Helical structures 9 helicene

Reviews of compounds having a helical structure, others than helicenes, have been given by Mislow 48) and by Voegtle 49). 

Macroscopical helical structures formed by cholesteric liquid crystals have been used as chiral helical media for the asymmetric synthesis of helicenes. 

Recently, series of carbon-sulfur [n]helicenes substituted with w-octyl groups at the a-positions of the terminal thiophene rings were prepared (Fig. 15.16) [64, 87]. The helical structures of [7]helicene 44 and [ll]helicene 45 were confirmed by X-ray crystallography. Multiple short S-S contacts were found, especially for racemic [ll]helicene 45. Asymmetric synthesis of [ll]helicene 45 provided enantiomeric excess of either the (-)- or the (+)-enantiomer for the monoannelation or, unprecedented, triannelation approach (Fig. 15.16). Also, selective diannelation of octathiophene 47, followed by monoannelation of decathiophene 46, provided an efficient synthetic route to (-)-[ll]helicene 45, avoiding protection/deprotection steps [64],... 

Grignard reagents Organomagnesium halides RMgX having a C-Mg bond, halohydrins Halo alcohols. For example, ethylene bromohydrin is BrCH2CH20H. helicenes Ortho-fused polycyclic aromatic compounds that have a helical structure. 

Helicenes are benzologues of phenanthrene in which a regular cylindrical helix is formed through an all-ortho annellation of the aromatic rings. The helical structure is a consequence of the repulsive steric overlap of the terminal aromatic nuclei. 

Helicenes are polycyclic aromatic compounds in which the helical structure is a consequence of a steric repulsion of the terminal aromatic nuclei. The conformation and chirality of helicenes have attracted the attention of many researchers over the years [39]. We chose to enlist helicenes as both pawls and springs because (1) they have an inherent helicity that we thought would favor the unidirectional rotation and (2) their relatively rigid structures resist deformation. That resistance to deformation is also a characteristic of springs. 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

Katz " surrounded the nucleus of an octaazaphthalocyanine complexed by copper or nickel with four helicene groups of (M) configuration (6.38). These groups give a global helical structure to the Pc (Figure 6.31). The CD shows that the helices are stacked along... 

In the previous section, we reported PMP derivatives bearing chiral side chains having a helicene-like helical structure through intrachain 7r-electron overlap interactions, which lead to a self-assembled whisker morphology due to interchain van der Waals interactions. However, each whisker of the PMP derivative is still randomly oriented, even if it has a self-assembled structure. It is therefore desirable to align the PMP derivative to construct a higher-ordered hierarchical structure and also to evolve an anisotropic nature in its potentially profound op-toelectrical properties. 

The first example of whiskers composed of helicene-like helical structures was published by Akagi et al., who used poly(m-phenylene) derivatives (Figure 5.18) with racemic or chiral side chains. ... 

A new approach to second-order nonlinear materials was reported by Verbiest et al. (56) in which chirality plays the key role. These authors investigated Langmuir-Blodgett films of chiral helicenes which lack features commonly associated with a high SHG response. The molecules adopt a helical structure on a solid support and this chiral supramolecular arrangement enhances the second-order NLO susceptibility by a factor of 30 when compared to the corresponding racemic mixture. An adequate description of the SHG response in a chiral system requires additional tensor elements. Experimental evidence was provided that those tensor elements which are only allowed in a chiral environment dominate the SHG response of the helicene system. 

Dissymmetry can also occur for materials that possess rotational symmetry elements. Typically dissymmetric materials that have two- or threefold rotational axes of symmetry can also exhibit optical activity. For example, consider the following materials 5,S-l,3-(2-methylbutyl)cyclobu-tane and helicene (I and II respectively in Fig. 4). Compound I contains two asymmetric carbon atoms, does not possess a superimposable mirror image, and is also optically active. However, this compound is of a higher symmetry class than asymmetric compounds because it has a simple twofold axis of rotation passing through the center of, and normal to, the cyclobutane ring system. Helicene, compound II, does not possess any chiral carbon atoms as they are all sp hybridized, and therefore each contains a mirror plane. However, helicene is still optically active because it has a helical structure that does not have a superimposable mirror image. This molecule also pos-... 

Helical phases have similar symmetry properties to those of a molecule of helicene in that they are dissymmetric because they possess a twofold (infinite) axis of rotation normal to the helical axis through the midpoint of the helical structure. As helical phases are dissymmetric and do not have supeiimposable mirror images, like helicene, they are also optically active. Thus a beam of plane polarized light traversing such an arrangement of molecules will be rotated by the helical ordering [16]. However, the specific rotation (a) is usually much greater for helical phases than for the sum of the specific rotation(s) of the individual molecules. Thus, in this case, as the optical activity is due to the helical macrostructure of the phase, rather than due to the sum of the optical activities of the individual molecules, it is termed/orm optical activity. 

The first representative of a new type of helical structure (419), having features of both helicenes and cyclophanes, has been formed by pyrolysis of a sulphone made from the bis(bromomethyl)-quaterphenyl (420a) and the corresponding dithiol (420b). An intramolecular carbene-insertion reaction involving the helicene skeleton has been reported in which l-formyl-[6]helicene toluene-p-sulphonylhydrazone (421) is converted into (422), the structure of which has been determined unambiguously by H and n.m.r. spectroscopy. 

Helical chirality results from the fact that clockwise and counterclockwise helices are non-superimposable. Helicenes (benzologues of phenanthrene) are typical samples of helical structures. With four or more rings, steric hinderance at both ends of these molecules prevents the formation of planar conformations, and helicenes rather adopt nonplanar, but helical and enantiomeric, structures with C2 symmetry (Figure 1.28). " ... 

If additional benzo units are likewise ortho-insed to the above pentacyclic system, helical structures result which can be specified as hexa-,hepta- etc. -helicenes. Since the current rules lead to confusing variable peripheral numberings for individual members of that compound class, a new consistent numbering scheme has been proposed according to which numbering always starts at one end of the helix and proceeds sequentially to the other. 

Carbon sulfur [7]helicene (M)-(-)-40, which has a relatively low turning angle but possesses two bulky bromine groups at the ends of the inner helix, has a free energy barrier for racemization of 39.0 kcal mol-1 (half-life of 11 h at 199 °C) [85], The relatively small helical turn in-plane (based on the X-ray structures of two racemic polymorphs) is apparently offset by two bulky bromine groups at the ends of the inner helix. 

The structural features of 6 are identical to those of a macroscopic ratchet. The helical chirality of the helicene pawl generates an energy profile for... 

Helices are chiral objects often encountered in nature, for example helical shells. The absolute configuration of these objects is designated P (plus) and M (minus) for right- and left-handed helices, respectively. A number of chemical structures resort to helicity, the most famous example being the class of molecules known as helicenes. Thus, the compound shown in diagram XXXVIII is (/>)-( + )-hexahelicene [33]. 

The reaction of optically active, helicene derivative 121 first with o-phenylenediamine and then with Ni(OAc)2 led to a helical polymer (Mn 7000) (122) having a unique ladder-type structure with Schiff base moieties immersed in the main chain (Figure 15) 206 -phg polymer showed red-shifted absorptions with respect to nickel salophene, the parent compound for the polymer, supporting the formation of a long conjugation system. Intense CD bands were reported for the polymer. 

For a given class of molecules it is interesting to explore whether the presence of certain functional groups can enhance fi. We survey calculations based on the AMI semi-empirical Hamiltonian and the TDHF scheme that explore how molecular structure, here for helical molecules such as helicenes and heliphenes, influences the magnitude of fi [54, 72, 73]. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

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