Rigid tertiary amine/saccharin adduct as halide-free organocatalyst for the cycloaddition of CO2 into epoxides

Abstract

Metal- and halide-free catalyst DABCO·HSac, i.e., the binary adduct of 1,4-diazabicyclo[2.2.2]octane (DABCO) and saccharin (HSac), in an ionic pair structure of DABCO-H+·Sac- is shown mild and efficient in promoting the cycloaddition of CO2 into epoxides (CCE) reactions. Under comparatively low temperature of 60 °C, ambient pressure of 0.1 MPa, and 10 mol% catalyst loading, DABCO·HSac catalyzed the cycloaddition of epichlorohydrin into CO2 with the conversion of 96% in 24 h. Terminal epoxides including epibromohydrin, glycidyl ethers, and alkylene oxides received 83–99% conversions, however, cyclohexene oxide and stilbene oxide were not viable under these mild reaction conditions. A catalytic cycle initiated by the saccharinate anion of its nucleophilic attack on methylene carbon of an epoxide ring with the iminolate oxygen was proposed as the key step, in which the Sac- anion functioned similarly to halide X- in other ring-opening mechanisms. The tertiary amine DABCO was protonated into an onium cation DABCO-H+ that worked as a hydrogen-bond donor in coordinating and activating the epoxide. A cocatalysis mechanism was suggested and validated by NMR titrations. Further control experiments by switch cation or anion of DABCO-H+·Sac- in structural analog cocatalysts certified both the onium DABCO-H+ and the saccharinate Sac- was indispensable in the catalytic cycle. The design of ionic pair organocatalysts illustrated the possibility of non-halide anions as the nucleophiles in wider halide-free catalyst discovery.