Atropisomeric N-Heterocyclic Carbene-Palladium(II) Complexes: Influence of the Backbone Substitution

Abstract

In order to facilitate the synthesis of NHC precursors as well as to incorporate new moieties, the influence of the NHC backbone substitution was investigated within the concept of atropisomeric NHC-metal complexes. A series of NHC precursors was prepared from new anilines and used to synthesize the corresponding Pd(allyl)Cl(NHC) complexes, most of the time as a mixture of diastereomers (meso and chiral). Chiral HPLC at preparative scale enabled to obtain enantiopure complexes in low to excellent yields. These complexes displayed good activity in the intramolecular alpha-arylation of amides and, as a function of the structure of the chiral catalyst, excellent enantioselectivities were reached (up to 96 % ee). The influence of the NHC backbone substitution was investigated for palladium-NHC complexes containing axial chirality. The two new series of atropisomeric Pd(NHC) complexes enabled the introduction of bulky moieties as ortho substituents of N-aryl groups. After resolution by chiral HPLC at preparative scale, enantiopure complexes successfully catalyzed the alpha-arylation of amides (up 96 % ee). image