Iron, organic derivatives ferrocene

Bis(cyclopentadienyl)iron derivatives, ferrocenes, are remarkably stable against heat and air and undergo various kinds of chemical reactions. They are usually prepared by the reaction of FeCh with an alkali metal salt of cyclopentadienyl or cyclopentadiene in the presence of base [51]. Although ferrocene derivatives basically have chemical reactivities similar to those of aromatic compounds, they have found only limited applications to organic synthesis so far. However, because chiral ferrocenylphosphines are capable of having both planar chirality and an asymmetry in the side chain in their rigid framework, they have been used recently in a number of asymmetric reactions. In this section, synthesis and developments of monocyclopentadienyl complexes and ferrocenylphosphines are described. The general chemistry of ferrocenes and half metallocene complexes is reviewed elsewhere [52-53]. 

There is a vast literature on the chemistry of ferrocene, but very little use has been made of this substance in organic synthesis. As ferrocene is readily available (Jolly, 1968) and undergoes a great variety of reactions such as acylation, alkylation, sulfonation, and metalation (Perevalova and Nikitina, 1972), a simple means of removing the iron at the end of the reaction sequence would provide a ready route to substituted cyclopentane derivatives. Some typical acylation reactions [which take place 3.3 x 10 times faster than with benzene (Rosenblum et al., 1963b) are illustrated in Eqs. (64) (Hauser and Lindsay, 1957), (65) (Rosenblum and Woodward, 1958), and (66) (Rosenblum et al., 1963a). 

In a similar manner, certain bis(arene)iron(II) salts react easily with nucleophiles and may be used in organic synthesis. For example, (mesityIene)2Fe(PF6)2 reacts with phenyl, terf-butyl, and vinyllithium to give either the compound XLVIII or the pseudo-ferrocene derivative XLIX, according to the stoichiometry of the reaction (Helling and Braitsch, 1970). 

Nitrate reductases (NaR) with an iron-sulfur center are used for nitrate conversion. Generally, nitrate is enzymatically reduced and NaR is in the oxidized form, which can be electrochemically reduced. However, the direct electron transfer between an enzyme and an electrode is strongly limited due to the fact that (1) the distance between the electrode surface and the redox active site of the enzyme, which is normally inside the globular protein, is large and (2) the orientation of donor to acceptor sites depends on the method of the immobilization of the enzyme at the electrode.Thus, low molar mass redox mediators including qui-nones, metal complexes, ferricyanide, derivatives of ferrocene, and organic redox dyes " have been used to facilitate the electron transfer between electrode and enzyme (Fig. 11.5). 

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