Nonplanar bridge conformation

In 1958, the synthesis of 2 was first reported X) the most satisfactory route was found to be the slow addition of butyllithium to a solution of l,2-i ,(6 -bromomethyl-2 -pyridyl)ethane (7) in ether to afford cyclophane 2 in 28 % yield. The X-ray analysis shows 2 to possess a nonplanar chair conformation in the crystalline state2. Also, the H NMR spectrum of 2 in deuteriochloroform shows the carbon bridge methylene signal to coalesce at 13.5 °C and to exhibit an A2B2 pattern at —40 °C. The calculated barrier to inversion for 2 is 14.8 kcal/mole. The coupling constants of the low temperature spectra are also consistent with a chair-like conformation 3>. 

The results for [16] annulene are similar. The compound was synthesized in two different ways, both of which gave 103, which in solution is in equilibrium with 104. Above -50°C there is conformational mobility, resulting in the magnetic equivalence of all protons, but at — 130°C the compound is clearly paratropic there are 4 protons at 10.565 and 12 at 5.35 5. In the solid state, where the compound exists entirely as 103, X-ray crystallography shows that the molecules are nonplanar with almost complete bond alternation The single bonds are 1.44-1.47 A and the double bonds are 1.31-1.35 A. A number of dehydro and bridged... 

Porphycenes 51 bearing substituents at ethylenic carbons have been prepared from 5,5 -acyl bipyrroles 53 (05OL1887) (Scheme 26). However, oxidation of the intermediate annulene 54 is difficult, owing to the presence of the alkyl chains on the alkene bridges. These reduce conformational flexibility, resulting in a near-nonplanar geometry, which resists oxidation and/or aromatization (08CSR215). 

Naphthol may form cyclic tetrameric condensation products with formaldehyde among which 101 has a C4 axis.194 However, since the OH groups are in exo positions in this case (in contrast to the calix[4]arene 97g derived from 2-naphthol) it seems difficult to fix a nonplanar conformation in these cases.195 A possibility would be the connection of two opposite oxygens by a (crown) ether bridge which would lead to a C2-symmetrical derivative. 

The [12]annulene 124 has been prepared. In solution, 124 undergoes rapid conformational mobility (as do many other annulenes), and above — 150°C in this par-tiuclar case, all protons are magnetically equivalent. However, at — 170°C the mobility is greatly slowed and the three inner protons are found at 5 while the nine outer protons are at 6 8. Interaction of the internal hydrogens in annulene 124 leads to nonplanarity. Above —50°C, 124 is unstable and rearranges to 125. Several bridged... 

Proton transfer occurs mostly in the original ground-state conformation that the complex has at the time of photoexcitation. We found, however, qualitative evidence of sub-picosecond photoinduced hydrogen bond reorganization of 7AI(H20)3 from the bridged-planar to cyclic-nonplanar isomer prior the proton transfer. 

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