Cross-Carboxylation of Methanol and Other Alcohols With CO2 Into Asymmetric Alkyl Methyl Carbonates Over a CeO2 Catalyst.

One-pot synthesis of asymmetric alkyl methyl carbonates from CO2, methanol (MeOH), and various alcohols was demonstrated using CeO2 and 2-cyanopyridine as a heterogeneous catalyst and dehydrating agent, respectively. The molar ratio of MeOH to the other alcohol and structure of the latter alcohol governed the distribution of three organic carbonates, that is, asymmetric alkyl methyl carbonate, dimethyl carbonate (DMC), and dialkyl carbonate. For primary alcohols such as ethanol, 1-propanol, and 1-butanol, an equimolar ratio of MeOH to the other alcohol afforded the highest distribution of the asymmetric alkyl methyl carbonate. In contrast, the low reactivity of 2-propanol due to steric hindrance from its bulky alkyl moiety enabled the preferential formation of isopropyl methyl carbonate (iPMC) with >80% distribution among the three carbonates of iPMC, DMC, and diisopropyl carbonate. The time-course study indicated the involvement of two reaction routes for synthesizing asymmetric organic carbonates: (i) direct cross-carboxylation of MeOH, the other alcohol, and CO2 and (ii) transesterification between DMC, which is formed via homo-carboxylation of MeOH and CO2, and the other alcohol. In the case of iPMC synthesis, the indirect route (ii) became dominant because of the low reactivity of 2-propanol.