Synthesis of Methyl Carbamates from Primary Aliphatic Amines and Dimethyl Carbonate in Supercritical CO2: Effects of Pressure and Cosolvents and Chemoselectivity

摘要

At 130 °C, in the presence of CO2 (5-200 bar), primary aliphatic amines react with dimethyludcarbonate (MeOCO2Me, DMC) to yield methyl carbamates (RNHCO2Me) and N-methylation sideproductsud(RNHMe and RNMe2). The pressure of CO2 largely influences both the reaction conversionudand the selectivity toward urethanes: in general, conversion goes through a maximum (70-80%)udin the midrange (40 bar) and drops at lower and higher pressures, whereas selectivity is continuouslyudimproved (from 50% up to 90%) by an increase of the pressure. This is explained by the multipleudrole of CO2 in (i) the acid/base equilibrium with aliphatic amines, (ii) the reactivity/solubility ofudRNHCO2ud- nucleophiles with/in DMC, and (iii) the inhibition of competitive N-methylation reactionudof the substrates. Cosolvents also affect the reaction: in particular, a drop in selectivity is observedudwith polar protic media (i.e., MeOH), plausibly because of solvation effects (through H-bonds) ofudRNHCO2ud- moieties. The reaction shows also a good chemoselectivity: bifunctional aliphatic aminesudbearing either aromatic NH2 or OH substituents [XC6H4(CH2)nNH2, X ) NH2, OH; n ) 1, 2], undergoudmethoxycarbonylation reactions exclusively at aliphatic amino groups and give the correspondingudmethyl carbamates [XC6H4(CH2)nNHCO2Me] in 39-65% isolated yields.