Helicene, fused

Sooksimuang T, Mandal BK (2003) [5]Helicene-fused phthalocyanine derivatives. New members of the phthalocyanine family. J Org Chem 68(2) 652-655... 

The logical development of the linear fusion of odd rings is described in [73], That report presents both the IUPAC nomenclature for molecules formed by the fusion of tricyclo through heptacyclo pentane modules and a binary code that simplifies the description of the complete set of fused tricyclo through hexacyclo pentane aggregations. The latter examples in Table 3 of Chapter 2, shall focus on cyclopentane as the module of interest. In a similar manner, the extrapolation of linear chains of cyclopentane modules to form the counterpart of helicenes, henceforth referred to as "helicanes", is described in [73]. 

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. 

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). 

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). 

Benzo-fused pyridinium systems also undergo the reaction and a new example, the photocyclisation of (153) to (154) in the presence of iodine, has been published. The quantum efficiency of photocyclisation of l,2-jbis-(a-naphthyl)ethylene to dihydropicene has been examined in micelles and the cyclisation of [5]-helicene (155) to dihydrobenzperylene (156) has been carried out at 4K (156) was characterised by its fluorescence and phosphorescence emission. The effect of different substituents on the photocyclisation of the symmetrically tetra-substituted stilbene (157) has been probed. 

Helicenes are extended fused polyaromatic hydrocarbons that have a helical or screw-shaped structure. 

Wynberg and co-workers have prepared helicenes composed of fused thiophene and benzene units from the photocyclization of alkene precursors [134], It was shown that the effect of substituting thiophenes for benzene rings in these structures led to a blue shift of the absorption maxima. Compound 61 can be converted to the circulene structure 62 in two steps and the latter compound represents another interesting class of fused thiophene stmctures [135]. Trithiophenes with a benzene core (63) can be prepared from diacetylene-fimctionalized bithiophenes via a five-coordinate rhodium(I) intermediate by the addition of elemental sulfur [136]. A related structure (64) was reported by Pei et al. [137], along with the elaborate helicene 65. The compounds were constracted via oxidative (FeCb) cyclization reactions of 1,2-dithienyl benzene derivatives. 

Photochemical trans-cis isomerization of diarylethenes followed by photochemical 6ic electrocyclization and oxidation is a standard protocol, and this protocol is widely applied in the synthesis of angularly fused aromatic compounds [4]. In particular, this strategy is widely used in the synthesis of helicenes, an important class of angularly fused polycyclic aromatic compounds (Scheme 16.11) [14]. 

Very recently, the enantioselective synthesis of a completely orfho-fused [ll]helicene-like molecule has been achieved via the rhodium-mediated intramolecular double [2+2+2] cycloadditions (Scheme 21.30) [34]. 

SCHEME 21.30 Enantioselective synthesis of completely ortAo-fused [ll]helicene-like molecule. 

The helicenes are a subclass of phenes consisting of benzenoid rings fused in a helical arrangement. These are informally named by a numerical prefix followed by helicene. 

Diene macrocycle 53 had, in fact, been previously synthesized in 1977 by Wen-nerstrom and Thulin via a Wittig reaction [23, 24]. It was subsequently irradiated in the presence of iodine to give the bi-4,5-phenanthrene cyclophane 57 (Scheme 6.9). Oxidation of cyclophane 57 by heating to 140 °C in a melt composed of NaCl and AICI3 provided the fused helicene 58 [23]. Attempts to further oxidize 58 to the fully closed derivative, however, were not successful. 

Helicenes Helicenes are fascinating molecules displaying a broad range of properties. The cobalt-catalyzed [2 + 2 - - 2] cycloaddition allows the construction of such angularly fused polycyclic compounds [68]. Stara et al. reported various applications, using CpCo(CO)2 or CpCo(C2H4)2 as a catalyst, like the one depicted in Scheme 1.35 [68b]. This topic is covered in detail in Chapter 10. 

Helicenes and helicene-like molecules possessing nonplanar ortho-fused scaffolds exhibit helical chirality. Enantiopure forms of these molecules are potentially applicable to optical or electronic functional materials and chiral reagents [1], Therefore, development of their practical enantioselective synthesis, which can introduce various substituents on their frameworks, is highly attractive. The classical nonasymmetric synthesis of helicenes is based on an oxidative photocyclization of stilbene-type precursors [2]. However, this method suffers from low product yields and low substrate concentrations. Thus, several nonphotochemical synthetic methods have been developed which have been applied to the synthesis of enantioenriched helicenes [3]. Among them, a [2 - - 2 - - 2] cycloaddition strategy is highly attractive because of its convenient operation and its applicability to the catalytic enantioselective synthesis. The synthesis of helicenes and helicene-like molecules by transition-metal-mediated [2 -I- 2 -I- 2] cycloaddition reactions, including enantioselective variants, is summarized in this section. 

Cataiytic transformation has also been accomplished. Kawaguchi et al. reported the palladium-catalyzed intramolecular 0-arylation of 2-[2-(pseudo)haloaryl]phenols [14]. 2-(2-Chlorophenyl)phenol (68) was converted to dibenzofuran (69) successfully in the presence of palladium(II) acetate/phosphine ligand, but the authors did not explore the scope of this reaction (Scheme 23.26). In addition, they accomplished the synthesis of fused polycyclic compounds such as dibenzo[(i,fi ]benzo[l,2-fe 4,5-fc jdifurans 70 [14] and oxa[7]helicene 71 [17] using this methodology (Scheme... 

The material in this chapter has been organised into ring-type and functional-group order so that required information may be rapidly obtained. Monocyclic systems and their alkyl derivatives are reviewed first, followed by related compounds, in order of their functional groups. This is followed by polycyclic systems (non-fused and fused). Derivatives of these are covered under the appropriate parent compound heading, appearing in the same order as that in the earlier sections. Later sections include more specialised systems such as helicenes and cyclophanes. Synthesis and reactivity are discussed collectively under the appropriate system heading. 

Helicens are polycyclic aromatic hydrocarbons with with nonplanar helical frames formed from orf/to-fused benzene or other aromatic rings [87]. The helicens and helicene-like molecules have for a long time attracted... 

Zhang, Y., Petersen, J.L. and Wang, K.K. (2007) Synthesis and structures of diindeno-fused l,12-diphenylbenzo[c]phenanthrene and l,14-diphenyl[5]helicene bearing severe helical twists. Organic Letters, 9(6), 1025-1028. 

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