endo-Selective Iron Catalysts for Intramolecular Alkyne Hydrofunctionalization

Metal–ligand cooperative iron complexes of the type [Fe­(Cp*)­(P^R₂N^Ph₂)­(MeCN)]­PF₆ (R = Cy, Ph) were synthesized and characterized. Both derivatives were active toward the intramolecular hydroamination of 2-ethynylaniline and hydroalkoxylation of 2-ethynylbenzyl alcohol. In the latter case, only the six-membered endo heterocycle isochromene was observed, which suggests that the alkyne is activated by a vinylidene mechanism. The reaction of [Fe­(Cp*)­(P^Cy₂N^Ph₂)­(MeCN)]­PF₆ with phenylacetylene indeed afforded a complex that was characterized as a vinylidene, which implicates an analogous intermediate in the hydrofunctionalization mechanism.