Unexpected chemoselectivity in the rhodium-catalyzed transfer hydrogenation of α,β-unsaturated ketones in ionic liquids

Abstract

Chalcone and some other α,β-unsaturated ketones were subjected to transfer hydrogenation catalyzed by the dimer [Rh(cod)Cl]₂ and the Wilkinson's catalyst in imidazolium-, ammonium- and phosphonium-based ionic liquids. In certain ionic liquids, the reduction of chalcone gave 1,3-diphenylpropan-1-one chemoselectively, in contrast with molecular solvents, which resulted in the formation of 1,3-diphenylpropan-1-ol. An accelerated reaction rate was observed in [emim][BuSO₄] and [emin][HeSO₄]. The observed chemoselectivity could be maintained applying a 5 molar equivalent excess of [bmim][BF₄] to chalcone in 2-PrOH. This phenomenon suggests that there is an interaction between the chalcone carbonyl group and the ionic liquid which prevents reduction of the carbonyl group. The catalyst [Rh(cod)Cl]₂ dissolved in an ionic liquid was successfully recycled at least three times, in contrast with Wilkinson's catalyst, which lost its activity on recycling.