Iridium complexes of dehydroamino acids: The kinetic resolution of racemic diphosphines and their application in catalytic asymmetric hydrogenation
Version of Record online: 21 SEP 2004
Copyright © 1991 Wiley-Liss, Inc.
Volume 3, Issue 4, pages 345–354, 1991
How to Cite
Brown, J. M. and Maddox, P. J. (1991), Iridium complexes of dehydroamino acids: The kinetic resolution of racemic diphosphines and their application in catalytic asymmetric hydrogenation. Chirality, 3: 345–354. doi: 10.1002/chir.530030421
- Issue online: 21 SEP 2004
- Version of Record online: 21 SEP 2004
- Manuscript Accepted: 13 MAY 1991
- Manuscript Received: 8 MAR 1991
- asymmetric hydrogenation;
- dehydroamino acid esters;
- enamide complexes;
- homogeneous catalysis
The reaction of chiral diphosphines with a configurationally pure cationic bis-enamide complex of iridium, bis(menthyl (Z)-α-benzamidocinnamate)-iridium tetrafluoroborate, is described. When the reactant ligand is racemic then kinetic resolution occurs with high specificity under the appropriate conditions. Since the iridium diphosphine complex is catalytically inactive in homogeneous hydrogenation, the residual enantiomer may be reacted with bis(norbornadiene)-rhodium tetrafluoroborate to produce an active catalyst. This effects the hydrogenation of methyl (Z)-α-acetamidocinnamate in optical yields comparable with those obtained separately with the enantiomerically pure ligand rhodium complex. The reaction of pure (+)- or (-)-enantiomer of bis(menthyl (Z)-α-benzamidocinnamate)-iridium tetrafluoroborate with enantiomerically pure diphosphines has been studied. Invariably one hand of the diphosphine reacts rapidly with a given enantiomer of the iridium complex to give a stable diphosphine iridium enamide complex in which the original configuration of the coordinated olefin is maintained. The other combination of enantiomers reacts much more slowly, in keeping with the kinetic resolution work, and produces an enamide complex which is unstable in solution, isomerising to a second diastereomer. Since the absolute configuration of the iridium bis-enamide complex has been established by X-ray crystallography, this experiment affords a method of determining the configuration of rhodium enamide complexes in asymmetric hydrogenation (assuming structural homology between Rh and Ir). In all cases the disfavoured enamide complex was the one involved in the catalytic pathway.