In Situ Anodically Oxidized BMIm-BF₄: A Safe and Recyclable BF₃ Source

The anodic oxidation of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF₄) efficiently generates BF₃ from BF₄^-. This Lewis acid, strongly bound to the ionic liquids, can be efficiently used in classical BF₃-catalyzed reactions. We demonstrated the BF₃/BMIm-BF₄ reactivity in four reactions, namely, a domino Friedel-Crafts/lactonization of phenols, the Povarov reaction, the Friedel-Crafts benzylation of anisole, and the multicomponent synthesis of tetrahydro-11H-benzo[a]xanthen-11-ones. In comparison with literature data using BF₃-Et₂O in organic solvents, in all the presented cases, analogous or improved results were obtained. Moreover, the noteworthy advantages of the developed method are the in situ generation of BF₃ (no storing necessity) in the required amount, using only the electron as redox reagent, and the recycling of BMIm-BF₄ for multiple subsequent runs.