Silica-supported chlorometallate(III) ionic liquids as recyclable catalysts for Diels-Alder reaction under solventless conditions

A range of Lewis acidic metal chlorides were tested as homogeneous catalysts in a model Diels-Alder reaction; AlCl3 and GaCl3 performed best in terms of conversion and endo: exo selectivity. Based on this outcome, the corresponding chlorometallate(III) ionic liquids were tested as homogeneous catalysts, without a decrease in performance compared to the chlorides. Finally, these ionic liquids were heterogenised by covalent tethering of the cation onto a multimodal porous silica support; such supported chlorogallate(III) ionic liquids are reported for the first time. Supported chlorometallate(III) ionic liquids were used as recyclable heterogeneous catalysts for the Diels-Alder reaction under solventless conditions. High yields (99%) and high endo-selectivities (95%) were obtained after very short reaction times (5 min) at near-ambient temperature (25 degrees C).