ETD

• boric acid
• CO2 fixation
• cyclic carbonates
• DFT
• phase behaviour
• styrene oxide
• tetrabutylammonium halides
• Walden analysis

The cycloaddition of CO2 to epoxides is a useful model reaction for converting an abundant C1 feedstock into value-added cyclic carbonates1. In this contribution, styrene oxide (SO) was converted into styrene carbonate (SC) using homogeneous tetrabutylammonium halide/boric acid systems under solvent-free, SO-rich conditions. The work connects three levels of evidence: physicochemical characterization of the reaction medium, reaction optimization and DFT-based mechanistic analyses. DSC phase-behaviour studies, conductivity, viscosity and Walden analysis2 showed that concentrated SO/TBAI/BA and SC/TBAI/BA mixtures are non-ideal associated liquids, where ion pairing, hydrogen bonding and phase boundaries influence the temperature/composition range where the mixture remains in the liquid state. In contrast, the investigated SO/TBACl/BA mixtures remained homogeneous for weeks at -25 °C, supporting their practical use as liquid catalytic media.
Catalytic screening identified TBACl/BA as the most effective practical system. Under excess CO2, the optimized 13:1:0.3 SO:TBACl:BA composition gave 98.5% SC yield after 4 h at 100 °C. Computational models with TBAX and BA indicate that the halide trend cannot be rationalized by considering only the intrinsic anion nucleophilicity: local solvation, ion pairing, BA-assisted activation and the stability/lability of ring-opened intermediates jointly determine the observed catalytic behaviour.