Asymmetric hydrogenation of indene carboxylic acids: stereochemistry of hydrogen addition

The stereochemistry of hydrogen addition to cr,B-unsaturated carboxylic aci ds was studied by means of hydrogenation of indene carboxylic acids and the ir derivatives. Experiments were carried out over Pd/Al2O3 in the presence and absence of cinchonidine as a chiral modifier. In all cases hydrogenatio n occurred via bottom side syn addition of two hydrogen atoms to the chemis orbed substrate. Formation of trans isomers, up to 72%, was observed due to C=C bond isomerization in the substrate, and to adsorption and hydrogenati on of the unsaturated molecule in a sterically unfavourable position. Adsor ption in 'upside down' position was promoted by N-bases. Hydrogenation of 3 -methylindene-2-carboxylic acid provided up to 45 % ee. Due to the high act ivity of Pd in C = C bond migration, a good ee can be achieved only when is omerization is negligible. (C) 2000 Elsevier Science Ltd. All rights reserv ed.