Frustrated Lewis pairs, highly active in heterolytic dihydrogen splitting, were used as active and very selective hydrogenation catalysts for organometallic substrates. Conjugated dienamines fixed at the ferrocene framework and at the zirconocene nucleus were selectively 1,4-hydrogenated under catalytic conditions at ambient temperature.
The title compounds, propynylferrocene, [Fe(C5H5)(C8H7)], (I), and (phenylethynyl)ferrocene, [Fe(C5H5)(C13H9)], (II), are stabilized by weak C—H⋯π interactions. The C[triple‐bond]C bond distances in these molecules are in the range 1.182 (3)–1.192 (3) Å. In (II), the ferrocenyl and phenyl groups are perpendicular, making an angle of 89.06 (13)°, which is a rare occurrence.
Deprotonation of 6-dimethylamino-6-methylpentafulvene (3a) with methyl lithium yielded the (1-dimethylaminoethenyl)Cp lithium reagent 4a. Its treatment with zirconium tetrachloride under carefully controlled reaction conditions gave the corresponding bis(enamino-Cp)ZrCl2 complex 5a. The product 5a is very sensitive to acid-catalyzed Mannich condensation. Photolysis of 5a (HPK 125, Pyrex filter) resulted in a rapid intramolecular [2+2]cycloaddition reaction to yield the cis-bis(dimethylamino)cyclobutylene-bridged ansa-zirconocene complex 7a in a photostationary equilibrium (5a:7a ≈ 30:70). After acid-catalyzed Mannich conversion of the minor component (5a to 6a) the new photoproduct 7a was isolated pure by fractional crystallization. The complexes 5a and 7a were both characterized by X-ray diffraction.
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