Highly Enantioselective Construction of Trifluoromethylated All-Carbon Quaternary Stereocenters via Nickel-Catalyzed Friedel–Crafts Alkylation Reaction

A highly
enantioselective Friedel–Crafts alkylation reaction
of indoles with β-CF3-β-disubstituted nitroalkenes
was achieved using a Ni­(ClO4)2–bisoxazoline
complex as a catalyst, which afforded indole-bearing chiral compounds
with trifluoromethylated all-carbon quaternary stereocenters in good
yields with excellent enantioselectivities (up to 97% ee). The transformation
of one of the products into first a trifluoromethylated tryptamine
and then a trifluoromethylated tetrahydro-β-carboline by sequential
nitro reduction and Pictet–Spengler cyclization were realized
with complete preservation of enantiopurity.