Carbamation, amine-dimethyl carbonate reactions

Aresta and Quaranta studied the reactivity of alkylammonium N-alkylcarbamates (RNH3)02CNHR towards a different acylating substrate, such as dimethyl carbonate (DMC) [62a, b]. Carbamate salts (RNH3)02CNHR (R = benzyl, allyl, cyclohexyl), prepared in situ from aliphatic primary amines and C02, reacted with DMC to afford N-alkyl methylcarbamates (Equation 6.6). The reaction requires mild conditions (343-363 K 0.1 MPa C02 pressure) and can be carried out in DMC used as solvent and reagent. At 363 K, carbamate esters were obtained in satisfactory yield (45-92%) with high selectivity, as side products such as ureas, N,N-dialkylcarbamate esters, and alkylated amines were formed in very small amounts. 

A fourth type of product, a carbamate RNHCOOR , can be obtained from primary or secondary amines, if these are treated with CO, O2, and an alcohol R OH in the presence of a catalyst. " Primary amines react with dimethyl carbonate in supercritical CO2 (see p. 414) to give a carbamate. " Carbamates can also be obtained from nitroso compounds, by treatment with CO, R OH, Pd(OAc)2, and Cu(OAc)2, " and from nitro compounds. " When allylic amines (R2C=CHRCHRNR 2) are treated with CO and a palladium-phosphine catalyst, the CO inserts to produce the p,y-unsa-turated amides (R2C=CHRCHRCONR 2) in good yields. " Ring-expanded lactams are obtained from cyclic amines via a similar reaction (see also, 16-22). Silyloxy carbamates (RNHC02SiR 3) can be prepared by the reaction of a primary amine with carbon dioxide and triethylamine, followed by reaction with triisopropylsilyl triflate and tetrabutylammonium fluoride. ... 

Carbonic add diesters are very attractive reagents and of great economic interest because they represent safe, nonenvironmentally acceptable alternatives to phosgene for carbonylation and carboxylation reactions. For example, methoxycarboxylation with dimethyl carbonate offers an eco-friendly alternative route for the production of carbamates and isocyanates, which are valuable precursors of ureas (see Sections 4.3.1 and 4.3.2) [781, 782]. The method is comparable, from an environmental point of view, with the transition metal catalyzed carbonylation of nitro compounds and amines with CO. 

Alternative sources for carbonyl groups were also investigated. Dimethyl carbonate, for example, efficiently replaces the dangerous CO/O2 mixture in the carbonylation of aliphatic amines to yield alkylcarbamates using ILs as reaction media and catalyst [80]. The solid carbamate can be recovered by simple filtration from the biphasic mixture of dimethyl carbonate/IL after reaction. The IL phase can be... 

The use of NBD-Cl for the fluorescence analysis of alkylamine-generating pesticides has been investigated [173]. A two-phase reaction system is employed for the hydrolysis and labeling of N-methyl- and N,N-dimethyl-carbamate pesticides. The residue is hydrolyzed in 0.1 M sodium carbonate and the liberated amine is treated with NBD-Cl in an organic phase (IBMK, isobutyl methyl ketone) above the aqueous layer. An aliquot portion of the organic layer is used for chromatography. The reactions involved are shown in Fig. 4.65. 

When weaker bases like potassium carbonate are used as catalysts, the reactions have to be carried out at high temperatures in an autoclave in order to achieve high conversion of the starting material. Table 3 shows results obtained with aromatic and aliphatic amines. Aliphatic amines, that are harder nucleophiles, give mainly the corresponding carbamates and methylcarbamates after 22 h, while aromatic amines react slower, and their softness is responsible for the formation of mono and dimethylated products. 

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