Transfer of Carbamate Group to Acylating Agents

The transfer of carbamate groups from metal carbamates to acylating agents, such as acyl halides and chloroformates, has been extensively studied by Calderazzo and coworkers. 

At ambient temperature, the reaction of metal N,N-dialkylcarbamates with acyl halides, R C(0)C1, or acetic anhydride, yields a carbamic-carboxylic mixed anhydride (Equation 6.4) which, on occasion, may undergo decarboxylation and convert into the corresponding amide R2NC(0)R [2]. 

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. 

The reaction in Equation 6.6 involves, as the key step, the O-methoxycarbonylation of carbamate anion to give a carbamic-carbonic mixed anhydride, RNHC(0)0C(0) OMe (Equation 6.7) [62d], which, under the working conditions, undergoes a very rapid decarboxylation and is converted into the carbamate product (Equation 6.8). 

With respect to the synthesis from amines, C02 and alkyl halides, the synthesis of carbamates from amines, C02 and alcohols (Equation 6.10) is not only a phosgene-free, but also a halogen-free process. Moreover, water forms as the only reaction coproduct. Whilst these features make the route very attractive from the point of view of environmental sustainability, unfortunately the reaction suffers from both thermodynamic and kinetics limitations. Kinetic impediments make necessary the use of a suitable catalyst which, moreover, must be water-tolerant in order to avoid deactivation by cogenerated H20. Several strategies have been explored to overcome these restraints, based mainly on the use of alcohols in a dehydrated form (for instance, as ortho esters or ortho carbonates) [63], or on the use of dehydrating agents [64, 65]. 

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