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Highly Enantioselective Michael Additions of Indole to Benzylidene Malonate Using Simple Bis(oxazoline) Ligands:  Importance of Metal/Ligand Ratio

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journal contribution
posted on 21.12.2006, 00:00 by Ramesh Rasappan, Markus Hager, Anja Gissibl, Oliver Reiser
Simple bis(oxazoline) ligands, especially azabis(oxazolines), can catalyze the copper-catalyzed addition of indoles to benzylidene malonates in up to >99% ee, provided that excess of chiral ligand is avoided. The paradigm followed in many asymmetric catalyses that an excess of chiral ligand with respect to the metal should improve enantioselectivity because a background reaction by free metal is suppressed, is not applicable here, which might call for revisiting some of the many copper(II)−bis(oxazoline)-catalyzed processes known.