Recent Progress in Asymmetric Allylic Substitutions Catalyzed by Transition Metal Complexes

Asymmetric allylic substitution catalyzed by an organometallic complex is a powerful method to create carbon-carbon and carbon-heteroatom bonds with high enantioselectivity, and is often applied to total syntheses of biologically active organic molecules. The reaction via a symmetrically substituted pi-allylpalladium intermediate has been widely studied using a variety of chiral ligands. However, the reaction of unsymmetrically substituted allylic compounds is still a challenging research subject, because nucleophilic attack generally takes place at the less hindered allylic carbon giving a liner product in the reaction of monosubstituted allylic substrate with palladium catalysts. Recently, other transition metal complexes have shown high potential in this type of reactions, in which a branched isomer is obtained as a main product by the substitution at the hindered allylic carbon. This review focuses on the recent progress in the asymmetric allylic substitutions of unsymmetrically substituted allylic compounds producing branched products with high regio- and enantioselectivities.