Schatz, A. and Rasappan, R. and Hager, M. and Gissibl, A. and Reiser, O. (2008) Dependence of enantioselectivity on the ligand/metal ratio in the asymmetric Michael addition of indole to benzylidene malonates: Electronic influence of substrates. Chemistry - A European Journal 14 (24), pp. 7259-7265.
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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.
|Additional information (public):||Schätz, Alexander; Rasappan, Ramesh; Hager, Markus; Gissibl, Anja; Reiser, Oliver|
|Institutions:||Chemistry and Pharmacy > Institut für Organische Chemie > Lehrstuhl Prof. Dr. Oliver Reiser|
|Subjects:||500 Science > 540 Chemistry & allied sciences|
|Refereed:||Yes, this version has been refereed|
|Created at the University of Regensburg:||Yes|
|Deposited On:||09 Feb 2009 10:04|
|Last Modified:||16 Nov 2009 14:09|