leaving only the free amine (also see Section 23-12E) ... 

Guanidine, H2N(G=NH)NH2, is the amidine of carbamic acid, H2N(CO)OH. Guanidine forms three types of complexes with metals cationic (in which the guanidinium cation is formed by taking up a proton), adducts with neutral molecules or coordination products with ionic salts, and substitution products. A brief account of each type is presented below. 

Ammonium carbamate is prepared from dry ice and liquid ammonia [14]. These conditions are very similar to the conditions under which we have observed the formation of amine salts. To some readers, ammonium carbamate may seem to be an exotic compound. In fact, it is manufactured industrially on a multiton scale, because on heating (usually at 100-185°C) ammonium carbamate is converted to urea and water [14-16]. Urea is important for both the agricultural and the plastics industries. The ammonium carbamate is not always isolated during urea preparation. Instead, the reactions are carried out under conditions where the carbamate is just an intermediate. Ammonium carbamate is only moderately stable and it gradually loses ammonia in air. Although the data are sparse, the rate of decomposition of carbamates in solution seems to decrease as the volatility of the parent amine decreases [17]. Free carbamic acids in solution do not decompose spontaneously to free amine and C02. Instead, the acid ionizes by reaction with water the proton is transferred from the hydronium ion to the amine and then decomposition occurs [17]. Acids catalyze the decomposition. 

C 161.5 in HSO3F-SO2CIF solution is only 0.9 ppm shielded from that of protonated carbamic acid. 

Sulfonylureas undergo hydrolysis as shown in the mechanistic scheme in Figure 64 (106). Under acid-catalyzed conditions, water addition leads to loss of an amine and formation of a carbamic acid derivative. Acid-catalyzed loss of CO2 from the carbamic acid derivative yields the corresponding sulfonamide. It was proposed that initial protonation is the rate-determining step in the hydrolysis. 

Fig. 8.5. Proton-induced decomposition of W-substituted carbamic acid C into carbon dioxide and a primary ammonium hydrochloride—a reaction occuring in the course of acidic hydrolysis of organic isocyanates.

Fig. 8.6. Proton-induced decomposition of /(/-substituted carbamic acid B into carbon dioxide and a primary ammonium trifluoroacetate—a reaction occuring during the acidic deprotection of a Boc-pro-tected amine.

Carbon disulfide is the dithio derivative of C02. It is only a weak electrophile. Actually, it is so unreactive that in many reactions it can be used as a solvent. Consequently, only good nucleophiles can add to the C—S double bond of carbon disulfide. For example, alkali metal alkoxides add to carbon disulfide forming alkali metal xan-thates A (Figure 7.4). If one were to protonate this compound this would provide compound B, which is a derivative of free dithiocarbonic acid. It is unstable in the condensed phase in pure form, just as free carbonic acid and the unsubstituted carbamic acid (Formula B in Figure 7.3) are unstable. Compound B would therefore decompose spontaneously into ROH and CS2. Stable derivatives of alkali metal xanthates A are their esters C. They are referred to as xanthic add esters or xanthates. They are obtained by an alkylation (almost always by a methylation) of the alkali metal xanthates A. You have already learned about synthesis applications of xanthic acid esters in Figures 1.32, 4.13, and 4.14. 

The Boc group is easily cleaved by brief treatment with trifluoroacetic acid (TFA), CF3COOH. Loss of a relatively stable ferf-butyl cation from the protonated ester gives an unstable carbamic acid. Decarboxylation of the carbamic acid gives the deprotected amino group of the amino acid. Loss of a proton from the fert-butyl cation gives isobutylene. 

Kinetic studies on the hydrolysis of [Ru(NH3)5NCO] to give [Ru(NH3)g] " and CO2 suggest that the reaction proceeds via N protonation of the coordinated NCO , followed by addition of HjO to give a carbamic acid complex, which subsequently loses HgO and CO2 to give the product (290). 

They also obtained the sequence of z-pentapeptides (carbamic acid) as well as the protonated molecules of these nonvolatile compounds. The presence of leucine in these compounds could be differentiated from that of isoleucine to some extent, but reproducibility of measurements remains a problem. 

Chemical interactions of CO2 with substrates, products or catalysts can also play a major role in defining rate and selectivity of a given reaction. This chemical influence must not necessarily be positive, making it even more important to remember that CO2 does not always provide an inert medium. For example, hydrogen carbonate and protons are generated in the presence of water (pH = 3), carbamic acids or carbamates are formed with basic N-H functionalities, and the coordination ability and reactivity towards various transition metal centers is well established [20]. One of the appealing prospects of the chemical reactivity of CO2 is its simultaneous use as solvent and Cj building block in metal-catalyzed processes. 

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