First Enantioselective Catalyst for the Rearrangement of Allylic Imidates to Allylic Amides

A series of Pd(II) complexes containing chiral diamine ligands were investigated as asymmetric catalysts for the rearrangement of allylic imidates to allyl amides. The best catalysts were cations obtained by dechlorination of dichloro[(S)-2-(isoindolinylmethyl)-N-methylpyrrolidine]palladium(II) (17) with silver salts in CH₂Cl₂. Catalyst 18 was studied thoroughly and shown by ¹H NMR and X-ray crystallography analysis to be a C₁ symmetric dimer (Figure 1). A series of related catalysts 22−27 having various counterions and anionic ligands were also prepared and studied as asymmetric catalysts for the rearrangement of allylic N-(4-trifluorophenyl)benzimidate 29 to allylic benzamide 30 (eq 4). Rearrangement of 29 in CH₂Cl₂ (48 h at 40 °C) in the presence of 5 mol % of 18 affords (−)-30 in 69% yield and 55% ee. Enantioselection is increased to 60% when an isomerically pure sample of 18 is employed. Chemical correlation of allylic benzamide 30 with (R)-norvaline established that (−)-30 has the R absolute configuration (Scheme 3). A cyclization-induced rearrangement mechanism (Scheme 1) requires that in the major pathway the imidate nitrogen attacks the re face of the olefin with Pd coordinated to the si face. These studies constitute the first report of asymmetric catalysis of the rearrangement of allylic imidates to allylic amides. However, significant hurdles remain to be overcome before the enantioselective rearrangement of allylic imidates becomes a practical route to enantioenriched nitrogen compounds.