Mechanism of Mercuration of Ferrocene:  General Treatment of Electrophilic Substitution of Ferrocene Derivatives

The mercuration of (pentadeuteriocyclopentadienyl)cyclopentadienyliron(II), 1, occurs preferentially at the C5H5 ring. The ratio of C5H5:C5D5 attack ranges from 5.2−6.5, depending upon the mercurating agent employed. In contrast to the Friedel−Crafts acetylation of 1, mercuration does not give rise to intramolecular interannular proton transfers. Under certain conditions, a complex, 6, can be isolated that contains both carbon−mercury and iron−mercury bonds. Taking these facts into account, a mechanism for the mercuration of ferrocene is proposed whereby precomplexation of the mercurating agent to the iron atom precedes the rate determining formation of the carbon−mercury bond with concomitant loss of H+. Combined with our previous work concerning the acetylation and proton exchange of ferrocene, this study allows the formulation of a general mechanism of electrophilic substitution of ferrocene, based on the reactivity of the electrophile and the basicity of the iron atom of the resulting ferrocene product.