Ferrocenyl group, stabilizing effect

The /3-silyl effect is attenuated in a-ferrocenyI-substituted vinyl cations because the strong electron-donating effect of the a-ferrocenyl group renders the cation centre less electron-deficient and thus lowers the demand for hyperconjugative stabilization by substituents at the /1-carbon149. 

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. ... 

As a stabilizing group for carbonium ions, a single ferrocenyl nucleus is almost as effective as three phenyl substituents, and this stabilization increases progressively as the metal atom is changed to ruthenium and then osmium. 

In the 1980s, the successful synthesis of elastomeric polyamides 120 of high molecular weight (d/n =10,000-18,000) was reported from the polycondensation of l,l -bis(/ -aminoethyl)ferrocene with diacid chlorides (Scheme 13). Also, polyureas 121 were prepared from the same ferrocene monomer and diisocyanates, and polyesters and polyurethanes were prepared from l,l -bis(/ -hydroxyethyl)ferrocene. However, the latter materials had much lower molecular weights and were characterized only by scanning electron microscopy. X-ray, and IR analyses. The introduction of ferrocenes in which the functional groups are separated from the cyclopentadienyl ring by at least two methylene units was crucial in order to reduce steric effects and to avoid the instability found previously in polymers of a-functionalized ferrocene due to the a-ferrocenyl carbonium ion stability. 

The H-D exchange rates of the methyl group in complexes FcCMeR are also sensitive to steric effects, e.g. k=2.1 x 10 , R=Me 3.1 x 10 , R=Et 2.7x 10 , R=Pri and 1.9 x 10 , R=BuS i.e. the rate increases with increase in the bulk of R cf. the rearrangement process). This corresponds to a decrease in the stability of the carbenium ion with increase in the bulk of R and thus provides a driving force for both reactions. This effect can be rationalized in terms of the Cais or Gleiter models for the ferrocenyl carbenium ions. 

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