Ferrocene moieties

Chiral ferrocenes have received niucli attenlion as ligands in metal-calalyzed reactions [39], bul tiieir use in copper cliemislry has been very limited [40, 41]. Hie ferrocene moiety offers die possibility of utilizing botli central and planar cliirality in die ligand. By analogy witli tlie copper arenetiiiolales described above, ferrocenyl copper complex 33 iSclieme 8.20) is extremely inleresling. 

Abstract The unique and readily tunable electronic and spatial characteristics of ferrocenes have been widely exploited in the field of asymmetric catalysis. The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst enviromnent. Instead, the Fe center can influence the catalytic process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. Of increasing importance are also half sandwich complexes in which Fe is acting as a mild Lewis acid. Like ferrocene, half sandwich complexes are often relatively robust and readily accessible. This chapter highlights recent applications of ferrocene and half sandwich complexes in which the Fe center is essential for catalytic applications. 

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. 

E0 of Compound ferrocene moieties A ° Reference Electrolyte electrode"1 Ref. 

Dendrimers with multiple ferrocene moieties at the surface usually show one-step multielectron redox reactions due to little internuclear interaction (141-143), while those with locally communicating ferro-cenyl groups have also been reported (141). 

The trans-azo bridge acts as a spacer assisting in the electron exchange between ferrocene moieties, similar to the vinylene bridge, as given for AE° of azoferrocene (25) in Table IV and its IT band feature (92,153). The mixed-valence cation of 25 formed in benzonitrile by le ... 

The effects of metal-to-metal electronic coupling are observed for several complexes where a ferrocene moiety is bound to transition-metal complexes with a -conjugated chain, 68 (167), 69 (168), and 70 (169). Tertiary amine-ferrocene conjugated molecules, 71, show two-step le oxidation, and their monocationic forms exhibit strong LMCT bands at 600-700 nm (170). 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

Figure 59 The carboranylenesiloxane (96) containing both the organometallic ferrocene moiety and the cross-linking diacetylene group. (Adapted from ref. 126.)...

There are, of course, metal-containing dendrimers that belong to more than one of the above-mentioned categories. Examples are the heptametallic dendrimer made of a central Fe(Cp)(C6Me6)+ core and coated with 6 ferrocene moieties [ 12], and the heterometallic dendrimers made of an organic core, containing up to 6 Pt(IV)-based organometallic species in the branches, and coated with up to 12 ferrocene units [13]. 

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

The one-pot synthesis of /1-lactams containing the ferrocene moiety 226 (Scheme 9.73) was described by Bonini [124], The reactions were performed under classical heating (3.5-24 h) and/or by using a focused microwave reactor (4—6 min). In this case, the stereochemical outcome of the reaction is the same with both procedures and microwave irradiation does not necessarily improve on the yields obtained by classical heating. 

Okayama, Japan. The synthetic procedures for S-D dyads and their reference compounds shown in Fig. 6 are illustrated in Fig. 7, where a naphthalene and a ferrocene moiety are used as an S and a D moiety, respectively. For comparison of photoinduced electron transfer rates between a single alkyl chain and a triple alkyl chain as the spacers of the S-D dyads with the same length of four-carbons, S-D dyads with a rigid spacer with a bicyclo[2.2.2]octane were synthesized [39]. The synthetic procedure for the S-D dyads with the rigid spacer is also shown in Fig. 8 [39], Other amphiphilic compounds and chemicals were commercially available. 

Stable monolayers of S-D amphiphilic dyads with a single alkyl chain spacer containing a naphthalene and a ferrocene moiety... 

The compounds [9] and [10] were synthesized in an effort to increase the through-space interaction between the ferrocene moiety and the bound cation (Scheme 3). These ligands were prepared by reaction of two equivalents of the corresponding mono-azacrown ether with bis(chlorocarbonyl)ferrocene. 

While examining the coordination properties of new di- and triaza-crown ether ligands containing multiple ferrocene moieties (Fig. 13) we discovered using H and 13Cnmr titration that these systems form selectively 1 1 complexes with ammonium cations (Beer et al., 1993c). 

Compound [25] was thus shown to have an unusual affinity for Ag+ cations. X-ray crystallographic determination of the structures of the free ligand, sodium and silver complexes were carried out and are shown in Fig. 17. The Ag-Fe distance in the silver complex of [25] is only 3.37 A, whereas the Na-Fe distance in the sodium complex is 4.39 A. This evidence together with the FAB MS data and UV spectroscopic data suggests that there may be an interaction between the silver cation and the iron present in the ferrocene moiety. 

The ligands discussed so far all contained C2 symmetry. An important new class of ligands having Ci symmetry was introduced by Togni [22] (see Figure 4.19). They can be easily made from an enantiomeric amine as the precursor in a few steps. Different substituents can be introduced at the phosphorus atoms. In addition to the chiral carbon atom the molecule has planar chirality as well. The chiral carbon atom is used to introduce the planar chirality, i.e. lithiation of the ferrocenyl amine takes place at a specific side of the amine at the ferrocene moiety. 

Alcohols have been converted into aldehydes mediated by a lipophilic / -cyclodextrin bearing a ferrocene moiety [139]. Efficient indirect in situ electroregeneration of NAD+ and NADP+ for enzymatic oxidations of butanol and 2-hexen-l-ol leading to the corresponding aldehydes using Fe bipyridine and phenanthroline complexes as redox... 

Bielawski et al. described the synthesis and properties of NHC 15 introducing a novel carbene architecture incorporating a l,l -disubstituted ferrocene moiety as backbone [71 ]. In its metal complexes [71,72] this carbene ligand supported a long-range electronic communication between the iron center and the carbene coordinated transition metal. 

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