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, H₂C=MCl₂, 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 FeCl₃ with n-butyllithium in various ratios at -78 °C. The presence of any resulting butylidene-iron(III) derivative, nPrCH=FeE (E = Cl, nBu), 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 nPrCH=FeE with the trapping agent. The products from, di-phenylacetylene and from, benzonitrile with D₂O workup are uniquely in accord with such a carbene precursor. A 3:1 ratio of nBuLi/FeCl₃ gave the optimal yield of nPrCH=FenBu, ca. 80%, from, the MBu₂FeCl 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 PhCH₂MgCl and FeCl₃ under similar conditions and trapping with diphenylacetylene provided evidence for the formation of PhCH=FeCH₂Ph in ca. 40%. These results support; the hope that alkylidene-iron(III) analogs of the Grubbs reagents may be accessible by this process.