Conformation cyclophane-type

Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. 

The preparation of thiamacrocycles of the cyclophane type by substitution of thiolates on suitable substrates usually leads to the desired products in good yields [55], favoured by the rigid group principle [56] and by application of the dilution principle [1]. On the other hand yields of conformative flexible thia-cycloalkanes under similar conditions as a rule are not satisfactory and often low... 

Lindner (171) developed his own tt-SCF MO force field that is similar to MMPI in construction. This program was applied to simulate racemization of metacyclophane (48) and hexahelicene (50). In metacyclophane the m-phenylene ring flips readily at room temperature. Two mechanisms can be conceived one operates by way of a high steric energy conformation (48b) the other involves a biradical intermediate (49). The calculated activation energies are 17 and 32 kcal/mol, respectively. The experimental value is 17.7 kcal/mol, in accord with the first mechanism (172). The structures and energies of seven types of cyclophane have been calculated (172). 

Fig. 10. Schematic representation of guest-binding behavior of hybrid assemblies formed with peptide lipid 9 and cage-type cyclophanes 7 and 8, indicating conformational change of the host cavity...

Examples in this series, whose cyclohexadiene units are all formally related by oxidation to meta (types V, VI and IX) and para (types VII and VIII) bridged cyclophanes of the kinds (192) and (193), respectively, appear not to exist in the literature (or they are at least rare and have escaped the attention of the authors of this review). Cyclophanes appear in standard textbooks and there is a recent example of a 2,5-bridged hexa-l,3,S-triene, namely (194), °° related by electrocyclization to a type VII 1,4-bridged cyclohexadiene. It is interesting that (194) is representative of a doubly orthogonal conformer of hexa-1,3,5-triene. However, the electrocyclization of hexatrienes to bridged 1,3-cyclohexadienes appears not to have been previously investigated. 

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... 

Wennerstrom and coworkers have synthesized numerous cage-type derivatives [33]. In 1982 they reported the synthesis of bicyclophanes 77-80, and showed that the cyclophanes were conformationally mobile such that twisted, helical conformations could be achieved, and, in most cases, were even favored. 

Compound 180 obtained unexpectedly along with phosphatripyrranes 175 gave corresponding cyclophanes 181 with 2,3-dihydrophosphole unit (Scheme 12.64). The interconversion between the in and out type conformers of 181 was sufficiently slow to isolate each of them. In the in-in type Pd(II) complex 182, two asymmetric cyclophane ligands coordinated to one palladium center have the same geometry and adopt partial cone conformations. The coordination mode of 181 is similar to those of 177 [158]. 

In the solid state, cyclophane 206 containing three phosphinine fragments adopts a partial cone-type structure. Two phosphorus-atom lone pairs point towards the top of the cavity and are located above the plane defined by three silicon atoms, the third one points below this plane [169]. Calix[4]phosphinines 207 adopt an opened-out partial cone conformation with the two opposing... 

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