12.21 Metacyclophane

This molecule (A, Structure 2) is of particular iuterest because the hydrogeu that is held over the ceuter of the beuzeue riug is realty squashed dowu iuto the riug. This is the type of system that cau give us iuformatiou regardiug the repulsive part of the vau der Waals curve betweeu hydrogeu aud carbou.  

Structure 2. /n-[3 ][7]Metacyclophane (stereographic structure with most hydrogens omitted for clarity). 

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Calixarenes (from the Latin ca/ x) may be understood as artificial receptor analogues of the natural cyclodextrins (96,97). In its prototypical form they feature a macrocycHc metacyclophane framework bearing protonizable hydroxy groups made from condensation of -substituted phenols with formaldehyde (Fig. 15b). Dependent on the ring size, benzene derivatives are the substrates most commonly included into the calix cavity (98), but other interesting substrates such as C q have also been accommodated (Fig. 8c) (45). 

An example of intramolecular replacement of fluonne is found m the thermal rearrangement of syn-S, 16-difluoro[2,2]metacyclophane In the cA-difluoro intermediate, one fluonne atom rearranges while the other is lost as HF in an interesting path to 1-fluoropyrene [85] (equation 47). 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

Scheme 30 A chiral [2.2]metacyclophane carbene complex in a benzannulation reaction...

Figure 1 shows the crystal structure of the 5,10 12,17 19,24 26,3-tetrakis (dimethyl-siladioxa)-l, 8,15,22-tetramethyl[l4]metacyclophane cavitand (7) which has an enforced cavity appropriately sized to include only slim linear guests 12b). This cavitand forms crystals of a 1 1 molecular inclusion complex with CS2, the guest species being almost entirely encapsulated within the host cavity 27). The crystal structure of the complexed... 

Structure 6.8 demonstrates a most extreme example of anisotropy. In this unusual metacyclophane, the predicted chemical shift (Table 5.8) of the methine proton that is suspended above the aromatic ring would be 1.9 ppm. In fact, the observed shift is -4 ppm, i.e., 4 ppm above TMS The discrepancy between these values is all down to the anisotropic effect of the benzene ring and the fact that the proton in question is held very close to the delocalised p electrons of the pi cloud. 

The chemistry of cyclophanes may be traced back to the work of Pellegrin on [2.2]metacyclophane in 1899 (1). Although the first... 

These structural data demonstrate that 12 is a rather less distorted molecule than [2.2]paracyclophane. However, a dramatic effect of the strong cr(Si—Si)—w interaction was observed in UV spectra as shown in Fig. 5. In the UV spectrum of phenylpentamethyldisilane, an intramolecular crfSi—Si)—7T charge-transfer band appears around 231 nm (11a, 12). Octamethyltetrasila[2.2]ortho- (15) and metacyclophane (16) show similar absorptions, but the band splits into two bands at 223 nm (e = 19,100) and 263 nm (e = 22,500) in 12. This type of red shift in the UV spectra occurs only in 12 among other polysilapara-cyclophanes such as 13 and 14. 

Similar to [2.2]metacyclophane (2), relative conformations of two aromatic rings in [2.2](2,5)heterophanes have been studied in detail... 

A one-pot reaction gave the two different organosilicon macrocyclic compounds concomitantly. The structures of these metacyclophanes,... 

The properties of the cyclophanes are best illustrated by the para-cyclophanes. In contrast to the metacyclophanes and metapara-cyclophanes 2>, where aromatic nuclei come into close proximity, there are in paracyclophane molecules two aromatic nuclei pressed one on top... 

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). 

Similarly, secondary phosphines and a,a -dibromo-m-xylene usually react to give diprotonated ditertiary diphosphines [44-48]. However, in the case of the fluorous secondary phosphine 12-Rfs (Scheme 2, bottom), dialkylation occurred to give the metacyclophane 13-Rfs. As detailed elsewhere [40], extensive efforts to adjust the stoichiometry to favor noncyclized products failed. Fortunately, the reduction of 13-Rfs with IiAlH4 gave some of the target ligand lO-Rfs. 

Clark (1988) calculated the stabilities of diverse species with odd-electron o bonds. Cataldo et al. (2001) produced evidence for the existence of the anion-radical with an intramolecular one-electron bond between two phosphorus atoms in a macrocyclic structure of the metacyclophane type. Dutan et al. (2003) observed a similar situation for the anion-radical of a di (m-silylphenyl-enedisiloxane) analog. 

An iron-catalyzed reaction between bis(2,4-dimethoxyphenyl)sulfide and sulfur monochloride in dilute chloroform led to a mixture of metacyclophanes in... 

Table 2. 4a,4b-Dihydrophenanthrenes bridged at position 4 and 5, derived from 12.2)metacyclophanes...

Fu, X. Hossain, M.B. Schmitz, F.J. van der Helm, D. (1997) Longithorones, unique prenylated para- and metacyclophane type quinones from the tunicate Aplidium longithorax. J. Org. Chem., 62, 3810-19. 

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