Planar bridge conformation

The chiral C2-conformer 42 of a ( )-cycloalkene can transform into the enantiomeric Cf-conformer 44 through a planar Cs-conformer 43, and this rope jump racemization can be prevented by anchoring one end of the unsaturated center onto the ring by means of an extra-bridge. This bridging creates a bicyclic anti-Bredt rule compound 45, revealing that all anti-Bredt rule compounds (45) with one double bond are necessarily asymmetric (C, symmetry) and have one asymmetric carbon atom. 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

In contrast, porphyrinogens 16 (Fig. 5) possess only 16 Ti-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28-30]. 

The [12]annulene (96) has been prepared. In solution this molecule exhibits rapid conformational mobility (as do many other annulenes), so that above a certain temperature, in this case — 150°C, all protons are magnetically equivalent. However, at — 170°C the mobility is greatly slowed and the three inner protons are found at 85 while the nine outer protons are at 68. Annulene 96 suffers from hydrogen interference and is certainly not planar. It is very unstable and above —50°C rearranges to 97. Several bridged and dehydro[12]annulenes are known. 

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

The molecular bridge is still balanced for Rg = Rx. But, as presented in Fig. 21, the variations of 7W as a function of Rg are different with our new circuit rules, as compared to that given by the Kirch off laws. Furthermore, the conformational variations used to balance the bridge do not permit to explore values of Rg below the planar conformation. This restricts the exploration of the possible 7W intensity as presented in Fig. 21. 

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