Cyclophane, planar-chiral

For [2.2]paracyclophane-4-carboxylic acid (25) as (—)(R) This result has been mentioned in a footnote in Ref. 1011 but seems never to have been published (see also Ref. 61). The chirality of this acid was correlated via its ( )-aldehyde with a levo-rotatory hexahelicene derivative which, according to the paracyclophane moiety at the terminal, had to adopt (A/)-helicity. Its chiroptical properties are comparable to those of hexahelicene itself101. For the (—)-bromoderivative of the latter the (A/)-helicity was established by the Bijvoet-method 102). In a later study, (—)para-cyclophane-hexahelicene prepared from (—)-l,4-dimethylhexahelicene with known chirality (which in turn was obtained with approximately 12% enantiomeric purity by asymmetric chromatography) confirmed these results. It should be mentioned that [2.2]paracyclophane-4-carboxylic acid (25) was the first planar chiral cyclophane whose chirality was determined 1041 (see also Ref.54 ). The results justmentioned confirmed the assignment (+)( ). 

For chemical correlations of planar chiral cyclophanes with centrochiral derivatives for the purpose of applying Horeau s method IOO), see the following section. 

Voegtle, F. and Pawlitzki, G. (2002) Cyclophanes from planar chirality and helicity to cyclochirality, in Takemura, H. (ed.), Cyclophane Chemistry for the 21st Century, Research Signpost, Trivandrum, India, pp. 55-90. 

Recent Theoretical and Experimental Advances in the Electronic Circular Dichroisms of Planar Chiral Cyclophanes... 

Keywords Chiroptical properties, Electronic circular dichroisms, Planar chirality, Quantum chemical calculations, Cyclophane... 

Apart from the role of cyclophanes as a model system for studying the electronic interaction between the aromatic moieties, chiral [2.2]paracyclophanes have also been utilized as planar chiral ligands in asymmetric catalysis. Recent advances and applications in this area have been reviewed [5, 6]. The synthesis of heterocyclic compounds based on [2.2]paracyclophane architecture, where the long-distance electronic communication and the planar chirality play significant roles in their application, has also been reported recently [7]. Although the preparation and application of chiral cyclophanes in asymmetric synthesis has attracted much attention for a long time, their chiroptical properties, especially the CD spectra, have rarely been paid attention or even completely ignored. 

When a 1 1 mixture of the solid and the liquid diastereomers of bridged [10] cyclophane nicotinamides 8 was heated, the solid (5p,3, S )-isomer became predominant over the liquid (f p,3 S)-isomer through the fast dynamic (rope-jumping) racemization and the subsequent spontaneous resolution via crystallization [23]. The CD spectra of these cyclophanes exhibit either two positive or two negative Cotton effects at 220 nm and 280 nm, primarily due to the planar chirality (Fig. 2). 

A planar chiral naphthalenediimide cyclophane (39) and its derivatives were prepared for their tunable intramolecular FRET properties. The enantiomeric enrichment of cyclophane 39 was accomplished by chiral HPLC (on a Daicel IA column) and the CD spectra of enantiomeric 39 were reported (Fig. 9) [51]. 

The CD spectral investigation of optically active bi[10]paracyclophanes (47), in which two planar and one axial chirality elements are incorporated, was reported in the literature [57]. The experimental CD spectrum of enantiomerically pure (RV,SP)-47, which was derived from the corresponding diastereomeric tetra-(S )-camphanoate, was found to be in good agreement with that calculated for the (aA)-isomer, but approximately the mirror image of the spectrum computed for the (aSHsomer (Fig. 11). Thus, the CD spectrum can be interpreted mostly in terms of the axial chirality, indicating that the effects from the planar chirality of cyclophane units were more or less cancelled out. 

Ishida Y, Iwasa E, Matsuoka Y, Miyauchi H, Saigo K (2009) An enantiopure cyclophane-type imidazole with no central but planar chirality. Chem Commun 3401-3403... 

KanomataN, Ochiai Y (2001) Stereocontrol of molecular jump-rope crystallization-induced asymmetric transformation of planar-chiral cyclophanes. Tetrahedron Lett 42 1045-1048... 

Gabutti S, Schaffner S, Neuburger M, Fischer M, Schafer G, Mayor M (2009) Planar chiral asymmetric naphthalenediimide cyclophanes synthesis, characterization and tunable FRET properties. Org Biomol Chem 7 3222-3229... 

Ma et al. [10] chose a different catalytic reaction to show the chiral inducement of the planar chiral [2,2]-paracyclophane functionalised NHC ligand. The rhodium(I) catalysed reaction between a boronic acid and the prochiral cyclohexenone results in the addition of the boronic acid s substituent to the olefinic double bond of cyclohexenone (see Figure 5.32). In the present case, the product is (5)-3-phenylcyclohexanone with 61-98 % ee. The lowest chiral resolution is observed with the least hindered [2,2]-para-cyclophane group. 

Figure 8. A polyamine dendrimer bearing planar-chiral cyclophane units (Vogtle et al.)...

The incorporation of arenes in small cyclophanes hinders rotation around certain bonds and can afford isolable enantiomers with appropriately substituted backbones, e.g., planar-chiral 11 (Liittringhaus) or helical-chiral 12 (Vogtle). Studies of the dynamic properties of small cyclophanes with intraannular substituents X (13) provided insight into the space occupancy of atoms or functional groups. ... 

Hochmuth, D.H. Koenig, W.A. Determination of the rotational energy barrier of planar-chiral cyclophanes using dynamic enantioselective gas chromatography and computer simulation. Liebigs Ann. 1996, 6, 947-951. 

In 2002, Shair and coworkers [44] utilized ethylene as an additive in the macrocyclic ene-yne metathesis to form the planar chiral cyclophane 80 [43]... 

SCHEME 21.22 Enantioselective synthesis of planar chiral cyclophanes. 

Interesting planar chiral tripodal cyclophanes have also been synthesized with high enantioselectivity by the cationic rhodium(I)/Me-Duphos complex-catalyzed [2+2+2] cycloaddition of branched triynes (Scheme 21.25) [29]. 

In this chapter, the intermolecular multicomponent aromatic ring construction reactions and intramolecular single-component aromatic ring construction reactions are described. Among them, the [2+2+2] cycloaddition and intramolecular hydroarylation reactions are the most widely employed and reliable method. Various polycyclic and sterically hindered aromatic compounds have been synthesized by this method. In the past 10 years, the asymmetric [2+2+2] cycloaddition and intramolecular hydroarylation reactions have been developed, which enabled the enantioselec-tive synthesis of sterically hindered chiral aromatic compounds, such as axially chiral biaryls, planar chiral cyclophanes, and helically chiral heUcenes. Details of the transition metal-mediated aromatic ring construction reactions are comprehensively covered in the recently published book... 

A recent publication [2] has suggested the readoption of an earlier proposal [7 ] according to which the molecule is treated overall as a case of planar chirality (cf. cyclophanes). In view of its widespread use, however, the Schlogl-Cahn-Prelog system is employed here [4]. 

Tanaka et al. examined chiral Rh-catalyzed intramolecular cycloaddition using triynes with substituents at both ends and obtained [7] to [lOJmeto-cyclophanes with high enantioselectivity. This is the first example of catalytic and enantioselective synthesis of planar-chiral cyclophanes using the [2+ 2+ 2]-cycloaddition approach (Scheme 8.14) [11a]. The tether structure of the 1,6-diyene moiety of the triynes affected the stereoselectivity, and the reaction of ester-tethered triynes realized almost perfect enantioselectivity (Scheme 8.15) [11a]. 

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