And helicenes

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

DeSt99a] M. Deza and M. I. Shtogrin, Primitive polycycles and helicenes, Uspechi Mat Nauk. 54-6 (1999) 159-160 (in Russian). English translation in Russian Mathematical Surveys 54-6,1238-1239. 

The extraordinarily strong chiral properties of [nfhelicenes provide an impetus for the development of synthetic approaches to nonracemic [nfhelicenes for applications as organic materials. From this point of view, asymmetric syntheses of functionalized long [n]helicenes (n > 7), and also [n]helicene-like molecules and polymers with novel electronic structures and material properties, are important. The properties of helicenes related to materials are relatively unexplored, compared with the more synthetically accessible n-conjugated molecules and polymers. Notably, redox states of helicenes are practically unknown [33, 34]. Assembly of helicenes on surfaces, their uses as liquid crystals, chiral sensors, ligands or additives for asymmetric synthesis and helicene-biomolecule interactions are in the exploratory stages [35-43],... 

The chiroptical properties of a hybrid of paracyclophane and helicene, [2.2] paracyclophanohexahelicene (38), were reported [50]. The introduction of [2.2] paracyclophane moiety caused a red shift and intensity enhancement of the absorption in the 240-280 nm regions, which was also reflected in the CD spectrum. It is noteworthy that the (M)-helicity was evoked in the hybrid 38 when started from (/ p)-[2.2]paracyclophanecarbaldehyde. 

Section 6 deals with the enumeration of catacondensed simply connected polyhexes, i.e. catacondensed benzenoids and helicenes. 

In the present section the catafusenes (catacondensed simply connected poly hexes cf. Sect. 5.1) are treated. However, in contrast to the Harary-Read numbers (first column of Table 7) we shall be interested in the numbers of unbranched and branched systems separately. The numbers of unbranched catafusenes (Table 10) are known from algebraic formulas (cf. Sect. 5.2), but now we are interested in the unbranched catacondensed benzenoids and helicenes separately. Likewise we shall treat the numbers of branched catacondensed benzenoids and helicenes separately. 

Table 16. Numbers of branched catafusenes benzenoids and helicenes...

For fibonacenes, l = 2. Their numbers split into the benzenoid and helicenic systems we write... 

Table 21. Numbers of fibonacenes benzenoids and helicenes (in parentheses)...

Kelly, De Silva, and Silva [32] reported a unidirectional rotary motor (1, Fig. 2) composed of a triptycene bearing an amine group on one blade, and of a helicene having an alkyl chain with a free alcohol at one end. The triptycene and helicene are... 

Twistoflex and helicene (Section 17.5) are two more aromatic compounds whose shape makes them chiral. 

The spin polarization transfer results demonstrate unequivocally that rotation occurs equally in both directions. That is, 37b does not behave as a ratchet. (More subtle issues, such as how we distinguish between uni/bidirectional rotation and helicene racemization, and why it is not necessary to resolve racemic 37b are addressed in the full paper [39b].)... 

Scheme 9 provides (using 47 for purposes of illustration) molecular detail for the concepts outlined in Fig. 21. In essence, the proof of principle for the molecular motor starts with 47a. Compound 47a is one of three low-energy rotational isomers (rotamers) about the axle connecting the triptycene and helicene components (47b is a second low-energy rotamer). Rotamer 47a is activated by reaction with phosgene to give the isocyanate 49. Isocyanate 49 is chemically armed to react with the OH group in the hydroxypropyl tether attached to the helicene, but, in the rotational ground state 49, the isocyanate...

The stereospeeifie Claisen transformation and the enantio-selective deearboxylation of ethylmalonic acid were found in the presence of cholesterylbenzoate and the formation of (S)-(-)-[Cr(ox)3] and (-)-helicene in photoreactions were reported on the (-)-rotating liquid-crystalline Tween-80, which is a polyoxyethylene-sorbitane-monooleate (n = 20)... 

Fig. 2 Sequence of events involved in the chemically powered unidirectional rotation of a compound made of triptycene and helicene components. (View this art in color at www.dekker.com.)...

Isomer Enumeration of Catafusenes, C4 +2H2n+4 Benzenoid and Helicenic Hydrocarbons. 

Synthesis of t-expanded coumarins (14CC9105), unsymmetrical sulfo-xanthenes (14CEJ8330), caged rhodamines with a 2-diazo-l-indanone unit (14CEJ13162) with fluorescence properties, and the synthesis, chemical stability, optical properties and representative applications of the cationic trian-gulenes and helicene dyes (14CSR2824) were studied. 

Let us now consider several possible isomers of the linear polyacene angular polyacenes and helicenes (see Fig. 3.13). In Table 3.12 the results obtained for the C22H14-C42H24 systems (containing 5-10 benzene rings) are presented for structures corresponding to systems A, B, and G for different numbers, n, of the benzene rings in the chain. 

The intramoleeular [2+2+2] cycloaddition of triynes is particularly effective for the synthesis of helicenes and helicene-like molecules. For example, the novel direct synthesis of fully... 

The cyclotrimerization can also be used to form unusual and challenging aromatic systems, including biaryls and helicenes (Scheme 11.22)." ° The triyne substrate 11.63 was constructed by a Sonogashira reaction of... 

Hoffan N (2014) Photochemical reactions applied to the synthesis of helicenes and helicene-like compounds. J Photochem C Phochem Rev 19 1-10... 

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-... 

Figure 3.2. Examples of toffl-condensed PAHs phenes and helicenes ...

Table 4.1. Comparison of the helix parameters of heliphenes and helicenes (calculated values given in parentheses).

A large number of CD spectra has been investigated recently by DFT/SCI treatments, including camphor derivatives, phenanthrene, and helicenes. Up to 25 excited states are sometimes required to predict such a spectrum and the agreement between computed and measured spectra is very... 

Combinatorial aspects for stereochemical features that do not fit the centron model of stereochemistry are also poorly understood at present. They include features like In-Out isomerism of large bicyclic systems, knotted structures, and helicenes. In some sense, they go beyond the local model to a more global property of the molecule. 

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. 

The most ideal synthesis of helicenes and helicene-like molecules is apparently catalytic enantioselective synthesis. In 1999, Stard, Stary, and co-workers developed the catalytic enantioselective synthesis of tetrahydro[6]helicene (R = H) by the Ni(cod)2/(5)-MOP complex-catalyzed complete intramolecular [2 - - 2 - - 2] cycloaddition of a triyne (Scheme 10.18) [5a]. A functionalized tetrahydro[6]helicene (R = OMe) was also synthesized using (5)-BOP as a ligand (Scheme 10.18) [5b]. Although both the product yields and ee values were moderate, this method enabled the one-step synthesis of the enantioenriched helicene precursors. 

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). " ... 

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