Ferrocene Isomer

Although the bonding in the proposed ferrocene isomer (Tj -CaHtlfTj -CgHelFe has been studied theoretically, synthesis of this molecule has not been reported (C. M. Brett, B. E. Bursten, Polyhedron, 2004, 23, 2993). For this molecule ... 

The high enantioselectivity again can be rationalized by enantioface-selective alkene coordination in 63 (Fig. 35). The olefin moiety is expected to bind trans to the upper imidazoline moiety [70,73] thereby releasing the catalyst strain. Coordination at this position may, in principal, afford four different isomers assuming the stereoelectronically preferred perpendicular orientation of the alkene and the Pt(II) square plane. In the coordination mode shown, steric repulsion between both olefin substituents and the ferrocene moiety is minimized. Outer-sphere attack of the indole core results in the formation of the product s stereocenter. 

In 1996, Fu et al. reported the S3mthesis of the planar chiral heterocycles 64, formally DMAP fused with a ferrocene core [82]. While the original synthesis provided racemic 64a in only 2% overall yield requiring a subsequent resolution by preparative HPLC on a chiral stationary phase, a recently improved synthesis furnished the racemic complexes 64 in 32-40% yield over seven steps. A subsequent resolution with di-p-toluoyltartaric or dibenzoyltartaric acid gave access to the enantiomers with >99% ee (28 14% yield for each isomer in this step) [83]. 

The l,l -diferrocenyl-VT electron donor molecule is structurally similar to diferrocenyltetrathiafulvalene but with the TTF moiety replaced by bis(vinylene-dithio)tetrathiafulvalene (VT) [76]. It has currently not been possible to separate the cis- and trans-isomers. The 1 1 polyiodide complex of l,l -diferrocenyl-VT was obtained through reaction with iodine. EPR and Mossbauer spectra indicate that in this charge transfer salt the VT moiety is oxidized while the ferrocene... 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

The bis-benzo-15-crown-5 ferrocene compound [7] containing two vinylic linkages was formed in a mixture of three isomeric components, the cis-cis, cis-trans and trans-trans isomers, which proved inseparable. However, the precedent of insignificant differences found between the magnitudes of the metal cation-induced anodic shifts in the ferrocenyl redox potentials of the respective separated cis and trans isomers [2a] and [2b] led us to use the same isomeric mixture of [7] throughout the subsequent FABMS and electrochemical group 1 and 2 metal cation complexation experiments,... 

When an unsymmetrical diyne is used, two regjoisomers of ortho and meta isomers are formed. Then, by choosing the bidentate phosphorus ligand (DPPE or DPPE (l,l -bis(diphenylphosphino)ferrocene), the ratio of their formation could be controlled from 1 4 to 7 1 (Scheme 11.2) [12]. 

The C90 cage has 46 possible constitutional isomers, out of which only five can be isolated. The electrochemistry of C90 shows two oxidations and six reductions. The redox potentials for C90 are given in Table 8.1. The first reduction potential appears at 0.49 V versus ferrocene/ferrocenium, thus making C90 the easiest to reduce among the empty cage fullerenes. 

A first group of compounds Ce Cg2, Gd Cg2, Y Cg2, and the major [C2v] and minor [Cs] isomers of La Cg2 and Pr Cg2 showed two oxidation steps, the first reversible and the second irreversible, even at scan rates up to 1 V/s. The low potential of the first oxidation step, close to that of the ferrocene/ferrocenium couple (see Table 8.3 and Fig. 8.4), made these compounds rather good electron donors. These compounds could also be reduced in four to six distinct steps, most of them reversible, and their reducing ability was found even higher than that of and similar to that of the major isomer of Cg2 (C2). Noticeably, all these compounds had a very low electrochemical HOMO-LUMO gap (A ,gap<0.50 V). In addition, similar UV/Visible spectra were obtained for all of them,28 suggesting also similar electronic structures. ESR showed that Y Cg229 and both isomers of La CX249 52 are radical species and consequently that the formal oxidation state of the metal in these structures is probably + 3. Therefore, their low HOMO-LUMO gap is probably a consequence of their open-shell electronic structure. 

Photochromic dithenylmaleimide 32 contains two ferrocene units.41 Electrochemical absorption spectral studies indicate that oxidation of ferrocenyl units can induce ring-opening reaction for the ring-closed isomer of 32. 

The major product is the heteroannular disubstituted derivative, l,l -diacetyl-ferrocene (I), while a very small amount of a homoannular isomer, 1,2-diacetylfer-rocene (II), is also obtained. The first acetyl group therefore appears to deactivate the substituted ring toward further electrophilic substitution, and the second acetyl group preferentially enters the opposite ring. 

The isolation of three isomers of this type is completely consistent with the concept that the two cyclopentadienyl rings in ferrocene are rotating in all but the crystalline state. From the amounts of each isomer formed and the number of nonequivalent ring positions, site reactivities can be calculated, the l -position being arbitrarily assigned a value of unity. 

The Friedel-Crafts alkylation of ferrocene with ethylene chloride has been the subject of considerable investigation. It was initially reported by Nesmeyanov and coworkers that the reaction produced diferrocenylethane and appreciable amounts of polymeric material (55). Rinehart and coworkers later proved that the low molecular weight product was l,l -diferrocenylethane (XVII) instead of the expected 1,2-isomer (103). The anomalous product is postulated to result... 

To realize the above system, it is required to design a polymer which reversibly changes the molecular properties, such as hydrophilicity, by the external stimulation. Many molecules are known to be reversibly transformed to other isomers by external stimulation, such as photons, electrons or chemicals. Table 1 lists a few examples. Azobenzene shows the property change by photoirradiation. It isomerizes from the trans to the cis form by ultraviolet irradiation, and the dipole moment increases from 0.5 to 3.1 deb ye. The polar cis form returns to the less polar tram form by visible irradiation. Electrochemical oxidation of ferrocene changes the hydrophilicity. When it is oxidized from Fe(II) to Fe(III), the hydrophilicity increases. The Feflll) state returns to the Fe(II) state by either electrochemical or chemical reduction. Host molecules also change the properties in the presence of suitable guest ions. Benzo[18]crown-6, for example, captures potassium ions in the cavity, and increases the hydrophilicity. 

Exercise 31-1 If the ferrocene rings in 3 were not free to rotate, how many different dichloroferrocene isomers would be expected (including chiral forms) How could the substitution method (Section 1-1F) be used to determine which of the isomers was which ... 

In the metallocene field, it has been observed that whereas 2-methyl-ferrocene fails to react, methylcymantrene (114) indeed forms the thienyl derivative 115 on treatment with the anil from 2-formylthiophene and aniline.55 It was also claimed that 5% of the cis isomer was isolated. 

S-Ketoesters, /3-ketophospbonates, and /3-ketosulfones have been used to alkylate ferrocene to afford the corresponding /3-ferrocenyl-a,/3-unsaturated derivatives in excess triflic acid262 263 [Eq. (5.98)]. The transformations are highly stereoselective, giving exclusively the (El-isomers this was explained by the exo-deprotonation of carbenium ion 71a of more stable conformation. Acetals of formylphosphonates and formylsulfones react in a similar manner. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

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