| Abstract
| - At 130 °C, in the presence of CO2 (5−200 bar), primary aliphatic amines react with dimethylcarbonate (MeOCO2Me, DMC) to yield methyl carbamates (RNHCO2Me) and N-methylation side-products (RNHMe and RNMe2). The pressure of CO2 largely influences both the reaction conversionand the selectivity toward urethanes: in general, conversion goes through a maximum (70−80%)in the midrange (40 bar) and drops at lower and higher pressures, whereas selectivity is continuouslyimproved (from 50% up to 90%) by an increase of the pressure. This is explained by the multiplerole of CO2 in (i) the acid/base equilibrium with aliphatic amines, (ii) the reactivity/solubility ofRNHCO2- nucleophiles with/in DMC, and (iii) the inhibition of competitive N-methylation reactionof the substrates. Cosolvents also affect the reaction: in particular, a drop in selectivity is observedwith polar protic media (i.e., MeOH), plausibly because of solvation effects (through H-bonds) ofRNHCO2- moieties. The reaction shows also a good chemoselectivity: bifunctional aliphatic aminesbearing either aromatic NH2 or OH substituents [XC6H4(CH2)nNH2, X = NH2, OH; n = 1, 2], undergomethoxycarbonylation reactions exclusively at aliphatic amino groups and give the correspondingmethyl carbamates [XC6H4(CH2)nNHCO2Me] in 39−65% isolated yields.
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