Chiral C ₂-Symmetric Cu(II) Complex Immobilized in Ionic Liquids: A Recoverable Catalytic System for Enantioselective Hetero-Diels-Alder Reaction

Abstract

Hydrophobic ionic liquids such as [Bmim]PF₆ and [Bmim]SbF₆ were used successfully as powerful media for bis(oxazoline)-copper-catalyzed asymmetric hetero-Diels-Alder reactions. The reactivities and stereoselectivities were comparable (endo/exo up to 97:3 and up to 94% ee, respectively) to those of the corresponding homogeneous reactions. Furthermore, the reaction was remarkably accelerated in [Bmim]PF₆ compared to that in dichloromethane at 3 (±1) °C and metal-ligand complexes were recovered and recycled up to eight times exhibiting almost the same reactivity and selectivity.

Key words

enantioselective - hetero-Diels-Alder reaction - ionic liquid - catalyst recycling - chiral bis(oxazoline)-copper complex

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Representative Procedure for Asymmetric hetero-Diels-Alder ReactionChiral bis(oxazoline) ligand 3 (13.6 mg, 0.046 mmol) and copper(II) trifluoromethanesulfonate (16 mg, 0.044 mmol) were put into a screw-capped vial (10 mL capacity, flat bottom), and ionic liquid (0.88 mL) was added. After the mixture was stirred vigorously for 3 h, the unsaturated ester (0.441 mmol) and enol ether (0.882 mmol) were added via syringe at 3 (±1) °C . After the indicated time, the reaction mixture was extracted with Et₂O until there was no detectable amount of product. And volatile components from the extracted solution were removed under reduced pressure. Then the crude product was purified by silica gel column chromatography to give the desired product. Recovered ionic liquid and metal-ligand complex were reused for recycling experiments after drying under reduced pressure.

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The ionic liquids were purchased from C-TRI Co., Ltd. (www.c-tri.com).