Dyes as efficient and reusable organocatalysts for the synthesis of cyclic carbonates from epoxides and CO2

Abstract

Inexpensive dyes available at the industrial scale, namely, rhodamine B (RhB), rhodamine 6G (Rh6G) and methylene blue (MB), were investigated as organocatalysts for the cycloaddition of CO₂ to styrene oxide to yield styrene carbonate under solvent-free conditions (80 °C, 10 bar CO₂, 24 h). Each of these dyes consists of a bulky cation, and a chloride anion that can act as a nucleophilic catalytic species in the target reaction. In order to prepare additional catalysts, the Cl⁻ containing dyes were ion-exchanged with KX (X = Br, I) to afford their counterparts with Br⁻ or I⁻ as the anion. Among this set of nine organocatalysts (three dyes, each with three types of halide), the highest yield of styrene carbonate was obtained with Rh6G-I, and trends were identified based on the nature of the organic cation and halide, with the latter having a much larger impact on the activity (I⁻ > Br⁻ > Cl⁻). Additionally, we explored the effect of adding H₂O as a green, inexpensive hydrogen bond donor acting as a co-catalyst, further optimising the styrene carbonate yield (96% with RhB-I and Rh6G-I in the presence of 50 mg H₂O). However, the activity of these organocatalysts was only modest if the reaction temperature was decreased to 45 °C. To tackle this limitation, we designed a tailored yet straightforward modification of RhB-I to synthesise a bifunctional organocatalyst bearing a hydrogen bond donor in proximity of the iodide anion (RhB-EtOH-I). This strategy proved successful and the RhB-EtOH-I catalyst achieved a major increase in styrene carbonate yield (29% after 18 h at 45 °C, 10 bar CO₂) compared to the RhB-I/H₂O catalytic system (7%). The RhB-EtOH-I catalyst was also versatile and promoted the conversion of a broad scope of epoxides with good to high cyclic carbonate yields under relatively mild reaction conditions (60 °C, 10 bar, 24 h). Although these dye organocatalysts were homogeneous, RhB-EtOH-I could be easily recovered by precipitation with diethyl ether and reused without any loss of catalytic activity. Additionally, we demonstrated that nanofiltration was an effective technique for removing the dye organocatalysts from the cyclic carbonate, affording a high purity product (≤0.1 ppm of RhB in propylene carbonate). The metal-free nature of the optimum organocatalyst (RhB-EtOH-I), its facile preparation and the low cost and commercial availability of its precursors, its promising activity under mild reaction conditions and its reusability are all assets in the context of green chemistry and for potential large-scale applicability.