Phenylcinnamic acid, 2-, enantioselective

Up to 72% ee has been achieved in the hydrogenation of a diphenyl-substituted reactant, (trans)-a-phenylcinnamic acid, with a Pd/Ti02 catalyst and cinchonidine at 1 bar in strongly polar solvent mixtures [49]. For aliphatic a,/ -unsaturated acids the enantioselectivities that can be attained are much lower. Therefore, for these type of substrates, homogeneous metal-catalysts are preferred. 

Cinchonidine modified catalysts, though, have been effectively used for several other enantioselective hydrogenations. Platinum-cinchonidine catalysts have been used for the hydrogenation of the a ketolactone, 36, to D-pantolactone (37) in 35% ee at complete conversion (Eqn. 14.26) while palladium-cinchonidine catalysts have been used for the enantioselective dehydrohalogenation of a,a-dichloro-2-benazapinone (38) (Eqn. 14.27) and the hydrogenation of (E) a phenylcinnamic acid (39) to (S) 2, 3-diphenylpropionic acid (40) in a 44% ee (Eqn. 14.28). ... 

Enantioselective hydrogenation of ( )-a-phenylcinnamic acid on cinchonidine-modified palladium catalysts influence of support... 

In the system Cnd-2-phenylcinnamic acid the alkaloid interacts with the acid via two hydrogen bonds Interaction of the C9-OH group is the crucial influence in the chiral induction. Substitution of OH for OMe will weaken the interaction between modifier and the substrate. A second hydrogen bond between the N-atom in the quiniclidine group and the carboxylic group was revealed by experiments of modification of Pd-titania catalyst with a mixture of (Cnd + 9-MeO-DHCnd). Therefore esterification of the carbox-yhc group resulted in complete loss of enantioselectivity. 

Nitta, Y., Kobiro, K. (1995) Solvent effect on enantioselective hydrogenation of ( )-fl//7/ja-phenylcinnamic acid with cinchonidine-modified Palladium catalysts, Zeft 165-165. 

Nitta, Y., Shibata, A. (1998) Enantioselective hydrogenation of E)-alpha-phenylcinnamic acid on Pd/Ti02 catalyst modified by cinchona alkaloids effect of modifier structure, Chem. Lett. 161-162. 

Nitta, Y. (1999) Importance of product desorption in enantioselective hydrogenation of ( )-a Aa-phenylcinnamic acid with a cinchonidine-modi-fied Pd/Ti02 catalyst. Effect of additives, Chem. Lett. 635-636. 

Nitta, Y., Kubota, T., Okamoto, Y. (2000) Preparation of cinchonidine-modified Palladium catalysts for the enantioselective hydrogenation of ( )-fl// /ifl-Phenylcinnamic acid. Bull. Chem. Soc. Jpn., 73,2635-2641. Tungler, A., Nitta, Y., Fodor, K., Farkas, G., Mathe, T. (1999) Comparison of chiral modifiers in the Pd catalysed hydrogenation of phenylcinnamic acid and isophorone, J. Mol. Catal. A. Chem. 149, 135-MO. 

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