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The mechanism of the electrophilic substitution reaction of ferrocene has been investigated using density functional theory. In particular, reactions with two hard electrophiles (protonation and acetylation) and one soft electrophile (mercuration) have been studied at the LDA and B-PW91 levels of theory using a triple-ζ STO basis set. A general description of the reactions has been obtained, leading to results in agreement with experiment. Acetylation is found to occur via exo attack, whereas mercuration follows an endo mechanism. In the case of protonation, evidence for a rapid equilibrium between metal-protonated and agostic ring-protonated ferrocene is obtained, and no clear conclusion concerning the exo or endo mechanism can be deduced. The calculated proton affinities corresponding to both metal-protonated and agostic ring-protonated structures are in excellent agreement with experiment.

A regio‐ and stereoselective route from benzene to trans‐disubstituted cyclohexadienes via complexation of the arene to the tricarbonylchromium group is reported. The key step involves an alkylation/carbonylation sequence of the anionic tricarbonyl (η5‐cyclohexadienyl)chromium complex (3) which is readily obtained by the addition of 2‐lithio‐2‐methyldithiane (2) to (η6‐benzene)tricarbonylchromium (1; cf. Scheme 6). In situ reaction of 3 with alkyl halides (in THF, THF/HMPA, THF/DMPU), followed by oxidation (I2, Ce(IV)) or ligand exchange (CO, Ph3P, Et3N) produced, with complete stereo‐ and regioselectivity, trans‐5,6‐disubstituted cyclohexadienes (15 examples). The cyclohexadiene substituent originating from the alkyl halide in all cases is an acyl group which shows that CO insertion into the metal–alkyl bond precedes reductive elimination to form the cyclohexadiene product. When, in the reaction of 3 with Mel, NHS3 was used to induce carbonylation und decornplexation, the isomerized, conjugated 1,4‐cyclohexadiene 13 was obtained almost exclusively. The electrophile selectivity in the reactions with 3 is consistent with a nucleophilic, SN2‐like mechanism with a high preference for primary iodides. Chloride, ketone, and ester functions in the electrophile are unreactive and are tolerated; a primary alkyl iodide reacted selectively in the presence of a secondary iodide. In one case, the trans‐configuration in a cyclohexadiene product 7 was demonstrated by the Diels‐Alder reaction with maleic anhydride. High facial selectivity was observed in this reaction giving a single diastereoisomer resulting from endo‐addition of the dienophile to the diene face carrying the acyl group. The anionic intermediate 3 was trapped with Ph3SnCl, and an X‐ray analysis of the resulting cyclohexadienyl [Cr(CO)3(R)] complex 15 provides evidence for electrophile addition to the metal. In the solid state, the Ph3Sn group in 15 is trans‐configurated to the dithianyl substituent. The ready access to [Cr(arene)(CO)3] complexes, the high selectivity of the reactions reported here and the mild decomplexation provide rapid access to Cyclohexadienes that possess functionality and are attractive for further transformation.

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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.