15-Ring helicene

The triplet quantum yield tplsc is for the helicenes nearly (1 — radiationless decay approximates klsc. The low value of kfl shows that the fluorescence of all the compounds occurs from the Lb state. The fluorescence rate (kfj of the helicenes with an odd number of benzene rings decreases more slowly than of the helicenes with an even number of rings. This is explained by the difference in molecular rigidity of the two groups. The even ring helicenes are symmetric about a naph-... 

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... 

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. 

That chiral molecules can be produced in a CPL field, either from achiral precursors by photo-activated synthesis or by preferential chiral photodestruction of a racemic mixture, is now well demonstrated and has been reviewed. [46] In all cases currently known, however, such processes have proved very inefficient. For example, asymmetric photochemical ring-closures of achiral helicene precursors induced by CPL have produced only about 0.2% e.e. in the products. Likewise, the CPL-induced photolysis of racemic camphor produced about 20 % e.e., but only after 99% photodestruction, and photolysis of D.L-glutamic acid produced only 0.22 % e.e. after 52 % photodecomposition. [71]... 

In general, only helicenes with six or more benzene or other rings will be treated. The lower homologs, dibenzo[c,g]phenanthrene (pentahelicene) and benzo[c]phen-anthrene will only be mentioned for comparison in some instances. 

The benzene rings in the helicenes are indexed by letters as shown. Sometimes helicenes are indicated by short nomenclature symbols, via the number of the con-... 

Ring closure of arylolefins into helicenes can be effected by irradiation of a dilute solution. In principle all solvents can be used that do not absorb at the wavelength of irradiation and do not react with the helicene precursor or the oxidizing agent. Methanol, hexane, cyclohexane, benzene are usual solvents. 

To examine the influence of annelated benzene rings on the helicity of helicenes several benzohexa- and benzoheptahelicenes have been synthesized. 

Another helicene containing a five membered ring (61) was formed from 5,6-diketo-hexahelicene as will be described in Section 8. Treatment of 61 with Li gives rise to the aromatic helicene 62 43). 

In general, the photosynthesis of heterohelicenes is similar to that of carbohelicenes. Because the nature, the number and the positions of the heterocyclic rings in a helicene can vary, the diversity of heterohelicenes is much larger than that of carbohelicenes. More than 30 different thiahexahelicenes are possible, but only a limited number is synthesized until now. Reports on heterohelicenes containing pyridine, pyrole and furane rings are even very few. 

The second mechanism seems highly unlikely as the ring opening of DHP must be much slower than the oxidation into helicenes with iodine 66). Moreover, the wavelength-dependence of the optical yield of [8]-heIicene does not correlate with that expected from the proposed CD-spectrum of the intermediate DHP62,65,67). This mechanism can also not explain the difference in optical yield of [6] from (1+4) and (2 + 3). 

Laarhoven et al. 711 have studied the influence of chiral solvents on the optical yield of [6]helicene, synthesized by photodehydi ocyclization of various precursors. Most thoroughly investigated was the photochemical ring closure of 2-styrylbenzo[c]-phenanthrene in optically active fluids (Table 5). 

From Table 8 it is obvious that the resolution always increases with an increase of the number of benzene rings and that riboflavine is a more powerful selector than the nucleotides, but not as good as TAPA. An interesting experiment shows that it is not always necessary to have the selector coated or bound to the solid phase but that it can sometimes be used as well, dissolved in the mobile phase. The n-dodecyl ester of N-(2,4-dinitrophenyl)-L-alanine is able to discriminate between the enantiomers of l-aza-[6]-helicene, when used as a chiral dopant in the mobile phase in HPLC on a reversed phase column 93) (see Table 9). The usefulness of this dopant must be due to the known ability of a dinitrophenyl moiety to form CT-complexes with polycyclic aromatic hydrocarbons the presence of a chiral site near this group causes resolution of helicenes, because the steric interactions in diastereomeric complexes will be quite different. 

For thiahelicenes, all having alternating thiophene and benzene rings, this regular course is not observed. This may be an indication that these helicenes are constructed... 

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. 

H NMR-spectroscopy has appeared a powerful method for the recognition of helicenes and for an analysis of their conformations in solution. This is especially due to the appearance of quasi first order spectra for the protons in the terminal rings, even at 90 MHz. 

By aromatization of the five-membered ring with Li the helicene 62 is obtained. In this compound the ring current in ring B is restored, but the distance between the terminal rings is enlarged compared to hexahelicene. By an accurate analysis of the shifts and comparison with the 8-values of the fluorenyl-anion the shifts of all protons except that of H(16) could be ascribed to the effects of i, ii and iii. The large downfield value of H(16) must be ascribed to the neighbourhood of the Li+-ion in the contact ionpair. 

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. 

Photoelectron spectra (PES) of helicenes have been measured to trace whether interactions between the orbitals of overlapping benzene rings occur. Evidence for such interactions has been found for cyclophanes 118, ll9) bands from symmetry-equivalent n-orbitals of the benzene rings in these compounds were split due to transannular jt-Jt-overlap. 

In Fig. 7 a plot of the wavelengths of a-, p- and (1-bands against the number of rings is given for carbo- and alternating thiahelicenes, together with some values for substituted helicenes. It shows the increase of A.max for all the bands. 

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,). 

The protons most strongly affected in hexahelicene are H(l) and H(8) suggesting that these protons are most strongly exposed to the ring current of the acceptor. A more detailed study using 2,15-dimethoxyhexahelicene (85) as the donor and R-TAPA as acceptor demonstrates that two diastereomeric complexes were present both in equilibration with the respective enantiomers of the helicene. The association... 

The difference between Es and ET is about constant for the helicenes with five to nine rings (Table 22), while the kisc increase by a factor of about 3 I25). This may suggest the participation of a quasi degenerate triplet state in the intersystem crossing. This most likely will be the 3Lb state since a direct spin-orbit coupling between Lb and 3Lb is allowed in the helicenes because of their C2-symmetry. It is proposed 125) that the increase in kisc reflects the departure from coplanarity when the amount of benzene rings increases. 

Differences between the geometrical structures of helicenes concern mainly the angles between the least square planes of subsequent benzene rings and the torsion angles of the bonds of the inner helix. In Table 24 some X-ray data are given. For heptahelicene two different crystals have been obtained. One of the crystals gave two... 

From the regularities mentioned here de Rango et al.I45) designed two general models for helicenes, viz. the triple helical model and the stair case model. In the triple helical model all C-atoms are located on one of three helices an inner helix with (n +1) C-atoms (n = number of benzene rings), a medium helix with (n +1) C-atoms and an outer helix with 2n C-atoms. In general, atoms of helicenes coincide very well with the best helices, obtained by computation from crystallographic data. 

Some aspects of the chemistry of helicenes require still more attention. Since the interpretation of the mass spectrum of hexahelicene by Dougherty 159) no further systematic work has been done on the mass spectroscopy of helicenes, to verify the concept of an intramolecular Diels-Alder reaction in the molecular ion. Though the optical rotation of a number of helicenes is known and the regular increase of the optical rotation with increasing number of benzene rings has been shown, the dependence of the rotation on the helicity is still unknown. The asymmetric induction in the synthesis of helicenes by chiral solvents, or in liquid crystals, though small, deserves still more attention because application to other organic compounds will be promoted when the explanation of observed effects is more improved. 

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