Reactions with dialkyl carbonates
Reactions with dialkyl carbonates reactions with dialkyl carbonates dimethyl carbonate. Most of the research in this area, has been devoted to the use of dimethyl carbonate (DMC). Dimethyl carbonate is a non toxic compound whose current industrial production (~ 12.000 t/y) takes place through clean technologies based on the oxidative carbonylation of methanol, and on the insertion of CO2 on epoxides (followed by transesterification). Thanks to its versatile reactivity, DMC can be used both as a carboxymethylating and as a methylating agent, in place of highly toxic and corrosive chemicals such as methyl halides (or dialkyl sulfates), and phosgene.
Besides its eco-friendly nature, DMC offers excellent synthetic solutions. In the field of methylation reactions for example, DMC allows unprecedented selectivity for the mono-C-methylation of CH2-active compounds, as well as for the mono-N-methylation of primary aromatic amines. Solid bases and zeolites are effective catalysts for these processes.
Base : Alkaline carbonates; X = CN, CO2Me, SO2R
Z : Alkyl-, alkoxy-, halide-, carboxy-groups
MY : Faujasites ( M= Li, Na, K, Cs )
W : Alkyl-, alkoxy-, OH, CONH2, CO2H, CH2OH
Methyl alkyl carbonates. Methyl alkyl carbonates (MACs, ROCO2Me), prepared by the transesterification of DMC with di- and tri-ethylene glycols mono-alkylethers, are excellent methylating agents yielding results comparable to those of DMC in terms of selectivity (Scheme 1). With respect to DMC (bp 90 °C), MACs have tunable boiling points and medium-to-low vapour pressures, which greatly help to simplify the experimental setup of reactions requiring high temperatures (120-200 °C, the case of many methylations of scheme 1): experiments can be carried out in normal glass labwares at atmospheric pressure, without autoclaves necessary for DMC-mediated processes.
Higher homologues of DMC, particularly diethyl and dibenzyl carbonates (DEC and DBnC), have been investigated under conditions similar to those for DMC. Both DEC and DBnC are good and selective alkylating agents. Special applications of these carbonates have also been highlighted in the synthesis of alkyl carbamates carried out in presence of scCO2 or of onium salts as catalysts.