Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 14, Issue 24, Pages 7259-7265Publisher
WILEY-BLACKWELL
DOI: 10.1002/chem.200800508
Keywords
asymmetric catalysis; enantioselectivity; homogeneous catalysis; Michael addition; oxazoline ligands
Categories
Funding
- DAAD
- International Doktorandenkolleg NANOCAT (Elitenetzwerk Bayern)
- Fonds der Chemischen Industrie
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Simple bis(oxazoline) ligands, especially azabis(oxazolines), call promote the copper(II)-catalyzed Michael addition of indoles to benzylidene malonates with up to > 99% ee (ee = enantiomeric excess), provided that the ligand/metal ratio is tuned meticulously With particular regard to the electronic properties of the Substrate. Despite a common paradigm followed in many asymmetric catalyses, an excess of chiral ligand is not always beneficial. In fact any excess of ligand has to be avoided to reach excellent enantioselectivities when electron-rich benzylidene malonates are used. On the contrary, malonates carrying all electron-withdrawing group require an excess of ligand for all Optimum ee value. A correlation of optical yields versus the sigma(I) values of several para substituents shows a signioid trajectory. In the presence of in additive. such as triflate, the significance of the ligand/metal ratio vanishes and very good enantioselectivities are achieved at any rate-no matter whether electron-donating or withdrawing substituents are present.
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