Novel Alkylidenating Agents of Iron(III) Derivatives by Base‐Mediated α,μ‐Dehydrohalogenation and Their Chemical Trapping by Cycloaddition

Studies of the reactions between group 4 metal chlorides (M = Ti, Zr, Hf) and methyllithium at –78 °C in toluene can lead to methylidene–metal complexes, H 2 C=MCl 2 , by a sequence of monomethylation, α‐carbon lithiation and α,μ‐elimination of LiCl. Here study of the preparation of alkylidene derivatives of iron was attempted by the interaction of FeCl 3 with n ‐butyllithium in various ratios at –78 °C. The presence of any resulting butylidene–iron(III) derivative, n PrCH=FeE (E = Cl, n Bu), was probed by adding chemical trapping agents, such as diphenylacetylene, benzonitrile, methyl benzoate and benzophenone. In each experiment the hydrolyzed products were consistent with a cycloaddition reaction of n PrCH=FeE with the trapping agent. The products from diphenylacetylene and from benzonitrile with D 2 O workup are uniquely in accord with such a carbene precursor. A 3:1 ratio of n BuLi/FeCl 3 gave the optimal yield of n PrCH=Fe n Bu, ca. 80 %, from the n Bu 2 FeCl precursor. When a 3:1 reaction mixture was simply brought to 25 °C and hydrolyzed, the purple alkylidene–iron complex decomposed completely to iron metal. A study of a 3:1 interaction of PhCH 2 MgCl and FeCl 3 under similar conditions and trapping with diphenylacetylene provided evidence for the formation of PhCH=Fe–CH 2 Ph in ca. 40 %. These results support the hope that alkylidene–iron(III) analogs of the Grubbs reagents may be accessible by this process.