Recent Advances in Catalytic Asymmetric Friedel-Crafts Reactions with Prochiral Trifluoromethylated Compounds

The incorporation of a trifluoromethyl (CF3) group can increase chemical and metabolic stability, lipophilicity, and membrane permeability of the molecules. As a consequence, the exploitation of efficient methods to construct chiral trifluoromethylated compounds is a highly significant task. Two strategies are often utilized to synthesize compounds bearing a CF3 group at the chiral center. The first strategy is the transfer of a CF3 group from a reagent. This strategy seems to be straightforward and practical, but direct enantioselective trifluoromethylation remains a challenge, and high enantioselectivities are rarely reached. An alternative strategy is the application of some established asymmetric catalytic reactions by using trifluoromethylated compounds as building blocks, which could provide various chiral trifluoromethylated products in high yields and enantioselectivities under mild conditions. Asymmetric Friedel-Crafts reaction has witnessed rapid development in recent years. Asymmetric Friedel-Crafts reaction using prochiral trifluoromethylated starting materials represents an important method for the access of trifluoromethylated derivatives of chiral aromatic compounds. Since Mikami's group identified that chiral BINOL-derived Ti(IV) complexs could be applied in the asymmetric Friedel-Crafts reaction with fluoral to obtain chiral trifluoromethylated compounds, considerable efforts have been dedicated to utilize other chiral organometallic catalysts (such as chiral Ti, Cu, Zn, Zr, Pd, Y, Yb, Ni, Ca complex, etc.) and chiral organo-catalysts-(such as cinchona alkaloids, chiral prolinol ethers, chiral phosphoric acids, etc.) in this type of catalytic reaction. Chiral trifluoromethylated compounds with a CF3-group at a chiral carbon center with, or without any heteroatom substituents were obtained in high yields and enantioselectivities. The components (especially CF3-containing starting material of trifluoromethyl ketones, imines and olefins), conditions and mechanism of the Friedel-Crafts reactions, and the synthetic utilities of Friedel-Crafts adducts are discussed in this review. Finally, the existing problem and limitation of this Friedel-Crafts reaction are summarized, and the key prospect of the chiral CF3-containing product-driven Friedel-Crafts reaction is also described. The aim of this review is to stimulate further developments in this area.