Dehydrative Direct CH Allylation with Allylic Alcohols under [Cp*Co^III] Catalysis

The unique reactivity of [Cp*Co(III)] over [Cp*Rh(III)] was demonstrated. A cationic [Cp*Co(III)] catalyst promoted direct dehydrative C-H allylation with non-activated allyl alcohols, thus giving C2-allylated indoles, pyrrole, and phenyl-pyrazole in good yields, while analogous [Cp*Rh(III)] catalysts were not effective. The high γ-selectivity and C2-selectivity indicated that the reaction proceeded by directing-group-assisted C-H metalation. DFT calculations suggested that the γ-selective substitution reaction proceeded by C-H metalation and insertion of a C-C double bond, with subsequent β-hydroxide elimination. The [Cp*Co(III)] catalyst favored β-hydroxide elimination over β-hydride elimination.