New Chiral N-Heterocyclic Carbene Ligands in Palladium-Catalyzed α-Arylations of Amides: Conformational Locking through Allylic Strain as a Device for Stereocontrol

New Enders/Herrmann-type chiral N-heterocyclic carbene (NHC) ligands have been developed and applied in asymmetric palladium-catalyzed intramolecular alpha-arylations of amides. The best ligands feature the bulky tert-butyl group and ortho-substituted aryl groups at the stereogenic centers. Aryl bromides readily react at room temperature and aryl chlorides at 50 degrees C. The highly enantiomerically enriched (up to 96% ee) 3-alkyl-3-aryloxindole products were obtained in generally high yields (> 95%) except in cases of steric congestion. The critical roles both of the bulky alkyl group and of the ortho-aryl substituent at the stereogenic center of the ligand were revealed in the crystal structure of a [Pd(eta(3)-allyl)(NHC-L*)(I)] complex. The ligand aryl location and orientation is fixed by conformational locking that minimizes A(1.3)-strain and enables optimal transfer of chiral information.