γ- and δ-Lactams through Palladium-Catalyzed Intramolecular Allylic Alkylation: Enantioselective Synthesis, NMR Investigation, and DFT Rationalization

The Pd-catalyzed intramolecular allylic alkylation of unsaturated amides to give gamma- and delta-lactams has been studied in the presence of chiral ligands. Ligand (R)-3,5-tBu-MeOBIPHEP (MeOBIPHEP=6,6'-dimethoxybiphenyl-2,2-diyl)bis(diphenylphosphine)) afforded the best results and allowed the cyclization reactions to take place in up to 94:6 enantiomeric ratio. A model Pd-allyl complex has been prepared and studied through NMR spectroscopic analysis, which provided insight into the processes responsible for the observed enantiomeric ratios. DFT studies were used to characterize the diastereomeric reaction pathways. The calculated energy differences were in good agreement with the experimentally observed enantiomeric ratios.