Simple and efficient water soluble thioligands for rhodium and iridium catalyzed biphasic hydrogenation

The activity of catalytic systems derived from the interaction between Rh(CO)2acac and [Ir(COD)Cl]2, respectively, with the water soluble thioligands (L)-Cysteine (1) and (S)-Captopril (2), was tested in the aqueous biphasic hydrogenation of some representative α,β-unsaturated compounds as 2-cyclohexen-1-one (I), trans-cinnamaldehyde (V) and [3-(1,3-benzodioxol-5-yl)-2-methylpropenal] (X), precursor of the fragrance Helional® (XI). The precatalyst Rh/Cap was able to hydrogenate cyclohexenone even at low pressure at 60 °C in a neutral medium while Rh/Cy required either higher pressure and temperature or an alkaline medium. The iridium based catalysts, Ir/Cy and Ir/Cap, showed an analogous trend though their activities were lower than those of the related rhodium catalysts. All the catalysts were easily recycled without significant loss of activity. The rhodium catalysts were also used in the hydrogenation of the above aldehydes V and X and their activity was strongly enhanced when ethylene glycol was used as organic solvent or co-solvent.

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► Rh/Cap and Rh/Cy were efficient hydrogenation catalysts in aqueous biphasic medium.
► Ir/Cy and Ir/Cap were less active than the related rhodium catalysts.
► All the catalysts were easily recycled without significant loss of activity.