Chiral helicene based

As mentioned previously, the helicene-based chlorophosphite M,M,R) 25a, which is easily prepared from the reaction ofenantiopure (M,M,R)-[5 -HELOL 24a with phosphorus trichloride, has been successfirUy used to analyze the enantiomeric compositions of a variety of chiral materials such as alcohols, amines, or carboxylic acids. The reaction is conducted in an NMR tube where the analytes are added to the chiral helicene-based chlorophosphite in CDCI3. After 2 h reaction, the diastereomers ratios generated in situ are determined by P-NMR spectroscopy. Notably, this attractive method has proved to be highly efficient for analyzing the enantiomeric purities of a wide range of molecules whose stereogenic centers are far away from functional groups that react with phosphorus atom. The authors... 

Fig. 17 Examples of intrinsically chiral molecules that associate into dimeric structures A clip racemic molecule provides only heterochiral dimers, while B helicene-based molecules give only homochiral dimers...

Interesting nonlinear optical properties were shown by chiral helicenes (5) forming LB films with larger second-order susceptibility than corresponding racemic structures. Nanotubes based on cyclic peptides were reported to form membranes with ion-selective channels. 

Before synthetic chiral stationary phases were developed, attempts were made to use naturally occurring chiral materials for the stationary phase. Quartz, wool, lactose and starch were inadequate but triacetylated cellulose has met with some success. The synthetic stationary phases introduced by Pirkle are able to interact with solute enantiomers in three ways, one of which is stereochemically dependent. Typically these interactions are based on hydrogen bonding, charge transfer (rc-donoi -acceptor based) and steric repulsive types. An independent chiral stationary phase therefore consists of chiral molecules each with three sites of interaction bound to a silica (or other) support. Early work in this area demonstrated that 5-arginine bound to Sephadex would resolve 3,4-dihydroxy-phenylalanine, and that direct resolution of chiral helicenes could be accomplished with columns packed with 2-(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)-propionamide or tri-P-naphthol-diphosphate amide. Amino acid esters have also been resolved with a silica bound chiral binaphthyl crown ether, but better separations are achieved with A-acylated amino acid derivatives with amino-acid derived chiral stationary phases. 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

An interesting example of the transfer of center chirality to helicity is the work by Ogawa et al., based on an asymmetric aromatic oxy-Cope rearrangement to provide nonracemic [5]helicenes (Fig. 15.8) [75]. The starting material with center chirality, bicyclo[2,2,2]ketone (-)-21 (>98% ee), was obtained by enzymatic resolution. In the annelation step, the phenanthrene derivative was subjected to aromatic oxy-Cope rearrangement, to afford a pentacyclic product in 47 % yield. The corresponding [5]helicene 22 was obtained in 7 % overall yield (> 98 % ee) after six steps. 

Two approaches to nonracemic [5]helicenes, starting from axially chiral binaphthyls, provided an early stereochemical correlation between axial and helical chiralities. The correlation was based on comparison of the signs of optical rotations the absolute configurations of binaphthyls were obtained from chemical... 

From electronic back to optical properties, in the final Chapter 15, Rajca and Miyasaka provide an overview of the synthesis of molecules with highly anne-lated, chiral, 3t-conjugated systems, primarily of the helicene-type. Tremendous progress has been made in the synthesis of helicenes however, the corresponding helical, ladder-type polymers remain a significant and attractive challenge. In addition to the synthesis, the configurational stability (barriers for racemization) and chirooptical properties are described and discussed in depth in this chapter. In this context, perspectives for isotropic materials with molecule-based chirooptical properties are outlined. 

Providing phthalocyanines with chiral bulky heli-cenes at the periphery (29) also results in helical aggregates.165 Aggregation in chloroform occurs upon addition of ethanol as was observed with UV—Vis spectroscopy, while CD spectroscopy revealed the helical nature of the columnar aggregates. Calculations have indicated that two phthalocyanines need to be rotated to allow a favorable intermolecular distance of 3.4 A, because of the bulky helicenes. As such, phthalocyanines provided with racemic helicenes cannot stack in such a defined manner, because of the steric hindrance of the racemic side groups. Aggregation of a smaller analogue of 29, a triphen-ylene-based porphyrazine, has been shown to occur as well.166... 

M. Miyasaka, A. Rajca, M. Pink and S. Rajca, Chiral molecular glass synthesis and characterization of enan-tiomerically pure thiophene-based [7]helicene, Chem. Eur. J., 10, 6531-6539 (2004). 

More recently, a polymer based on the helicene motif has been prepared. The key step in the s)mthesis of a helical polymer based upon a helicene is the condensation of a chiral [6]helicene that has salicylaldehyde functionality at each end with 1,2-phenylenediamine in the presence of a Ni salt. This gives the chemical structure shown to the right (bonds enormously stretched for clarity of presentation). The ORD spectra of structures of this kind display extraordinarily large rotations, and the circular dichroism... 

Binaphthyl based structures are not the only chiral groups that have been incorporated into the backbone of chiral macrocydes. Recently, there has been an increasing number of reports of macrocydes that are chiral due to the incorporation of a helicene moiety [62]. One of the earhest stmctures to be reported was from Wennerstrom and coworkers [63] and linked directly two [5]hehcenes to give macrocycle 169, which came to be known as propdhcene. 

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. 

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