This contribution is dedicated to Prof. Dr. Herbert Mayr on the occasion of his 70th
birthday.
Abstract
Stereochemically flexible 2,2(-bis(diphenylphosphino)biphenyl (BIPHEP) ligands were
modified with chiral α-substituted carboxylic acid auxiliaries in the 3- and 3′-position.
The resulting central-to-axial chirality transfer to the stereochemically flexible
chiral axis of the BIPHEP core was investigated as well as complexation of these
diastereomeric ligands to iridium(I). Solid-state structures of both ligand diastereomers
and a diastereomerically pure iridium(I) BIPHEP complex were obtained. Thermal equilibration
of the resulting iridium(I) complexes was studied to investigate the stereodynamic
properties of the BIPHEP ligands. The iridium(I) complexes without and after pre-catalysis
warming in solution — which induces a shift of the diastereomeric ratio — were applied
for asymmetric hydrogenation of a prochiral α-substituted acrylic acid, resulting
in temperature-controlled bidirectional enantioselectivity of iridium catalysts for
the first time. In both cases, enantioenriched (R)-naproxen as well as (S)-naproxen — after re-equilibration of the catalyst at elevated temperatures — was
obtained by using the same catalyst.
Key words
asymmetric catalysis - bidirectional enantioselectivity - chiral auxiliaries - stereodynamic
catalysts - supramolecular chemistry -
tropos ligands
