Bio-Inspired Metal-Free Catalysis: Natural Sugars Enable Efficient CO2 Conversion into Cyclic Carbonates

The consistent increase in atmospheric CO2 concentration, mostly driven by the global combustion of fossil fuels, is considered one of the primary contributors to the increasing severity of environmental problems, like climate change and global warming. Attending to this issue requires innovative strategies that transform CO2 into a valuable resource. In this work, we report a sustainable and fully metal-free approach for the synthesis of cyclic carbonates via the direct coupling of CO2 with epoxides, using natural sugars as readily available, non-toxic organocatalysts in combination with tetrabutylammonium iodide (TBAI) as a cocatalyst. Seven representative mono- and disaccharides were screened, employing styrene oxide as a model substrate under mild reaction conditions (80 °C, 20 bar CO2, 2 h). Among them, D-xylose exhibited the best catalytic performance. The robustness of this catalytic system was further demonstrated through the efficient transformation of a wide range of terminal, internal, and biomass-derived epoxides into their corresponding cyclic carbonates with high yields and selectivity (up to 99%). Additionally, a set of computational simulations based on density functional theory (DFT) calculations was carried out to gain insight into the atomistic mechanisms involved in this chemical transformation. We identified that the hydroxyl groups of the sugar catalyst play a pivotal role in activating the epoxy ring-opening process, leading to cyclic carbonate formation. This bio-inspired strategy provides a green, cost-effective, and scalable pathway to produce key precursors for organic chemistry, contributing to the development of a circular carbon economy and the advancement of sustainable chemistry.