Ferrocene nucleus

In 3, the amino functional group is two methylene units removed from the ferrocene nucleus. It appears from the instantaneous and quantitative formation of h from 3 that this feature minimizes steric effects and also enables 3 to undergo the Schotten-Baumann reaction readily without the classical a-metallocenylcarbenium ion effects providing any constraints. The IR spectrum of showed the characteristic N-H stretch at 3320 cm" (s), the amide 1 (carbonyl) stretch at 1625 an - -(s), the amide II (N—H) stretch at 1540 cm (s), and the amide III band at 1310 cm 1(m). In addition, characteristic absorptions of the ferrocenyl group were evident at 1100 and 1000 cm l (indicating an unsubstituted cyclopentadienyl ring) and at 800 cm"l. 

The same reaction but with trimethylacetaldehyde results in the frans-adduct exclusively in 100% yield and in 97% ee. The use of the gold catalyst is essential for the high selectivity silver and copper catalysts are much less effective. The length of the side chain between the diethylamino group and the ferrocene nucleus is also an important factor in the selectivity. 

The same reasons for the interest in incorporating ferrocene units into polymers also provided motivation for the synthesis of dendritic macromolecules of well-defined size and structure containing ferrocenyl units. An important additional rationale for the construction of ferrocenyl dendrimers is provided by the fact that such macromolecules raise the possibility of combining the unique and valuable redox properties associated with the ferrocene nucleus with the highly structured macromolecular chemistry. This may provide access to materials of nanoscopic size possessing unusual symmetrical architectures, as well as specific physical and chemical properties which would be expected to differ from those of the ferrocene-based materials prepared to date. 

Stronger conditions of oxidation lead to complete rupture of the cyclopenta-dienyl-iron bond. Attempts to nitrate ferrocene using ethyl nitrate in the presence of sodium alkoxides led to considerable amounts of nitrocyclopentadienylsodium and iron oxides (118). Treatment of ferrocene with bromine or chlorine is also reported to result in destruction of the ferrocene nucleus, the products containing pentahalocyclopentanes (64). 

One structure that seemed attractive was ferrocinnamic acid, a ferrocene derivative carrying an acrylic acid chain. The substrate examined was the p-nitrophenyl ester of this acid (I) (12). It was clear from the known binding constants of ferrocene (4) and of p-nitrophenyl acetate that the ferrocene nucleus should be the one... 

Czarniecki also has examined another cinnamic acid derivative (12), in an attempt to explore the relevance of our ideas about binding geometry. Since the f-butylphenyl group binds to j3-cyclodextrin in these mixed solvents about as well as a ferrocene nucleus does (4), Czarniecki examined substrate III, the p-nitrophenyl ester of cinnamic acid carrying a ra-t-butyl group. The methoxyl group is present on the substrate for synthetic simplicity. This substrate binds nicely to /3-cyclodextrin in our medium, and again an Eadie plot shows that a one-to-one complex is formed. However, the acceleration in this case is only 1200-fold compared with hydrolysis of the substrate by the same buffer, so this is approximately 600 times less effective as a sub-... 

The yields are lower in these more difficult cases. It was demonstrated that the metathesis conditions do not exert any oxidative stress on the substrates. Dimerization of 27 furnished spectroscopically pure 32 in 99 % yield after precipitation from methanol. Under oxidative stress, the ferrocene nucleus would have been oxidized and paramagnetic material would have been isolated. 

A number of organometallic polymers containing metallocenes and metallocene analogues has been well-known for some time2. Because of the features of high-temperature stability and radiation resistance of the ferrocene nucleus, ferrocene-containing polymers are of special interest. Basically, these polymers may be divided into two classes the metallocene moiety is either located in a pendant group or in a backbone of the polymer chain. The former polymers have been synthesized by vinyl polymerization of vinyl metallocene monomers such as vinylferrocene. The latter polymers have been prepared by polycondensation of l,l -disubstituted metallocenes or metallocene dihalides with a.w-disubstituted monomers, and fell into two main types, (A) and (B). 

Poly(vinyl ferrocene) was first prepared by Arimoto and Haven in 1955, (3 ) a process for which was patented in 1958 ( ). Vinyl ferrocene was found to be much less reactive than styrene, but still readily pol3mierlzable with azo initiators. Peroxide initiators were, in general, not effective, causing the ferrocene nucleus to oxidize instead (1 ). 

Now let me come back to primary substitutions at the ferrocene nucleus. Together with Vil chevskaya, we phosphorylated ferrocene and its derivatives to triferrocenylphosphine oxides [263, 264). An unusual reaction, discovered in collaboration with Perevalova and Yur eva, was the direct cyanation of ferrocene with hydrocyanic acid in the presence of ferric chloride [265,272). Initially, cyanide attacks the iron atom of the ferricinium cation, then the cyanide group transfers to the ring while the iron is simultaneously reduced. The reaction was termed by us as the ricochet (from the metal to the nucleus) substitution it may be applied to many substituted ferrocenes and to the ruthenocenium cation [273), and it is now the simplest route to ferrocene carboxylic acids through their nitriles. Further, ferrocene was studied in acid-medium reactions with oxo compounds. With aldehydes [274), the reaction was complicated by the transformation of ferro-cenylalkyl carbinol formed Initially via the carbonium ion, followed by transformation to a radical which, in its turn, was coupled to 1,2-bis-(ferrocenylalkyl)ethane (27.5). The reaction with acetone led to 2,2-di-ferrocenylpropane (276). 

Very soon we could deduce that electrophilic substitution rules are transformed when the benzene nucleus is replaced by the ferrocene nucleus. Actually, substituents of the first kind (ortho and para orientants) direct an electrophile to the same cyclopentadienyl ring while those of the second kind (meta orientants) direct electrophiles to the other. We showed this to be true for ricochet substitutions as well. However, the first/second type boundary, as determined for ferrocene, differs from that found in the benzene series, the electron-accepting type embracing an essentially greater number of substituents in the former case (see below). 

However, the transannular effect of the substituents was shown to operate essentially through an inductive mechanism. The effects were studied most extensively with the redox potentials of the X-substituted ferrocene versus the X-substituted ferricinium system. The potentials of the ferrocene derivatives offer a parallel to the activity of the ferrocene nucleus as a function of the substituents. Alkyl ferrocenes are oxidized more readily than ferrocene and they are more active in their electrophilic substitutions. Halogens, COOH, COOCHj, NHCOCH3, HgCl, CH OH, C Hj, to say nothing about RC(0) and CN groups, are electron acceptors... 

The ferrocene nucleus proved to be absolutely unable to undergo the rearrangements characteristic of the benzene ring, so the Claisen rearrangement of allyl ferrocenyl ether 291), the benzidine rearrangement of hydrazo-ferrocene 294-296), and the Sommelet rearrangement in the ferrocene series were all unsuccessful. 

It has also been observed that the regression lines for T- and 2-substituted methylferrocenes, obtained from the plot of the chemical shift values of the -CHj protons vs. electronic effect through the ferrocene nucleus. The ratio of the slopes (in percentage terms) gave a value of 28%, which agreed reasonably well with the value obtained by Butter and Beachell (22%) (11). 

The very strong electron-donating power of the ferrocene nucleus is clearly demonstrated in the extraordinary stability of the a-ferrocenyl carbonium ion (X). Questions have been raised concerning the mode of stabilization of such ions, i.e., (1) whether the77-orbitals of the ring overlap the empty p-orbital (resonance), (2) whether the f-orbitals on iron participate or overlap the empty p orbital, and (3) whether there is ring movement. 

The organometallic acrylates and methacrylates containing the ferrocene nucleus undergo ready radical-initiated homo- and copolymerization. Unlike the unusual kinetic behavior of vinylferrocene, the homopolymerization of ferrocenylethyl acrylate 13 (Scheme 10-4) and ferrocenylethyl methacrylate 14 (Scheme 10-4) was found to be first-order in monomer and half-order in initiator, similar to that of their organic analogs. In these monomers, the vinyl groups are removed from the influence of the ferrocene nucleus. Monomers developed using this concept will also be discussed later. 

One of the most well-studied characteristics of the ferrocene nucleus is its ability to undergo a reversible one-electron oxidation (57). Polymers such as poly(vinylferrocene) possess electroactive ferrocenyl moieties which are well-separated from one another by insulating organic units. The ferrocenyl units in these polymers are essentially noninteracting, and a single reversible oxidation wave is detected by cyclic voltammetry (55). The observation that the poly(ferrocenylsilane) 15 (R = R = Me) exhib-... 

Substituted ferrocenes also form monoanions at very negative potentials electron addition is genuinely associated with the ferrocene nucleus rather than with an electroactive substituent. The E° value for [Fe(f/-C5H4Ph)2] is — 2.62 V, and [FeCp2] itself shows a quasi-reversible reduction at —2.93 V in dmf (452), with a peak separation of 250 mV at — 37°C(v = 1 Vsec-1). Exhaustive electrolytic reduction of ferrocene derivatives yields solutions containing the substituted cyclopentadienide anions the latter may be used in the syntheses of other cyclopentadienylmetal complexes (453). Ferrocenes are also finding use as mediators in electron-transfer reactions, especially at electrode surfaces (454-456). 

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