Halide-free squaramide–phenolate organocatalyst for the cycloaddition of CO₂ into epoxides

The cycloaddition of CO₂ into epoxide (CCE) reaction was the few routes viable industrially in catalytic fixation of carbon dioxide into value-added chemicals in which cocatalyst halide was predominant. To obviate the harmful corrosion of halide anions to steel reactors, and to alleviate the environmental burden of halogen waste, halide-free catalyst was desirable. We propose a category of one-component H-bond donor (HBD) and nucleophilic anion (HBD–anion) bifunctional organocatalyst as representative halide-free catalyst in CCE reactions. Squaramide (Sq) in conjugation with phenolate constitutes a typical HBD–anion catalyst featured with the squaramide as the HBD moiety and the phenolate as the nucleophilic anion moiety. Series of pre-catalyst in structure of squaramide–phenol (Sq–PhOH) was designed and the active catalyst Sq–phenolate (Sq–PhO) was generated from the corresponding Sq–PhOH by deprotonating of the weakly acidic hydroxyl of the phenol by super strong Brønsted base. In an optimal N-benzyl squaramide para-phenol (Sq–PhOH-p) and P2 phosphazene (t-BuP₂) combination, CCE reactions proceed under mild conditions of 120 °C, and atmospheric pressure of 0.1 MPa, by 2.5 mol % loading of catalyst Sq–PhOH-p/t-BuP₂. Terminal epoxides were transformed with excellent conversions (86–99 %) and high selectivity (85–97 %) in 8 h. Bifunctional activations mechanisms were proposed and validated. ¹H and ¹³C NMR titrations certified the coordination of H-bond donor squaramide to epoxide, and FT-IR spectra observed formation of phenyl carbonate anion from phenolate and carbon dioxide, these suggested unprecedent CCE reaction composed capture of CO₂ in carbonate anion followed by its intramolecular nucleophilic attack on the HBD coordinated epoxide as the key catalytic steps. The squaramide–phenolate catalytic dyad exemplified halide-free HBD–anion organocatalyst of wide scope for carbon dioxide activation and fixation.