Homogeneous asymmetric hydrogenation using phosphine ligands of transition metals

A process for the preparation of an enantiomerically enriched 2-substituted succinic acid derivative of formula 2, which comprises asymmetric hydrogenation of the dehydro precursor of formula 7 or 8, or a mixture thereof

• wherein R1 and R2 are independently selected from the group consisting of H, a salt-forming cation, and an organic group of up to 30 C atoms, and R3 is selected from H or an organic group of up to 30 C atoms, and wherein R4 is CHR5R6, R5 and R6 each being H or any other group consistent with the definition of R4, wherein said asymmetric hydrogenation takes place in the presence of a transition metal complex of a chiral phosphine ligand having the formula
• wherein n is 0 to 6 and R represents at least one non-hydrogen organic group of up to 30 C atoms;
• wherein said transition metal is selected from the group consisting of rhodium, ruthenium, and iridium;
• conversion of the isomerised compound 9
• into a compound readily separable from the hydrogenation product 2;
• and removal of said separable compound.