Benzene rings, bent

As discussed above, olefins were initially proposed as approaching the oxo-Mn center along path a, with an orientation directing their bulkier substituent away from the C3(C3 )-substituent (see Scheme 14). It is noteworthy that path a runs over the downwardly bent benzene ring. Later, Jacobsen et al. proposed path b for the epoxidation with (26) possessing /-butyl groups at C5 and... 

Neutral hexakis(methylsulfonyl)benzene (see Scheme 6.23) adopts a chair conformation. On the contrary, the tube conformer appears to be inherent in the corresponding anion-radical. The methylsulfonyl fragments at positions 1 and 4 of the bent benzene ring are nonequivalent. Moreover, one methylsulfonyl moiety is nonequivalent to all of the other five (Fabre et al. 2002). Scheme 6.23 depicts an intuitively constructed picture. Localization of spin-charge density within one methylsulfonyl group causes the attraction of the other from position 4. This makes the tube conformation the most stable in the case of hexakis(methylsulfonyl)benzene anion-radical. 

The photoisomerization of benzene into Dewar benzene, benzvalene and fulvene is a well-known process, and in 1995 the interconversion between the [4] paracyclophane (3) and the 1,4-bridged Dewar isomer (4) in a matrix at 77 K was described. The analogous Dewar isomers (5) and (6) have been synthesized by conventional means in order to investigate the properties of the derived substituted [4]paracyclophanes and the kinetic stabilization of this skeleton by substituents which sterically hinder reactions at the bridgehead sites. Irradiation under matrix isolation conditions of (5) and (6) yields the corresponding cyclophanes which have widely differing half-lives (1 min at — 90°C to 0.5h at — 20°C) for their cycloreversions the authors also discuss in detail the aromaticity of the extremely bent benzene ring in the [4]paracyclophanes. 

X-ray structure analysis showed that macrocycle 57 was essentially planar, with the twist angle of the benzene rings from the plane of the macrocycle being less than 2°. Most of the strain was seemingly contained in the triple bonds, as these were bent from linearity by 10.1° to 12.3°. Despite its strained nature, the macrocycle showed remarkable stability. Decomposition occurred above 300°C on attempted melting. No reaction was observed between 57 and cyclopenta-diene at room temperature. 

FIGURE 2. The possible bent configuration of the benzene rings for the concerted formation of benzidine... 

The anion-radicals from aromatic nitro compounds preserve the second-order axis of symmetry. The analysis of superfine structure of the ESR spectrum of the nitrobenzene anion-radical reveals equivalency of the ortho and meta protons (Ludwig et al. 1964, Levy and Myers 1965). With the anion-radical of nitrosobenzene, the situation is quite different. This was evidenced from the ESR data (Levy and Myers 1965, Geels et al. 1965). Following electron transfer, the bent nitroso group fixes in the plane of the benzene ring to a certain extent. This produces five different types of protons, since both meta and ortho protons become nonequivalent. The nonequivalence of the ortho and meta protons has also been established for the anion-radicals of acetophenone (Dehl and Fraenkel 1963) and 5-methylthiobenzoate (Debacher et al. 1982 Scheme 6.17). 

The structure of the paracyclophanes explains many of the alterations that occur in the absorption spectra. In [2.2] paracyclophane, the inter-ring separation varies between 2.8 and 3.09 A, and the benzene rings are bent out of the plane by 13°. This forced overlap lowers the first excited singlet state level by about 5800 cm-1, as shown... 

Distortion is less in the [3.3] paracyclophane, where the inter-ring separation varies between 3.1 and 3.3 A, and the benzene rings are only bent by 6°. In the crystal, the hydrocarbon links adopt the chair conformation, and the benzene rings are displaced by about 0.5 A from the sandwich structure, although the rings remain parallel. Some ground-state overlap occurs between the rings, since the UV absorbance extends to the red of 305 nm. The peak of the broad fluorescence emission of [3.3] paracyclophane appears at the same position as that of [2.2] paracyclophane. 

Cram, D. J. Cram, J. M. Cyclophane Chemistry Bent and Battered Benzene Rings. 

The present review will be based on the classical views of aromaticity [14-16], confining it to two dimensions, although it has been customary to ascribe the exceptional stability of icosahedral closo-carboranes to their three-dimensional aromaticity [17, 18], The discovery of fullerenes, nanocones, and nanotubes has opened new vistas for bent and battered benzene rings. Also, it is possible to obtain benzenic rings... 

Cram, D.J. and Cram, J.M. (1971) Cyclophane chemistry bent and battered benzene rings. Accounts of Chemical Research, 4, 204-213. 

For the description of the bent benzene, the flat benzene ring was the starting point. For this system, there is a clear a n separation. There is a clear distinction between the doubly occupied orbitals for the a-core and the 6 non-orthogonal p-orbitals for the 7i-system. 

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