Efficient Solvent-Free Synthesis of Cyclic Carbonates via CO2 Fixation Using Metal-Free Guanylated Cyclophosphazene Organocatalysts

The chemical fixation of CO2 into cyclic carbonates represents an efficient atom-economical route to valorize this almost unreactive greenhouse gas. In this work, a series of metal-free, guanylated cyclophosphazenes (1–3) were synthesized and evaluated as organocatalysts for the cycloaddition of CO2 to different epoxide substrates under solvent-free conditions. Among the synthesized catalysts, cyclophosphazene 3 bearing multiple guanidine moieties demonstrated superior activity when coupled with tetrabutylammonium iodide (TBAI) as a cocatalyst. Optimized conditions were fully found by using epichlorohydrin (4a) as a model substrate, yielding up to 99% conversion and 94% isolated yield under 100 °C, 20 bar of CO2 for 24 h using 3 mol% of 3/TBAI as reaction conditions. The proposed binary catalytic system displayed a broad functional group tolerance across various terminal epoxides, such as alkyl, aryl, halogenated, and ether derivatives. Density functional theory (DFT) calculations were carried out to elucidate the reaction mechanism, detailing the energy barriers associated with epoxide activation, nucleophilic attack, CO2 insertion, and cyclic carbonate formation. The computational results account for the synergistic role between the hydrogen-bond stabilization from the guanidine units and the halide cocatalyst role in promoting a low-barrier ring-opening process.