Halogen bonding catalysis for the [3+2] cycloaddition reactions of epoxides with CO₂, and other heterocumulenes

Halogen bonding catalysis was introduced into the [3+2] cycloaddition reactions of epoxides with CO₂, and other heterocumulenes. Series of 2-iodoimidazolium halides, a typical halogen bond donor (XBD) featured with C–I bond, were synthesized and evaluated in the [3+2] cycloaddition reactions. An optimal XBD catalyst, 1,3-bis-(2,6-diisopropylphenyl)-2-iodoimidazolium bromide, achieved 96 %, 94 %, and 61 % conversions in the cycloadditions of epoxides with CO₂, isocyanates, and CS₂, respectively, under 80 °C. Arrays of epoxides were transformed into the corresponding five-membered heterocycles of cyclic carbonates, oxazolidinones, and cyclic dithiocarbonates by high yields and selectivity. ¹H NMR and ¹³C NMR titrations of the XBD catalyst on the styrene oxide indicated that the halogen bond donor coordinated with the substrate through halogen bonding between iodine atom of C–I moiety and oxygen of the epoxide. Possible mechanism was proposed, in which halogen bond donor activated the epoxides by halogen bonding and halide counter anion attacked the carbon of methylene to facilitate the ring-opening of the epoxides. Halogen bonding catalysis was firstly extended to general [3+2] cycloaddition reactions of epoxides with CO₂, isocyanates, and CS₂.