Reactions of chlorine substituted (E)-2,3-diphenylpropenoic acids over cinchonidine-modified Pd: Enantioselective hydrogenation versus hydrodechlorination

The effect of the chlorine position on the C–Cl bond hydrogenolysis and the enantioselective hydrogenation of Cl substituted (E)-2,3-diphenylpropenoic acid derivatives has been studied over cinchonidine-modified Pd/Al2O3. In contrast to the fast hydrodechlorination of the β-phenyl-para-Cl substituted acids the Cl on the α-phenyl ring was barely hydrogenolized. These observations were interpreted by the different arrangements of the two phenyl rings on the surface, with the α- and β-phenyl rings adsorbed tilted and parallel, respectively. The results confirmed the beneficial effect of the α-phenyl-ortho-substituents on the chiral discrimination, thus the 2,3-diphenylpropionic acids substituted by Cl on the α-phenyl ring could be prepared in good yields and optical purities. The conclusions were used for the rational design of an acid, i.e. (E)-2-(2-methoxyphenyl)-3-(3,4-difluorophenyl)propenoic acid, which afforded the best optical purity (ee up to 95% at 295 K) described until now in this heterogeneous system.

Graphical abstractThe differences in the hydrogenations of Cl substituted (E)-2,3-diphenylpropenoic acids in different positions over Pd modified by cinchonidine were interpreted by the arrangement of the phenyl rings on the metal surface.Figure optionsDownload full-size imageDownload high-quality image (64 K)Download as PowerPoint slideResearch highlights▶ The enantioselective hydrogenation/hydrogenolysis of Cl substituted (E)-2,3-diphenylpropenoic acids over Pd-cinchonidine catalyst was studied. ▶ The fast hydrodechlorination of the β-phenyl-para-Cl was in contrast with the almost exclusive hydrogenation of acids having the Cl on the α-phenyl ring. ▶ These were interpreted by the tilted and parallel arrangements of the α- and β-phenyl rings on the surface. ▶ The conclusions were used for the rational design of an acid affording up to 95% ee at 295 K.