Halide-free pyridinium saccharinate binary organocatalyst for the cycloaddition of CO2 into epoxides

Cycloaddition of CO₂ into epoxides (CCE) reaction was the few chemical processes viable commercially in valorizations of carbon dioxide. The produced cyclic carbonate was valuable in electrolyte of lithium batteries and so on, however, halide anion was monopoly as cocatalyst in catalytic CCE reactions. To avoid the harmful halide in corrosion of process equipment, the detrimental halide in electrolytes, and the waste halide as environmental burden, halide-free catalysis for CCE was highly desirable. In design of halide-free catalytic platform, we propose binary adducts of food sweetener saccharin and strong pyridine base in ionic pair pyridinium saccharinate exemplified by PPy·Sac (3). CCE reaction of epichlorohydrin at mild conditions of 40 °C, 0.1 MPa, by 10 mol% loading of catalyst 3 in 24 h were evaluated. Terminal epoxides including glycidyl ethers, alky epoxides, and styrene oxide were transformed into their corresponding cyclic carbonates by excellent yields (80 to 90% up). Internal epoxides of hexene and stilbene were not viable under these mild conditions. The key role of saccharinate anion in cocatalysis in both the nucleophilic attack on epoxide in initiating the ring-opening, and as good leaving group in ring-closure was validated. ¹H NMR titration experiment of PPy·Sac on epoxide verified the activation of the substrate by H-bonding, the capture of intermediates in mass spectroscopy certified the ring-opening by Sac, and the control experiment by switching the cation and anion of PPy·Sac supported the indispensable of the cation and anion in the cocatalysis. In view of the wide application of halide anions in nucleophilic catalysis and substitutional reactions, diversity design of anions of appropriate nucleophilicity and nucleofugality would be useful in practical catalysis.