Disubstituted Ferrocene Derivatives

The thermal characteristics exhibited by the series 11 [18] and 14 [20] proved that interesting liquid crystal properties can be obtained with symmetrically 1,T-disubstituted ferrocene derivatives. The structure of the organic unit, which is combined with the ferrocenyl moiety, must be carefully chosen. 

Compounds 15 [21, 22] and 16 [21] represent the first 1,3-disubstituted ferrocene-containing liquid crystals (Fig. 9-12). (In a recent review [5d], the structures of 15 (n = 6 — 8) and 16 were incorrectly drawn.) 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

Ferrocene derivative 16 showed an enantiotropic nematic phase between 204 °C and 235 °C. This result indicated that a biphenyl system, when associated with the ferrocenyl moiety substituted in the 1,3-positions, was also propitious to liquid crystal formation (compare 16 with its l,T-isomeric analogue 7 (n = 6)). 

No mesomorphic behavior resulted from family 17 [22] (Fig. 9-12). This observation is of interest, as it showed that a limit exists for obtaining liquid crystallinity even in the case of 1,3-disubstituted ferrocene derivatives. 

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Ugi has coined the term stereorelating synthesis for the sequence lithiation/reac-tion with electrophiles [62,118], and used this technique as a method for the chemical correlation of the structure and for the determination of the enantiomeric purity of many 1,2-disubstituted ferrocene derivatives obtained either by resolution or by asymmetric synthesis (for a compilation, see [118]). It is important to note that all stereochemical features discussed above for central chiral compounds, such as retentive nucleophilic substitution, remain valid when more substituents are present at the ferrocene ring and the conversion of functional groups in planar chiral ferrocenes can be achieved by the same methods as described. 

Among the l,l -disubstituted ferrocene derivatives, the bifunctional thio- and selenoethers, fc(SMe)2 and fc(SeMe)2, have found particular attention as chelating ligands (Scheme 5-34) [32,226-230], being comparable in this respect with 1,1 -bis(diphenylphosphinyl) ferrocene, fc(PPh2)2 (cf Chap. 1). 

We started our research on disubstituted ferrocene derivatives late in 1989. At this time, only the paper describing compounds 7 [13] was known. 

Fig, 9-7, Isomeric 1,1 -disubstituted ferrocene derivatives which differ in the orientation of the external ester functions. 

Fig. 9-12. First 1,3-disubstituted ferrocene-containing liquid crystals (15 and 16) and non mesomorphic 1,3-disubstituted ferrocene derivatives 17.

Fig. 9-19. Unsymmetrically l,l -disubstituted ferrocene derivatives reported by Thompson, Goodby and Toyne.

We designed and studied [27] a family of unsymmetrically l,l -disubstituted ferrocene derivatives obtained by combining the organic units A and B (above), used to prepare the families 13 and 14, respectively within the same molecular framework. Structures 25 (Fig. 9-21) led to remarkable mesomorphic properties. All derivatives exhibited liquid crystal properties. Compound 25 (n = 11) gave rise to an enantiotropic smectic A phase. Complex 25 (n = 12) showed an enantiotropic... 

The oxidation potentials of substituted ferrocenes are, perhaps not unexpectedly, found to be a sensitive function of the number and type of ring substituents. The formal oxidation potentials of a considerable number of mono- and disubstituted ferrocene derivatives have been determined titrimetrically 19, 20, 31, 35, 38), as well as by chronopotentiometry 5, 25, 27, 28, 29, 62, 63) and polarography 2, 39, 56, 57, 58). The oxidation potentials of these substances appear to correlate rather well with disubstituted ferrocenes, although Nesmeyanov and his coworkers have more recently claimed 35) that a better correlation is obtained with the Taft constants 53, 54). 

In an analogous manner, l,l -ferrocenylene dibromide, fcBr2 [279] (cf. Scheme 5-3), has been used to prepare l,l -disubstituted ferrocene derivatives such as fc(N3)2... 

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