Hydrogen‐Bond DonorAssisted Cycloaddition of CO ₂ to Epoxides Catalyzed by Schiff Base Derived Ionic Liquids: Catalytic, Mechanistic, and Kinetic Studies

ABSTRACT

Four new salicylaldehyde, o ‐vanillin, 5‐nitrosalicylaldehyde and benzaldehyde derived Schiff base ionic liquids (ILs) with bromide counterion ( IL‐Sal , IL ‐ Van , IL‐NSal, and IL ‐ Bz ) have been facilely synthesized. The co‐existing phenolic‐OH and counteranion, Br ⁻ in IL‐Sal , IL‐Van , and IL‐NSal are employed as bifunctional organocatalysts for epoxide‐CO ₂ coupling reaction, where ‐OH activates the epoxide via hydrogen bonding and Br ⁻ promotes ring opening. Experimental evidence from controlled experiments, FT‐IR and NMR spectroscopy corroborates a catalytic mechanism involving catalyst‐epoxide hydrogen bonding. A control experiment using IL‐Bz , lacking phenolic‐OH, shows inferior activity compared to IL‐Sal [cf. substrate: butyl glycidyl ether; conversion: 9% (cat. IL‐Bz ); 90% (cat. IL‐Sal )], confirming the crucial role of ‐OH as HBD in epoxide activation. Further, NMR spectroscopic investigation reveals H‐bonding via downfield shifts with slightly reduced intensity, supported by FT‐IR through shifting of the O‐H stretching band. Kinetic studies indicate first‐order kinetics with rate constants of 1.2 × 10 ⁻⁵ s ⁻¹ , 2.2 × 10 ⁻⁵ s ⁻¹ and 2.7 × 10 ⁻⁵ s ^{− 1} at 40°C, 60°C, and 80°C, respectively. Activation parameters are Δ H ^# = 7.50 ± 0.40 kJ·mol ⁻¹ and Δ S ^# = −244.5 ± 1.21 J·mol ⁻¹ ·K ⁻¹ . The catalyst also shows good recyclability with conversions of 94%, 89%, and 85% in three consecutive cycles.