Asymmetric Synthesis of Fluorinated Allenes by Rhodium-Catalyzed Enantioselective Alkylation/Defluorination of Propargyl Difluorides with Alkylzincs.

The reaction of propargyl difluorides R1 CF2 C≡CR2 with alkylzincs R3 ZnCl giving axially chiral fluorinated allenes R1 FC=C=CR2 R3 with high enantioselectivity (up to 99 % ee) was found to be catalyzed by a chiral diene/rhodium complex. A key step in the catalytic cycle is selective elimination of one of the enantiotopic fluorides at the β-position of an alkenyl-Rh intermediate, which is generated by regioselective addition of R3 -Rh onto the triple bond of the starting difluorides.