Cyclophane symmetry

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. 

The photoproduct derived from 3, the new tricyclic [4,4]cyclophane 4, has 5-type cyclobutane rings and has no alternating axis of symmetry, showing it chiral, and the oligomers are considered to have a zig-zag shaped rigid chain structure of alternative (IS, 2S, 3R, 4R ) and (1R, 2R, 3S, 4S ). 

Halogen atoms, like any other substituents, modify the symmetry of the framework to which they are attached. Unlike many others, however, they introduce no conformational complications of their own since, in a mechanical sense, they are cylindrically symmetrical (locally CJ about the bond axis. (See, e.g., the discussion of Cram and coworkers17 of the numerous point symmetries potentially available in bromine substituted [2.2]-para-cyclophanes.)... 

To study bridgehead reactivity, our group (89) prepared a cyclophane of C3 symmetry 74 a comparison with triphenylmethyl chloride indicated that the bridgehead chloride of 74 is conspicuous in its striking lack of reactivity in SnI displacement reactions and in free radical formation. 

Another interesting cyclophane of an unusually twisted structure is the recently prepared [2.2.2](l,2,4)(l,2,5)cyclophane (76) (92). Its C2 symmetry reflects its unique structural feature in that this molecule is built up by combining two 1,2,4-trisubstituted benzene moieties (C, symmetry) with their enantiotopic faces of the same chirality face-to-face, as discussed for 75. 

Cram and co-workers (93) once called [2.2] paracyclophane (77) one of the rigid cyclophanes that illustrate stereochemical principles, and they demonstrated various ways to desymmetrize its intrinsic DJjl symmetry by substitution. 

In their pioneering work in multilayered cyclophane chemistry, Longone and Chow (97) prepared a mixture of four-layered [2.2]paracyclophanes 82 and 85 by making use of the elegant procedure of Cram, which involves coupling of the p-xylylene intermediate 80 derived by a 1,6-Hofmann elimination of the corresponding quaternary ammonium base. There are available two alternatives for coupling the racemic 80 of C2 symmetry either between enantiomers of opposite chirality or between enantiomers of same chirality. The former should provide meso-82 of C2h symmetry, whereas the latter should yield 85 of D2 symmetry. 

Loops correspond to compounds like ansa cyclophanes, but equally well they might induce the design of novel annelated benzenes. Simple and polycycles manifest the familiar classes of cyclophanes and cryptophanes commonly synthesized by practitioners of supramolecular chemistry [12,13]. The Platonic graphs are a subset of the polycyclic, but, because of their potential for high symmetry and structural simplicity, they form an especially provocative group. 

The C3 symmetry of the cup-lilie trimers indicates that they are chiral typically being formed as a racemate. In some cases, the cups are resolvable via suitable dia-stereomers. as the enantiomers are somewhat stable to racemization via conformational inversion of the nine-membered cyclophane ring (AG 298 = 110-1 i5 kJ/mol). Chirality in the cup-shaped units implies that two types of cryptophanes exist Joining two cups of opposite... 

Often symmetry operations cannot be used in a simple way to classify chiral forms because, e.g., the molecule consists of a number of conformations. Therefore, independent of the symmetry considerations, a chemical approach to describe chiral molecules has been introduced by the use of structural elements such as chiral centers, chiral axis, and chiral planes. Examples for a chiral center are the asymmetric carbon atom, i.e., a carbon atom with four different substituents or the asymmetric nitrogen atom where a free electron pair can be one of the four different substituents. A chiral axis exists with a biphenyl (Figure 3.2) and chiral planes are found with cyclo-phane structures [17]. Chiral elements were introduced originally to classify the absolute configuration of molecules within the R, S nomenclature [16]. In cases where the molecules are chiral as a whole, so-called inherent dissymmetric molecules, special names have often been introduced atropiso-mers, i.e., molecules with hindered rotation about a helical molecules [18], calixarenes, cyclophanes [17], dendrimers [19], and others [20]. 

In earlier research by one of us pronounced orbital symmetry effect on intramolecular charge-transfer absorption and emission spectra were reported from studies on cyclophanes containing aromatic D/A pairs in a close-contact sandwich type arrangement. The pairs 12(5,5)/12(4,8) and 13(6,6)/13(3,8) epitomize the plethora of D/A cyclophanes investigated in these studies (see Fig. 9). 

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