CO2-based monomers: a detailed investigation of the carbonation reaction over diols for the synthesis of (a)cyclic carbonates promoted by organic dual systems

The present research aims at investigating in details the reaction between carbon dioxide and a diol to afford cyclic and acyclic carbonates in moderate to good yields. Thanks to its chemical structure, CO2 has been identified as a promising reagent for the synthesis of organic carbonates as it is non-toxic, cheap and an abundant C1 renewable feedstock, and could potentially replace phosgene based carbonates produced industrially. Prior research has been focused on the epoxide/CO2 coupling to afford cyclic carbonates in good yields. Yet, this coupling suffers from limitations such as restricted substrates and exclusive synthesis of 5-membered cyclic carbonates. In order to extend the products availability through Carbon Dioxide Utilization, more recent research has been carried out with new substrates such as the use of diols. Thus, efforts are still needed to identify cheaper and more efficient protocols for the selective synthesis of cyclic and acyclic carbonates from the coupling of CO2 with diols. In this context, the use of organic bases/alkylating agent dual system as promoters and low pressure and temperature conditions (P<1MPa, r.t) seem to be a good alternative to afford organic carbonates. Thanks to an in-situ ATR-IR spectroscopy monitoring, a detailed understanding of the synthesis of propylene carbonate from propylene glycol and CO2 promoted by organic bases and the addition of co-reagents is proposed. Reaction intermediates are identified, and kinetic profiles determined for various parameters (pressure, temperature, solvent…). Two dual systems, one combining DBU and an alkyl halide (e.g Bromoethane), and another one with TEA and TsCl were compared. Different substrates were also tested such as 1,x diols (2 ≤ x ≤ 4) and Isosorbide. The selectivity toward the cyclic carbonate or the side products is discussed. Density Functional Theory (DFT) was also performed in this work to understand and validate the reaction pathways proposed via ATR-IR monitoring. Structures of transition states were optimized and comparisons between several reagents and/or paths were calculated, giving capital information about further enhancement of the CO2-diol conversion into carbonates.