Bridged annulenes,

Azaadamantanes biocide, 1, 400 mechanism, 1, 401 photoelectron spectroscopy, 2, 142 5-Azaadenines reactions, 5, 878 synthesis, 5, 901 8-Azaadenosines synthesis, 5, 895 Azaallyl cations generation, 7, 73 Aza[10]annulenes bridged... 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

Annulenes, bridged, 38,123-124, 333-334 Anthracene Diels-Alder addn. to, 93 —, 9-(chloromethyl)- mutagen, 341-342 Antibiotics synth., 310-329 (See also Griseo-fulvins, 74, 86)... 

The n.m.r. spectrum of (XV) indicates that all the charge resides on the periphery, with little, if any, on the central double bond [192]. This in turn suggests that the best description of this compound is as a [12]annulene bridged by a carbon-carbon double bond. Treatment with deuterium oxide causes addition of deuterium at the 2- and 6-positions of the pentalene moiety. 

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). 

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

Dieckmann cyclization thiophenes from, 4, 874-875 Thiaannulenes structure, 7, 715 twice-bridged structure, 7, 718 Thiaannulenes, didehydro-structure, 7, 718 Thia[17]annulenes restricted structure, 7, 720... 

The bridged annulene 5,10-epoxy[10]annulene behaves as an aromatic 1 Ore-system rather than an oxepin and reacts with copper(II) nitrate in acetic anhydride to give a 1 1 mixture of 1-and 2-nitro-5,10-epoxy[10]annulene 2 and 3.154... 

Bromine can also undergo addition to the nonaromatic double bond of dibenz[ft,/]oxe-pin.160,161 Oxygen-bridged annulenes that incorporate an oxepin structure have been shown to undergo 1,4-addition of bromine at — 78°C.159... 

Cyclazines may be considered as nitrogen-bridged annulenes rather than as hetarenes and, therefore, are only briefly mentioned in this volume. The chemistry of [3.3.3]cyclazines 1 and aza analogs, e. g. 3, is covered comprehensively in Houben-Weyl, Vol. E7b, p 205 IT. For another recent review, see ref 1. Subsequently, the chemistry of individual types of cyclazines has been reviewed, in particular 2.2.x]cyclazines,1 2 [2.2.2]cyclazines,3 [2.3.x]cyclazines4 and [2.2.3]-cyclazines.5 Moreover, the synthesis and the reactions of [2.2.3]cyclazines and the question of the effect of bcnzannulation have been addressed specifically.6... 

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... 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

The known bridged [18]annulenes are also diatropic as are most of the known... 

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

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. 

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... 

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