Asymmetric transfer hydrogenation of carbonyl compounds catalyzed by rhodium nanoparticles

• Stable, chirally modified Rh(0) nanoparticles are formed in situ.
• These nanoparticles are catalysts for transfer hydrogenation reactions.
• Yield and enantioselectivity depend on the chiral modifier and reaction conditions.
• Enantiomeric excesses (ee) for products up to 58% ee.
• The size of the nanoparticles influences their catalytic activity.

Transfer hydrogenation of acetophenone and methyl benzoylformate with 2-propanol has been studied on rhodium nanoparticles obtained by the reduction of rhodium(I) complexes in the presence of optically active compounds, including chiral diamine S,S-DIODMA, (4S,5S)-N4,N4,N5,N5,2,2-hexamethyl-4,5-bis-(aminomethylene)-1,3-dioxolane, quaternary diammonium salt (4S,5S)-(-)-N1,N4-dibenzyl-2,3-dihydroxy-N1,N1,N4,N4-tetramethylbutane-1,4-diammonium dichloride, and (8S,9R)-(-)-cinchonidine. Increasing the modifier to Rh ratio leads to an increase in the enantiomeric excess (ee) of the reaction products. The greatest ee values (43.8% for (R)-1-phenylethanol and 58.2% for the methyl ester of (R)-mandelic acid) were achieved at (8S,9R)-(-)-cinchonidine/Rh ratios of 9:1 and 3:1, respectively.

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