Helicene

If compounds have the same topology (constitution) but different topography (geometry), they are called stereoisomers. The configuration expresses the different positions of atoms around stereocenters, stereoaxes, and stereoplanes in 3D space, e.g., chiral structures (enantiomers, diastereomers, atropisomers, helicenes, etc.), or cisftrans (Z/E) configuration. If it is possible to interconvert stereoisomers by a rotation around a C-C single bond, they are called conformers. 

The Bucherer carbazole synthesis was pivotal in the preparation of the first hexahelicene 37a. Reaction of 2,7-dihydroxynaphthalene 35 with phenylhydrazine and sodium bisulfite afforded helicene 37a although in low yield. More recently, the synthesis was extended to the preparation of 37b using 2,5-dimethylphenylhydrazine 36b. ... 

It is also interesting that the hypothetical helicene 255 has been incorporated in a study of the jt -electron spectra of a series of infinite helicenes composed of various structural units (77BAU2532). A compound 256, closely resembling an indolo-[2,3-c]carbazole but possessing an additional six-membered ring between the... 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

Scheme 17 Angular versus linear benzannulation of helicenes...

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

Enantiomers (M)- and (P)-helicenebisquinones [32] 93 have been synthesized by high pressure Diels-Alder reaction of homochiral (+)-(2-p-tolylsulfo-nyl)-l,4-benzoquinone (94) in excess with dienes 95 and 96 prepared from the common precursor 97 (Scheme 5.9). The approach is based on the tandem [4 + 2] cycloaddition/pyrolitic sulfoxide elimination as a general one-pot strategy to enantiomerically enriched polycyclic dihydroquinones. Whereas the formation of (M)-helicene is explained by the endo approach of the arylethene toward the less encumbered face of the quinone, the formation of its enantiomeric (P)-form can be the result of an unfavourable interaction between the OMe group of approaching arylethene and the sulfinyl oxygen of 94. 

An analogous type of lithium-induced zipper cyclization was observed with PAM 5, affording the mesolDL pair of helicenes (Scheme 11) [34]. PAM 5 also reacted with dicobaltoctacarbonyl to give a tetracobalt cluster in which only two of the triple bonds have been complexed [35]. 

Recently, Carreno, Urbano and coworkers were also able to synthesize almost enantiopure [7]helicene bisquinones 4-58 and 4-59 (96% ee) by reaction of the sulfoxide (S,S)-4-55 with the diene 4-56 in dichloromethane at -20 °C (Scheme 4.12) [19]. This six-step domino process includes a double Diels-Alder reaction, sulfoxide elimination, and aromatization of the rings B and F of the intermediate 4-57 to give 4-58, which could be oxidized to the fully aromatized 4-59. 

Many molecules are chiral - that is, these molecules are not superimposable on their mirror images. One way for molecules to obtain chirality is to have so-called asymmetric carbons (that is, a carbon atom with four covalent bonds, none of which are equivalent). But there are many other structures that are also chiral, ranging from helicenes through simple twisted molecules such as biphenyl. 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

Figure 6.4 Graphical representation of mode of association of helicene 33 into helical columns. (Katz, T. J. Angew. Chem. Int. Ed. Engl. 2000, 39, 1921-1923, reproduced by permission of John Wiley Sons, Inc.)...

Metal phthalocyanines functionalized with four helicenes (62) have also been reported to form chiral columnar aggregates.76 In chloroform solutions of these metal phthalocyanines aggregation into columns occurred upon addition of ethanol, as was observed by UV-Vis spectroscopy. CD spectroscopy revealed that the chromophores within the columnar aggregates are in a chiral environment, implying that the chirality of the peripheral helicenes has been transferred to the supramolecular aggregates. These phthalocyanines stack with a typical intermolecular distance of 3.4 A, and calculations have indicated that to allow this distance the two phthalocyanine moieties have to be rotated because of the bulkiness of the helicenes. It can easily be imagined that a phthalocyanine provided with both R and S helicenes cannot stack in such a defined manner because of the steric interactions between the nonconform helicenes. 

A last example of a dopant whose chirality has been investigated by the LC technique is represented by helicenes and related molecules. Once again, compounds very different spectroscopically, such as 46-49, and hence hardly comparable with chiroptical techniques, are very similar in shape and give helical twisting powers of the same sign and of comparable intensity80 the twisting powers of helicenes have been successfully calculated by the shape model of Ferrarini et al.73... 

The systems discussed up to now all showed chiral susceptibilities that were of the same order of magnitude or smaller than the achiral susceptibility components. The system that we discuss in this section has chiral susceptibilities that dominate the nonlinear optical response.53 The material is a chiral helicenebisquinone derivative shown in Figure 9.22. In bulk samples, the nonracemic, but not the racemic, form of the material spontaneously organizes into long fibers clearly visible under an optical microscope. These fibers comprise columnar stacks of helicene molecules.54,55 Similar columnar stacks self-assemble in appropriate solvents, such as n-dodecane, when the concentration exceeds 1 mM. This association can be observed by a large increase in the circular dichroism (CD) of the solutions. 

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