Bifunctional Resorcinarene-Based Organocatalysts Bearing 5‑Imino Pyridyl and 4‑OH Groups for CO2 Cycloaddition to Epoxides.

The cycloaddition of carbon dioxide to epoxide (CCE) reactions requires binary catalysts in which halide cocatalysts are predominant. However, halide is notorious for its corrosion to steel reactors, and halide residue is environmentally harmful. In the design of a halide-free catalyst for CCE reactions, we proposed resorcinarenes installed with the 5-(imino pyridyl) catalytic group, in cooperation with 4-OH on the same resorcinol unit, as a bifunctional organocatalyst. A series of resorcinarene pyridyl (R-Py) catalysts was designed and evaluated in CCE reactions of styrene oxide, of which the optimal catalyst R-Py2 converted 15 terminal epoxide, 3 internal epoxides, and commercial epoxide bisphenol A diglycidyl ether into the corresponding cyclic carbonates in high yields and nearly quantitative selectivity (91-99%). Noticeably, epichlorohydrin transformed into its cyclic carbonate at atmospheric pressure of CO2, at 100 °C, in 24 h with 99% conversion and 99% selectivity. The catalyst was recycled five times, while the conversion remained at 96%. Control experiments with analogue catalysts R-Ph (5-(imino phenyl)) and R-Qu (5-(imino 8-quinolyl)) and the 4-OH masked analogue catalyst R-Py2 (4-OMe) showed no conversions or decreases, suggesting the indispensable pyridyl and phenolic OH groups in the bifunctional catalysis. These exemplified a first case of a calixarene-based halide-free organocatalyst for CO2 cycloaddition and suggested an avenue of halide-free ion pair catalyst design for carbon dioxide activations.