Alkene Hydroformylation Catalyzed by Rhodium Complexes in Ionic Liquids: Detection of Transient Carbene Species

D/H exchange reactions at the imidazolium cation of an ionic liquid were observed in hydroformylation reactions of 1-octene catalyzed by [Rh(acac)(CO) 2 ](phosphine ligand) dissolved in 1- n -butyl-3-methylimidazolium- d 3 N -bis(trifluoromethanesulfonyl)imidate at 75 °C under 5 atm (constant pressure) of CO/H 2 (1:1). This D/H exchange occurs preferentially at the C2 position and occurs essentially in the presence of the phosphine ligand and the rhodium precursors, and it is more pronounced in the presence of bases, even a very weak one such as methanol. The presence of phosphine is also essential for the hydroformylation reaction. This result suggests the formation of N -heterocyclic carbenes (NHCs) in the biphasic hydroformylation reactions promoted by Rh complexes in imidazolium ionic liquids. These carbene species are easily displaced by the other species present in the ionic liquid, and they do not cause any significant changes in the catalytic activity or selectivity of the hydroformylation reaction.