Paratropic, bridged annulenes

The same effect of benzoannelation is also seen for bridged annulenes thus a benzomethano[10]annulene [149] and a benzometheno-[10]annulene [259] both show reduced diatropicity and a benzo-methano[12]annulene [260] shows reduced paratropicity compared to their non-annelated analogues. 

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... 

The methylene-bridged bisdehydroaza[19]annulene (215), a hetero[4w— l]annulene with Ann electrons, has been synthesized . Comparison of the H-n.m.r. spectrum of 215 with that of the open-chain analogue 214 indicates that 215 is a paratropic 20tc electron system just as carbocyclic [4 i]annulenes . 

Both the symmetric 1,4 9,12- and the unsymmetric 1,4 7,10-dioxido-[16]annulcnes are strongly paratropic [214,215]. The oxygen bridges in these annulenes help to fix the annulene ring in a coplanar conformation and prevent conformational interchange of the position of the hydrogen atoms [215]. 

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