Catalytic olefin cyclopropanation using μ-oxo-bis[(salen)iron(III)] complexes

A series of μ-oxo - bis[(salen)iron(III)] complexes was prepared from the reaction of the corresponding salen ligands with FeCl ₃·6H₂O in the presence of NEt₃, and characterized by elemental analysis, infrared spectroscopy, and mass spectrometry. These air-stable complexes catalyzed the cyclopropanation of olefins with ethyl diazoacetate in good yields. The catalytic activity of these μ-oxo dimers, in the cyclopropanation of styrene, was examined as a. function of the diamine backbone and the substituents in the 3,3′- and 5,5′-positions of the phenyl rings on the ligands. Solvent variation, catalyst loading, and styrene concentration were investigated to determine the optimal reaction conditions. The complex [Fe(3,3′,5,5′- ^tBu₄salen)]₂O (1d) was identified as the most efficient catalyst in the series, which successfully catalyzed the cyclopropanation not only of styrene but also of less reactive substrates such as α-methylstyrene, α-(trifluoromethyl)styrene, 1,1-diphenylethylene, methylenecyclohexane, and n-butyl vinyl ether and internal olefins such as trans- and cis-β-methylstyrene and ethylidenecyclohexane.