| Popis: |
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/Al 2 O 3 . 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. |
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