Electrophilic substitution reactions of ferrocene

The Chemistry of Organometallic Compounds, Macmillan, New York, NY, 1967. 

Coates and K. Wade, Organometallic Compounds, Vol. I, Methuen and Co., London, 1967. 

Wakefield, Organomagnesium compounds in organic synthesis, Chem. Ind. (London), 450 (1972). J. M. Brown, Organolithium reagents in synthesis, Chem. Ind. (London), 454 (1972). 

Grignard reagents. Compositions and mechanisms of reaction, Q. Rev. Chem. Soc., 259 (1967). 

Wakefield, The Chemistry of Organolithium Compounds, Pergamon Press, Oxford, 1974. 

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It is also an oxidation addition reaction, where Br- has increased the oxidation state of Mn from 0 to + 1. The preparation and electrophilic substitution reactions of ferrocene. 

Volume 13 of this serial publication comprises six chapters of which four deal with general accounts of compound classes 1-azirines (F. W. Fowler), phenanthridines (B. R. T. Keene and P. Tissington), tri-thiapentalenes (N. Lozac h), and heterocyclic ferrocenes (F. D. Popp and E. B. Moynahan). The other two chapters are concerned with particular aspects of the chemistry of groups of heterocyeles the tautomerism of purines (B. Pullman and A. Pullman) and quantitative aspects of the electrophilic substitution reactions of five-membered rings (G. Marino). 

The mechanism of electrophilic substitution reactions in ferrocene is illustrated below... 

The density functional approach has been used in an investigation of the protonation of ferrocene, ruthenocene, and osmocene in the gas phase, and the general conclusion is that the addition of a proton to the carbon atoms in the cyclopentadienyl ring is favored in ferrocene, whereas metal protonation is favored with ruthenocene and osmocene. The results obtained from these calculations were used in the interpretation of electrophilic substitution reactions of metallocenes. The basicity of the ligand group l,l -bis(diphenylphosphino)metallocene has also been examined. This is an important aspect for catalysis because it has proved difficult to obtain results which... 

As mentioned above, ferrocene is amenable to electrophilic substitution reactions and acts like a typical activated electron-rich aromatic system such as anisole, with the limitation that the electrophile must not be a strong oxidizing agent, which would lead to the formation of ferrocenium cations instead. Formation of the CT-complex intermediate 2 usually occurs by exo-attack of the electrophile (from the direction remote to the Fe center. Fig. 3) [14], but in certain cases can also proceed by precoordination of the electrophile to the Fe center (endo attack) [15]. 

Although dibenzenechromium is thermally quite stable, it is less so than ferrocene and melts with decomposition at 285° to give benzene and metallic chromium. Furthermore, it appears to lack the aromatic character of either benzene or ferrocene as judged by the fact that it is destroyed by reagents used for electrophilic substitution reactions. 

Like the CSH5 ring in ferrocene and CpMn(CO)3 (see later), the C4H4 ring undergoes typical aromatic electrophilic substitution reactions. There is quite an extensive organic chemistry of the derivatives. Finally, it is possible to obtain free cyclobutadiene for use in in situ organic reactions by oxidative decomposition ... 

The study of relative site reactivities for aromatic electrophilic substitution reactions has provided useful information for assessing homoannular electronic effects within the ferrocene system. Ferrocenes of the type (I)... 

It is well known that electron-donating groups attached to benzene greatly enhance the ring s reactivity towards electrophilic substitution reactions, whereas electron-attracting groups decrease such reactivity markedly. Like most aromatic systems, ferrocene undergoes electrophilic substitution reactions readily. Care must be exercised because of the ease of oxidation of ferrocene to the ferricinium cation. Thus ferrocene cannot be nitrated, chlorinated, or brominated. Acetylation under a variety of conditions (63) has been successful, however. Under intensive Friedel-Crafts conditions further acetylation of acetylferrocene took place exclusively... 

Ferrocene behaves like an aromatic compound activated for electrophilic substitution reactions. Thus, only minor modifications of experimental procedures developed for aromatics are necessary to obtain ferrocene derivatives (a useful review on general methods is given by Schldgl and Falk [42]). For central chiral ferrocenes, resolution of the racemate is a frequently applied technique. Traditionally, resolutions are best achieved by salt formation between a chiral acid or base and the... 

Ferrocene, Fe(Ti5-C5H5)2, and related cyclopentadienyl complexes of transition metals in fact are far more thermally stable, less reactive substances than ionic cyclopentadienides, and have an extensive derivative chemistry that is typically aromatic in that their C-H bonds can undergo such electrophilic substitution reactions as Friedel-Crafts alkylation or acylation, nitration, and so on. Moreover, as a substituent, the ferrocenyl group (ri -f sl l5)Fc(ri -( 5l I4) (=R) is even more effective than a phenyl substituent in stabilizing carbenium ions [RCH2]+. The redox and photochemical properties of many metaUocenyl residues make them versatile substituents with many chemical and materials applications. ... 

Electrophilic attack on coordinated polyenyl ligands is uncommon, but electrophilic attack on the Ti -cyclopentadienyl ligands in certain complexes has become synthetically valuable. For example, the acetylation of ferrocene (Equation 12.75) is a reliable Friedel-Crafts process " and generates derivatives used to make many ferrocenyl ligands. Similar electrophilic aromatic substitution reactions of haloboranes on ferrocene (shown in Equation 12.76) and on the anion generated from deprotonation of tungstenocene dihydride (Equation 12.77) leads to borylcyclopentadienyl complexes. " ... 

An interesting recent development involves the suggested aromaticity of cyclobutadieneiron tricarbonyl [6-19]. This complex is aromatic in the sense that it undergoes electrophilic substitution reactions that parallel those of ferrocene. Figure 6-6 depicts the complete reaction schemes followed for electrophilic substitution and characterization of the reaction products. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

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