[bmim][Br] as an Inexpensive and Efficient Medium for the Barbier-Type Allylation Reaction Using a Catalytic Amount of Indium: Mechanistic Studies

Barbier-type allylation reactions of aldehydes and ketones have been carried out with both unsubstituted and -substituted allyl bromides using only a catalytic amount (0.1 equiv.) of In metal in [bmim][Br], but not in H2O, organic solvents, or other room-temperature ionic liquids. The reactions did not require any metal activator and proceeded chemo- and regioselectively. The results of time-dependent H-1 NMR studies suggested that besides acting as a solvent, [bmim][Br] also activates the In metal surface by electron polarization to generate both CH2=CHCH2-In (I) and CH2=CHCH2-InBr2 (II) from allyl bromide and In. Of the active allylating intermediates, species II was regenerated by the in situ reduction of InBr3 with an imidazolium-based N-heterocyclic carbene, both produced during the process, accounting for the catalytic action of the In metal.