title: New chiral N-heterocyclic carbene ligands in palladium-catalyzed alpha-arylations of amides: conformational locking through allylic strain as a device for stereocontrol.
abstract: 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.
authors.affiliation: Department of Organic Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211, Geneva 4, Switzerland.
authors.forename: Yi-Xia
authors.initials: YX
authors.lastname: Jia
authors.forename: Dmitry
authors.initials: D
authors.lastname: Katayev
authors.forename: Gérald
authors.initials: G
authors.lastname: Bernardinelli
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Seidel
authors.forename: E Peter
authors.initials: EP
authors.lastname: Kündig