Of ferrocenes

The first page of this chapter displayed an electrostatic poten tial map of ferrocene You may wish to view a molecular model of it on Learning By Modeling... 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Examine the molecular model of ferrocene on Learning By Modeling Does ferrocene have a dipole moment Would you expect the cyclopentadienyl nngs of ferrocene to be more reactive toward nucleophiles or electrophiles Where is the region of highest electrostatic potential... 

This experiment examines the effect of reaction time, temperature, and mole ratio of reactants on the synthetic yield of acetylferrocene by a Eriedel-Crafts acylation of ferrocene. A central composite experimental design is used to find the optimum conditions, but the experiment could be modified to use a factorial design. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

Properties of ferrocene-containing polymers have been improved by inclusion of pyrazole systems in the backbone. The synthesis of (748) was achieved by condensation of bis()3-diketoferrocenes) with aromatic dihydrazines to give polyhydrazones that were later cyclo-dehydrated (B-80MI40408). 

DETERMINATION OF FERROCENE IN GASOLINE BY ATOMIC-ABSORPTION AND PHOTOCOLORIMETRIC... 

The quality of the iron used in preparing the ferrous chloride has a marked effect on the yield of ferrocene. The checkers employed Rascher and Betzold (730 N. Franklin, Chicago, 111.) 300-mesh iron powder, reduced by hydrogen. When 40-mesh iron filings were used, the yield of ferrocene was much lower (ca. 33%). 

The methods of preparation of ferrocene have been reviewed by Pauson and by Fischer. Ferrocene has been made by the reaction of ferric chloride with cyclopentadienylmagnesium bromide, by the direct thermal reaction of cyclopentadiene with iron metal, by the direct interaction of cyclopentadiene with iron carbonyl, by the reaction of ferrous chloride with cyclopentadiene in the presence of organic bases such as diethyl-amine, by the reaction of ferrous chloride with sodium cyclo-[lentadienide in liquid ammonia, and from cyclopentadiene and... 

Although not so generally applicable for the preparation of dicyclopentadienyl metal compounds as the sodium cyclopenta-dienide method, the amine procedure represents the simplest preparation of ferrocene. The amine procedure can also be employed for dicyclopentadienylnickel (about 80% yield), using nickel bromide obtained by the action of bromine on nickel metal powder and 1,2-dimethoxyethane as the solvent. The method of preparation given here is a modified version of that originally described. ... 

Structure of ferrocene elucidated organometallic chemistry burgeons Nobel Prize awarded jointly to E. O. Fischer and G. Wilkinson 1973,... 

Tlie importance of bis(cyclopeniadienyl)irou (Fe(jj -C5H3)2( in the developmenl of organo-metallic chemistry has already been alluded to (p. 924). Tile compound, which forms orange crystals, mpl74°, has extraordinary thermal stability (>500°) and a remarkable structure which was unique when first established. It also has an extensive aromatic-lype reaction chernisiry which is reflected in its common name ferrocene The molecular structure of ferrocene in the ciysialline slac features two parallel cyclopentadienyl rings at one lime Ihese... 

Biologically, iron plays crucial roles in the transport and storage of oxygen and also in electron transport, and it is safe to say that, with only a few possible exceptions in the bacterial world, there would be no life without iron. Again, within the last forty years or so, the already rich organometallic chemistry of iron has been enormously expanded, and work in the whole field given an added impetus by the discovery and characterization of ferrocene. 

The structure of ferrocene and an MO description of its bonding have already been given (p. 937). The rings are virtually eclipsed as they are in the analogous ruthenocene (light-yellow, mp 199°C) and osmocene (white, mp 229°C). 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

Next, consider how Fe " interacts with C5H5. Examine the geometry of ferrocene, Fe(C5H5)2. Are the FeC distances all the same, or does iron bond more strongly to some carbons than to others Are the CC bond distances all the same Which of the above models, the electrostatic or covalent, gives the better description ... 

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

Interaction of iron(II) chloride with the lithium salt of R4B2NJ (R = Me, Et) gives sandwiches 61 (R = Me, Et) (67ZAAC1, 96MI4), resembling in electronic properties those of ferrocene (99ICA(288)17). The n- rf-) complex stems from the further complex-formation of 61 (R = Me, Et) with mercury(II) salts via the unsubstituted nitrogen atom. 

The reaction is carried out under argon in a 2-liter three-necked flask fitted with a mechanical stirrer, reflux condenser, 250-ml pressure-equalizing addition funnel, and gas inlet and outlet. After purging with argon, the flask is charged with a solution of 89 g (0.48 mole) of ferrocene in 1 liter of dry tetrahydrofuran. The solution is next heated to 45°, and there is added dropwise with stirring, 155 ml (0.29 mole) of an -butyllithium solution (15% in heptane-pentane, 2 1, Foote Mineral Co.) during a period of 75 minutes. The resultant solution is maintained at 45° for an additional 2 hours, then is cooled to —77° by means of an external Dry Ice-chloroform bath. 

Singer and co-workers have investigated the acylation reactions of ferrocene in ionic liquids made from mixtures of [EMIMJI and aluminium(III) chloride (Scheme 6.1-5) [9, 10]. The ionic liquid acts both as solvent and as source of the Friedel-Crafts catalyst. In mildly acidic (X(A1C13) > 0.5 [EMIM]I/A1C13, the monoacetylated ferrocene was obtained as the major product. In strongly acidic [EMIM]I/AlCl3 X(A1C13) = 0.67 the diacylated ferrocene was the major product. Also, when R = alkyl, the diacetylated product was usually the major product, but for R = Ph, the monoacetylated product was favored. 

Scheme 6.1-6 Arene exchange reactions of ferrocene in [BMIM]CI/AICl3.

In a recent continuation of the work on dediazoniation of 2-(2 -propenyloxy)ben-zenediazonium salts (10.55, Z = 0, n= 1, R=H) in the presence of ferrocene, Beckwith et al. (1992) found that 3-ferrocenylmethyl-2,3-dihydrobenzofuran (10.65) is formed. The results are consistent with a mechanism involving electron transfer and dediazoniation followed by homolytic attack on the ferrocenium ion. This investigation resolved a long-lasting dispute regarding the heterolytic or homolytic character of the formation of arylferrocenes from arenediazonium ions (for literature since 1955 see Beckwith et al., 1992, references 1-7). 

RELATIVE REACTIVITIES OF REACTION OF FERROCENE DERIVATIVES WITH MeCOCl... 

Nesmeyanov et a/.545 used a mixture of ferrocene, deuterated trifluoroacetic acid and benzene in the molar ratios 1 2 20 in a preliminary investigation of the reactivity of ferrocene and its derivatives. At 25 °C, rate coefficients were 1,620 x 10-7 (ferrocene) and 19.3 xlO-7 (acetylferrocene). In a subsequent publication by Alikhanov and Shatenshtein543 these values were altered to 1,600 x 10-7 and 1.5 x 10 7, respectively, and a value of 0.77 x 10"7 added for 1,1-diacetylferrocene. Under the same conditions, toluene gave a value of 0.3 x 10-7 so that the activating effects of these compounds relative to benzene can be approximately determined. 

Nesmeyanov et a/.546 have also measured the effects of substituents in deuteration of ferrocene by deuterated trifluoroacetic acid in dichloromethane at 25 °C. Rate coefficients were measured for ferrocene and its derivative in a range of such acid mixtures, the composition of which was omitted, and in some cases the rate of exchange for ferrocene was calculated on the basis of a linear relationship between log and —H0. Results including the calculated knl values are given in Table 161. It should be noted that, in discussing those results, the authors quoted the incorrect partial rate factors for dedeuteration of toluene arising from the use of the incorrect data for benzene (see p. 199). This should be taken into account... 

A relative reactivity of ferrocene benzene of 105-106 has been quoted557 following a kinetic study of the deuteration of ferrocene in acetic acid-trifluoroacetic acid mixtures at 25 °C, but the value is entirely in error, being based on two faulty assumptions. The data are given in Table 166 and a linear plot of log Art versus —H0 was extrapolated to — H0 = 5.0, a rate coefficient of 1.3 x 10 1 being obtained. This was compared to the Gold and Satchell value for dedeuteration... 

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