Double Helicenes

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

Diphenanthro[3,4-c 3, 4 -l]chrysen (755) belongs to type A. Two optically active and one meso structures theoretically are possible. The lower-melting racemic mixture and the higher-melting meso compound were obtained experimentally. The latter is thermodynamically more stable because both helix windings lie on facing 

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 

Diphenanthro[4,3-a 3, 4 -ojpicen 140 can be regarded as the lower benzologue of hexaheliceno[3,4-c]hexahelicene 139 The thermodynamically more stable d,I-compound obtained in the synthesis has been ascertained by X-ray analysis 

140 crystallizes monoclinically in the space group P2,/c. Both halves of the molecule are arranged similar to hexahelicene, but the deviation from a perfect symmetry of both molecular halves is appreciable. This may be due to environmental differences in the crystal packing (Fig. 53b). 

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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. 

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... 

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. 

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 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). 

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. 

The cyclization of arynes can be applied to the synthesis of various polycyclic aromatics. Hexabenzo[a,c,g,/,w,o]triphenylene (260) was obtained as a single product in 39 % yield from 9,10-didehydrophenanthrene generated from 259 [80]. Cyclization of 3,4-didehydrophenanthrene 261 gave rise to the polycyclic compound 262, which has a double helicene structure, in 26 % yield [81]. 

Racemate and mesomer forms of the interesting elaborated thiophene-based naphthalene-cored double helicenes 18 have been synthesized using a one-pot photocycUzation of l,l,2,2-tetrakis(dithieno[2,3-l 3 2 -[Pg.9]

A more elaborated example using the same methodology is shown in Scheme 16 in which the simultaneous formation of two ortho-condensed arene rings has been realized. In this case the yield of the double helicene 38 from the precursor 39 is significantly higher than in the previous example (20100L456). 

Tetra(3-thienyl)ethene (30) in the presence of iodine upon 313 nm irradiation led to (31) as the main product, while 350 nm irradiation afforded the tetrathienonapththalene (32) in 20% yield/ Irradiation of (33) in the presence of iodine in henzene afforded the racemate (34) and the meso-isomer (35) of the thiophene-hased naphthalene-cored double helicenes/ The enantiomers (34) were isolated hy using a semipreparative scale chiral column. The oxidative photocyclization of (36) in a batch photoreactor gave the aza[6]helicene (37) in 19% yield, while in a flow-micro-photoreactor the yield was up to 55%/ ... 

Pena et al. reported the synthesis of double helicenes by the palladium-catalyzed complete intermolecular homo-[2- -2-1-2] cycloaddition of 3,4-didehydrophenanthrene (Scheme 10.12) [13a]. 1,2-Didehydrotriphenylene could also be employed for this reaction to give the corresponding triphenylene-based double helicene, although the product yield was low (Scheme 10.12) [13b]. 

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