Annulene oxido-bridged

The H NMR spectrum of 7.80 was also recorded by Elix. While it was of insufficient quality to allow an extract assignment of structure vide supra), it did allow Elix to conclude that, as expected given its 4n n-electron formulation, compound 7.80 is not aromatic as judged by the absence of any substantial macrocyclic ring-current effects. This conclusion reached, it should be noted, that the formally 4n + 2 pentaoxa[30]pentaphyrin-(2.2.2.2.2) isomers also failed to display any kind of substantial ring-current effects. This led Elix to conclude that there is no fundamental difference in electronic character between large 4n and 4n + 2 7r-electron oxido-bridged annulenes. ... 

Sz/w-1,6 8,13-bridged [14]annulenes having two oxido bridges 1180, 188], one oxido and one methane bridge [189] and one amido and one raethano bridge [190] have also been prepared but not their anti--isomers. These compounds are thermally stable, and their n.m.r. spectra show that they are diatropic. The 1,6-methano-8,13-oxido compound is acetylated by acetic anhydride in the presence of boron trifluoride to give a mixture of 2-, 9- and 10-acetyl derivatives [189] the dioxido compound has been acetylated, brominated and nitrated, in each case providing the 2-substituted product [191]. 

Another type of oxido-bridged annulene which has been prepared... 

The preparation of l,6-oxido[10]annulene, described simultaneously by Sondheimer and Shani4 0 and by Vogel, Biskup, Pretzer, and Boll, is illustrative of the rather general synthesis of aromatic 1,6-bridged [10]annulenes starting from 1,4,5,8-tetrahydronaphthalene. Apart from the present compound, the following bridged [10]annulenes have thus far been obtained by this approach l,6-methano[10J-annulene,2-8-9 the ll,ll-dihalo-l,6-methano[10]annulenes,9 10 and 1,6-imino [ 1 OJannulene.11... 

A final sub-group within the bridged 10 7r-electron systems includes those compounds bridged by heteroatoms. The synthesis and n.m.r. investigation of l,6-oxido-[10]annulene 137-139,142) and many of its derivatives 17,125,138,140) have accordingly been executed. The n.m.r. spectrum 17 of l,6-oxido-[10]annulene itself (30, see Table 6) consists of an AA BB system centered at t 2.65 (in CCI4 also quoted at t 2.52... 

The molecular structure of oxy-bridged annulenes has been studied using a range of semiempirical methods including HF/STO-3G, HF/4-31G, MP2/STO-3G, MNDO, and correlation corrected MNDO <86JA4768>. The barrier to inversion, at the ether oxygen, of l,6-methano-8,13-oxido[14]-annulene (11) was predicted to be 27 kcal mol . ... 

In the dioxido system, a bridge flipping of the dihydro precursors, 41 42b, was accomplished by heating at 80—100°C for about forty hours. In the present case, synthesis of the < f -l,6-methano-8,13-oxido-[14]annulene has not been possible, because the aw i-dihydro precursor, presumably as the open form 45 a, (c.f. 41), as derived from the anti-tetrabromide, isomerizes spontaneously to the syn compound 45 b at room temperature 159). Consequently, sy -l,6-methano-8,13-oxido-[14]-annulene, 39, is the ultimate product from the anti precursors. 

---