Ferrocene substitution reactions

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 lithium benzamidinates Li[PhC(NR)2] (R = Cy, Pr ) and Li[2,4,6-(Cp3)3C6H2C(NCy)2] have been prepared analogously. Reaction of FcLi (Fc = ferrocenyl) with 1,3-dicyclohexylcarbodiimide ( = DCC, Scheme 6), followed by addition of water, afforded the ferrocene-substituted amidine Fc(NCy)NHCy in 50% yield. The amidine is readily deprotonated by LLN(SiMe3)2 or NaN(SiMe3)2 to yield the alkali metal amidinates, Li[FcC(NCy)2l and Na[FcC(NCy)2l in high yields. ... 

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]. 

Numerous chemical reactions have been carried out on ferrocene and its derivatives.317 The molecule behaves as an electron-rich aromatic system, and electrophilic substitution reactions occur readily. Reagents that are relatively strong oxidizing agents, such as the halogens, effect oxidation at iron and destroy the compound. 

Ikeda and co-workers examined ferrocene-based ligands 11a and 11b in the same allyhc substitution reaction of racemic acetate 136 (Fig. 9.44, Table 9.24). This catalytic system resulted in quantitative yield of 137 with ee of 96 and 99%, respectively. 

Ruthenocene is an example of a stable x-bonded organometallic compound which undergoes substitution reactions similar to those displayed by ferrocene. Because ruthenocene has heretofore been relatively unavailable, its chemistry has not been extensively studied. 

Another very important ring substitution reaction of ferrocene is its ability to undergo metalation with organolithium and organosodium compounds. Metalation with n-butyllithium in ethyl ether, first reported by Nesmeyanov and coworkers (63) and independently by Benkeser, Goggin, and Schroll (3),"leads to rather low yields of ferrocenyllithium (XXXI, M = Li) and l,l -ferrocenylenedi-lithium (XXXII, M = Li). It was subsequently shown by Mayo, Shaw, and... 

We have seen how a substituent on one cyclopentadienyl ring in ferrocene markedly affects subsequent ring substitution reactions. While certain transformations of this type are at least partially determined by steric factors, it seems certain that resonance and inductive interactions also play a very important role in determining orientation. Electronic transmissions both within and across the cyclopentadienyl rings are not fully understood, since the precise nature of the bonding in ferrocene is still uncertain. 

In the solid state ruthcnocenc and osmocenc prefer the eclipsed, pentagonal prismatic- structure and in solution exhibit a chemistry similar to that of ferrocene. The other metal loccncs. on the other hand, are quite different chemically, none of them showing the typical aromatic substitution reactions of ferrocene. 

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. 

Ferrocenes substituted with silicon-silicon groupings are prepared by (1) the reaction of lithiated ferrocenes with an appropriate chloropolysilane and/ or (2) lithiation of a cyclopentadienyldisilane followed by treatment with ferrous chloride. The following give examples (116,117) ... 

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... 

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 ... 

L3039>, and diarylbenzo[c]selenophenes <2005TL7201>. The reversible redox reactions of ferrocene-substituted benzo[f]selenophenes 15 were also studied and established a new type of multistep reversible redox system <2006TL2887, 2007JOM(692)60>. 

Ferrocene substituted amldinate complexes are also known. For example, reaction of fer-rocenyl lithium with DCC affords the lithium amldinate 44, which on hydrolysis gives the... 

Ferrocene reacts with acetyl chloride and aluminum chloride to afford the acylated product (287) (Scheme 84). The Friedel-Crafts acylation of (284) is about 3.3 x 10 times faster than that of benzene. Use of these conditions it is difficult to avoid the formation of a disubstituted product unless only a stoichiometric amount of AlCft is used. Thus, while the acyl substituent present in (287) is somewhat deactivating, the relative rate of acylation of (287) is still rapid (1.9 x 10 faster than benzene). Formation of the diacylated product may be avoided by use of acetic anhydride and BF3-Et20. Electrophilic substitution of (284) under Vilsmeyer formylation, Maimich aminomethylation, or acetoxymercuration conditions gives (288), (289), and (290/291), respectively, in good yields. Racemic amine (289) (also available in two steps from (287)) is readily resolved, providing the classic entry to enantiomerically pure ferrocene derivatives that possess central chirality and/or planar chirality. Friedel Crafts alkylation of (284) proceeds with the formation of a mixture of mono- and polyalkyl-substituted ferrocenes. The reaction of (284) with other... 

The cyclopentadienyl anion 13 is an efficiently resonance-stabilized anion in which all the carbon-carbon bond lengths are equal (Figure 1.9). It forms stable compounds, of which ferrocene (14) is an example, which undergo aromatic substitution reactions such as sulfonation and acetylation. 

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