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

Abstract

a b s t r a c tThe activity of catalytic systems derived from the interaction between Rh(CO) 2 acac 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 ␣,␤-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.

pH-Responsive Gas–Water–Solid Interface for Multiphase Catalysis

“…Solid catalyst-containing gas–water multiphase reactions are widely used in laboratory synthesis and industrial fabrication of various fine chemicals via hydrogenation, oxidation, hydroformylation, and biochemical processes. − Because of the extremely low solubility of gases in water, e.g., H 2 and O 2 , the catalytic efficiency of these multiphase reactions is usually significantly suppressed. − To address this limitation, many methods have been developed as follows. Introducing a cosolvent or raising the gas pressure is employed to increase the gas molecule concentrations in liquids. ,, Organic solvents may be added to form water-in-oil Pickering emulsions, in which the continuous oil phase can increase gas molecule concentrations in the whole systems and the larger oil–water interface benefits the reaction. ,, Obviously, these methods require extra additives or impact process safety and do not enable gas–water–solid phases to contact directly. Alternatively, engineering strategies such as bubbling fluidized beds, packed bubble columns, “tube-in-tube” techniques, and microbubble generators have been exploited to increase the gas–liquid interface area. − The work of the Mase group on bare micro- or nanobubbles is particularly noteworthy, ,, although little mention is made of their sizes.…”