Helicene, double

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. 

The interesting hydrocarbon (104), having both aromatic and anti-aromatic character, could be isolated in low yield by treatment of a bis-ylide with freshly distilled glyoxal.96 Precursors for the preparation of helicenes, e.g. (105), have been synthesized in one-step reactions. The yield of the double Wittig reaction is better using a di-aldehyde rather than a bisphosphonium salt.97 The [12]-annulene (106) is obtained from the reaction of the bis-ylide derived from (107) and a di-aldehyde.98 The condensation of (107) with two moles of 3-methylpent-2-en-4-yn-l-al (108) was also successful.99... 

Table 2. Synthesis of Benzo- and Double Helicenes from Aryl Olefinic Precursors...

The double helicenes 52, 54 and 56 have been synthesized because they may occur in a racemic and meso form and to study possible isomerization between stereoisomers. 

By chromatography on A1203 the racemic mixture and the meso form could be separated. The racemic form melted at about 320°, resolidified and melted again at the melting point of the meso form 35,37). From the other double helicenes only one isomer could be isolated. 

Perhaps the most intriguing observation of helicenes is the unexpected ease with which these compounds racemize thermally. According to Martin 1031 three pathways for the thermal racemization can be considered 1) via bond breaking 2) via an internal double Diels-Alder adduct, and 3) via a direct inversion. Martin rejected the first possibility because it is not in accordance with kinetic data for the racemization. He could exclude the second possibility in an elegant way by using appropriately substituted derivativesl03>. 

Another demonstration of the remarkable flexibility of helicenes is found with the double helicene 55. Dissolved in naphthalene it racemizes at 210 °C (more than 230 °C below its melting point) with about the same rate as hexahelicene at this temperature. From this observation it must be concluded that the racemization does not occur via the meso-form which has the terminal rings at one side of the central ring, but, due to the large number of bonds, via a high-vibrational state in which the terminal rings are at a non-hindering distance. Not only the racemizations mentioned in Table 11, but also the thermal racemization of [10] and [11] helicenes has been carried out successfully. 

Table 18. Chemical Shift (in PPM) of Some Protons of Double Helicenes and Bis-2,2 -Hexahelicyl... 

From the NMR-data for dl 33 and meso 33 it can be concluded that in the racemic form both hexahelicenyl moieties are almost perpendicular, whereas in the mesoform the central biphenyl moiety is planar 25). The three double helicenes 54, 55 and 56 could only be isolated in one form. For 55 this was the racemic form having the terminal rings at opposite sides of the central part, as could be deduced from the isolation of optically active samples. From racemization experiments (vide infra) it could be deduced that the mesoform can hardly exist. Comparison of the NMR spectra of 54 and 56 with that of 55 suggests that 54 and 56 are also racemic forms. The chemical shift of H(16) and H(17) in 56 has the lowest value found for protons in carbohelicenes. 

A substituent at C(l) of hexahelicene 22) and heptahelicene 20) results in a batho-chromic shift of the a- and p-band and a loss of fine structure. A bridge over the terminal rings of heptahelicene (57) has the same effect. The spectra of the double helicenes are more structured than the carbohelicenes with the same number of benzene rings (see Fig. 6,). 

Double Helicenes and Chiral Polycyclic Aromatic Hydrocarbons... 

Double and triple helicenes are Jt-conjugated molecules, which consist of two or three annelated helicene-like subunits. Such structures may also be considered as examples of chiral polycyclic aromatic hydrocarbons (PAHs). 

The first double helicenes, consisting of head-to-tail annelated [6]helicenes, were prepared by oxidative photocyclization of stilbenes to helicenes [21]. Recently, double helicene 48, in which two benzene rings are shared by [5]- and [7]helicene, was prepared via nonphotochemical Pd-catalyzed cydotrimerization of 3,4-didehydrophenanthrene derived from 47 (Fig. 15.17) [88]. The H NMR spectrum for racemic 48 showed better agreement with the calculated spectrum for the diastereomer with homochiral versus heterochiral helicenes. The characterization of double helicenes did not indude X-ray crystallography [21, 88],... 

Recently, a conjoined double helicene, in which two hydrazine-based [5]heli-cenes are highly annelated in their mid-sections, was reported [89]. The conjoined double helicene 51-D2 possesses two homochiral hydrazine-based [5]helicene-like fragments annelated in their mid-sections. The synthesis of conjoined double he-licene 51-D2 from 49 consists of two annelation steps (1) Friedel-Crafts diannela-tion to provide pentacyclic diamine 50 and (2) one-step oxidative coupling forming one CC and two NN bonds [90], to give dodecacydic structure 51-D2 (Fig. 15.18). 

Fig. 15.17 Palladium-catalyzed cyclotrimerizations of didehy-drophenanthrene leading to double helicenes.

Fig. 15.18 Synthesis and X-ray structure of conjoined double helicene 51-D2. Each of the two homochiral [5]helicene-like fragments is shown in stick-and-ball format.

Conjoined double helicene 51-D2 was obtained only as a racemate. Its barrier for racemization is expected to be significantly greater than the barrier of... 

D. Pena, A. Cobas, D. Perez, E. Guitian, L. Castedo, Dibenzo[ ,o]phenanfhro[3,4-sjpycene, a Configurationally Stable Double Helicene Synthesis and Determination of Its Conformation by NMR and GIAO Calculations, Org. Lett. 2003, 5, 1863-1866. 

K. Shiraishi, A. Rajca, M. Pink, S. Rajca, i-Cori jugated Conjoined Double Helicene via a Sequence of Three Oxidative CC- and NN-Homocouplings, J. Am. Chem. Soc. 2005, 127, 9312-9313. 

Figure 3 shows the forms of all circulenes with h < 9 as dualists. All of them are planar (non-helicenic) monocirculenes. They include the unique coronoid at h = 8 and the five coronoids at h = 9 (cf Table 2). The dicirculenes with h < 12 are depicted in Fig. 4. Also these systems are planar, but contain no double coronoid.

Ogawa, Y., Ueno, T., Karikomi, M., Seki, K., Haga, K., Uyehara, T. Synthesis of 2-acetoxy[5]helicene by sequential double aromatic oxy-Cope rearrangement. Tetrahedron Lett. 2002, 43, 7827-7829. 

The lUPAC name of hexahelicene is phenanthro[3,4 d]phenanthrene In a simplified nomenclature for the helicenes introduced by M. S. Newman, the number of the annellated aromatic rings is set in brackets before the helicene name hexahelicene = t6]helicene. Today, according to the building blocks composing the helicene, one differentiates carbohelicenes, heterohelicenes, double helicenes, bihelicenyls, metallohelicenes and cyclophanohelicenes. The names dehydrohelicene and circulene are used for some related compounds (v.i.). 

The double helicenes can be devided into three groups according to their annellation pattern. 

Hexaheliceno[3,4-c]hexahelicene (J39) is an example of type B of the double helicenes. Theoretically one meso and two optically active molecules are expected here. In the synthesis only one singlet isomer, most probably the racemic compound, is formed exclusively. The racemic mixture should be energetically favoured, because the terminal aromatic rings are situated on different sides of the molecule, with respect to the central naphthaline unit... 

The successful synthesis of [13]helicene (105) may be regarded as a token of the tremendous progress made in recent helicene chemistry. From the standpoint of symmetry, however, this exquisite three-layered molecule shares the same C2 symmetry with [6] helicene, the simplest among this class of compounds. Fusing two C2 [n] helicenes (104) will generate double helicenes either of Cj symmetry 105 or of C2 symmetry 106, depending on the way of combination of the enantiomers. The preparation of several double helicenes of Cj and C2 symmetry has been reported (106). 

Scheme 46. Efficient synthesis of [6]helicenes (91) by double Diels-Alder reaction [146]...

The synthesis of HBT (73), which contains three [5]helicene units, illustrates the power of the cyclotrimerization of polycyclic arynes for the synthesis of he-licenes. More examples are shown in Table 2. Again, Pd2(dba)3 is the catalyst of choice for trimerization of the asymmetric arynes 77-79, which are generated from the corresponding o-(trimethylsilyl)aryl triflates 74-76. In the reactions of 1,2-didehydronaphthalene (77) and 1,2-didehydrophenanthrene (78), mixtures of regioisomers are obtained, whereas 84 is the only isomer isolated from the cyclotrimerization of 79. Compounds 80 and 82 contain a [5]helicene unit, while compound 84 is the first example of a double helicene formed by a pen-tahelicene and a heptahelicene with two rings in common. 

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