Enantioselective Michael Additions of Indole to Benzylidene Malonates

Significance

The reported study contradicts the general trends previously observed in Cu-catalyzed reactions. Indeed, background reactions often occur faster with a ligand-free metal than with coordinated species, leading to erosion of enantioselectivity. Thus, an excess of ligand is usually used. Previous studies had concluded that BOX ligands like 4 were unsuitable for the above reaction (79% ee). It was now shown that, for the specific system presented, the BOX ligands are among the best ligands (99% ee), provided that the ligand is not used in excess versus the metal catalyst. The parent ligand 5 increased the reactivity, giving the desired chiral adduct 3 in 97% yield (instead of 89% with 4).