Reactions of Cyclopentadienyl Complexes

Associative substitution reactions of dative ligands in cyclopentadienyl complexes can occur by a decrease in the haptidty of the Cp ligand from to These ring-slip reactions, which occur more rapidly for indenyl complexes than for cyclopentadienyl complexes, are discussed in Section 5.5.3. This increase in rate is called the indenyl effect.  

Electrophilic attack on coordinated cyclopentadienyl rings, particularly those in ferrocene, is well established. This process occurs in a similar fashion to electrophilic attack on arenes and was used to establish the binding mode of the Cp ligand in ferrocene (see Equation 3.88). These reactions of the Cp ligand with electrophiles are described in more detail in Chapter 12, which covers electrophilic attack on coordinated ligands. Friedel-Crafts acylation, formylation, aminomethylation, and mercuration are all known. -  

However, electrophilic aromatic substitution on ferrocene presents two complications (1) its susceptibility to one-electron oxidation, which precludes halogenation, nitration, or direct (H SO ) sulfonation and (2) competition between the Fe and the ring as the initial site of electrophilic attack. It is now clear from H NMR spectroscopy of the product of the protonation of ferrocene with a superacid at -122 °C that protonation of ferrocene initially occurs endo. The detailed structure of the protonation product has not been unambiguously established, but calculations imply that the proton is associated at least as much with the cyclopentadienyl carbons as with the iron.  

Ferrocenes greatly stabilize positive clrarge a to a cyclopentadienyl ring. This stabilization of positive charge allows the ferrocenyhnethyl acetate in Equation 3.89 to serve as a precursor for stereoselective exclianges of the acetate by phosphines at the pseudo-benzylic 

CpjTiCl. Cleavage of the methyl C-H bonds of coordinated Cp also occurs frequently. An example of the activation of the Cp methyl groups to form dinuclear products is 

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