Substrate dependent ligand acceleration in enantioselective hydrogenation of (E)-2,3-diarylpropenoic acid on cinchonidine-modified Pd/C

• Enantioselective hydrogenation was performed with a variety of phenylcinnamic acid derivatives.
• Cinchonidine was a chiral modifier through the study.
• Up to 93% enantiomeric excess (ee) was obtained depending on the substituent.
• Ligand acceleration was observed overcoming suppression effect of the substrate adsorption.

The hydrogenation of substituted 2,3-diarylpropenoic acids over cinchonidine-modified Pd/C showed obvious ligand acceleration effects with the substrates yielding over 90% enantiomeric excess (ee). 2,3-Di(methoxyphenyl)propenoic acid exhibited profound ligand acceleration, and its hydrogenation rate relative to the rate on unmodified Pd/C was 370%, yielding 91%ee. 3-(p-Fluorophenyl)-2-(o-methoxyphenyl)propenoic acid showed 92%ee with an activity increase of 230%. It is considered that the difference is caused by the ortho-methoxy substituent on the α-phenyl ring. The substituent effect of the ortho-substituent on the α-phenyl ring demands the presence of an electron-donor substituent on the β-phenyl ring. A remarkable substituent effect was observed with 3-(p-methoxyphenyl)-2-(o-methoxyphenyl)propenoic acid, which attained up to 93%ee. The factors affecting enantioselectivity results are analyzed for each substituent.

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