Chiral N-Heterocyclic Carbene-Copper(I)-Catalyzed Asymmetric Allylic Arylation of Aliphatic Allylic Bromides: Steric and Electronic Effects on γ-Selectivity

Chiral N-heterocyclic carbene ligands were electronically and sterically tuned to improve gamma-selectivity in copper(I)-catalyzed asymmetric allylic arylation of aliphatic allylic bromides with several aryl Grignard reagents. High gamma-selectivity was realized when either the aryl group of the Grignard reagent or the aryl group on the N-substituent of the carbene ligand was electron-deficient or when either the car bene ligand or allylic bromide was bulky. The results indicated that electron deficiency and steric hindrance of the initially formed sigma-allyl copper intermediate enhance the rate of the reductive elimination to give gamma-products as major isomers.