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

✍ Scribed by Yi-Xia Jia; Dmitry Katayev; Gérald Bernardinelli; Thomas M. Seidel; E. Peter Kündig

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 419 KB
• Volume: 16
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.201000031

✦ Synopsis

Abstract

New Enders/Herrmann‐type chiral N‐heterocyclic carbene (NHC) ligands have been developed and applied in asymmetric palladium‐catalyzed intramolecular α‐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 °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(η^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.