Walphos versus Biferrocene-Based Walphos Analogues in the Asymmetric Hydrogenation of Alkenes and Ketones

Two representative Walphos analogues
with an achiral 2,2″-biferrocenediyl backbone were synthesized.
These diphosphine ligands were tested in the rhodium-catalyzed asymmetric
hydrogenation of several alkenes and in the ruthenium-catalyzed hydrogenation
of two ketones. The results were compared with those previously obtained
on using biferrocene ligands with a C2-symmetric 2,2″-biferrocenediyl backbone as well as with those
obtained with Walphos ligands. The application of one newly synthesized
ligand in the hydrogenation of 2-methylcinnamic acid gave (R)-2-methyl-3-phenylpropanoic acid with full conversion
and with 92% ee. The same ligand was used to transform 2,4-pentanedione
quantitatively and diastereoselectively into (S,S)-2,4-pentanediol with 98% ee.