Methylene-Bridged -Annulenes

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

An interesting general synthesis using repeating anellation procedures has been developed by E. Vogel (W. Wagemann, 1978). First cyclohepta-3,5,7-triene-l,3- 

Spherical, pentagonal dodecahedrane is the thermodynamically most stable polycycloalkane. It is the so-called CJ0H2o stabilomer . It should therefore be available by thermodynamically controlled, e.g. acid-catalyzed, isomerization of less stable CjoHg,-isomers. Experiments along this line, e.g. treatment of the basketene photo-dimer with Lewis 

The strained undecacyclic pagodane framework was obtained in a series of 14 one-pot operations with an overall yield up to 24% from commercial isodrin. The key steps are (i) a benzene-benzene [6 + 6]photocycloaddition, and (ii) a domino Dieis-Alder reaction. 

As a consequence of the rigid face-to-face orientation, there are strong electronic interactions between the benzene rings in the dibenzo-anellated isodrin derivative. Irradiation with 254-nm UV light gave rise to a 7 3 equilibrium mixture of the educt with the [6 + 6]cycloaddition isomer. At an irradiation wavelength of 300 nm the cycloaddition wa completely reversed. 

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Considering this situation Becby and Sondheimer have carried out the first synthesis of a bisdehydroaza[17]annulene having a methylene bridging chain . ... 

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 . 

The methylene-bridged bisdehydrooxa- (240) and thia- (241a,b) [17]annulenes, which were found to be diatropic, have been prepared according to the following reaction sequence ... 

Methano[10]annulene is rather like naphthalene but with the middle bond replaced by a methylene bridging group. 

Aza[10]annulene is best observed in the methylene-bridged structure of Figure 6.15. Several derivatives of this system are known. Here, there are two trans double bonds, with the two inner H replaced by the bridge. The bridge holds the ring in a planar conformation, and these annulenes show the NMR and X-ray properties consistent with cychc electron delocalization. 

When the conjugated systems are not monocyclic, the situation becomes a little less clear. Naphthalene, for example, has ten electrons but you can also think of it as two fused benzene rings. From its chemistry, it is very clear that naphthalene has aromatic character (it does substitution reactions) but is iess aromatic than benzene itself. For example, naphthalene can easily be reduced to tetralin (1,2,3,4-tetrahydronaphthalene), which still contains a benzene ring. Also, in contrast to benzene, all the bond lengths in naphthalene are not the same. 1,6-Methano[10]annulene is rather like naphthalene but with the middle bond replaced by a methylene bridging group. This compound is almost flat and shows aromatic character. 

An X-ray crystal structure of annulene-dione (64) indicates an anti,anti configuration between the methylene and sulfur bridges.102 Diprotonation gives highly localized positive charges in the dication (65), mainly due to unfavourable p-orbital overlap. 

The simplest polycycles stem from two triply-bridged points. These systems are exemplified by Trost et al. s 4,8-dihydrodibenzo[oi,g7z]pentalene (69) [87, 88], a precursor for a purturbed [12]annulene dianion, and Mislow et al. s double-bridged biphenyl derivatives (generally shown as 70) [89], where X is methylene, carbonyl, or various heteroatoms. Other longer bridged biphenyls include the triple-bridged cyclophanes 71 made by Hubert and Dale [90], their unsym-metrical relatives by Cram and Reeves [91], and the recent polyalkynyl cyclo-phane (72) made by Rubin et al. [92], a proposed fullerene precursor. 

The n.m.r. of the anti isomer of 1,6 8,13-dimethano-[14]annulene 127,159,2446,247) 3Q does reflect the olefinic character expected from a consideration of its geometry. The perimeter protons absorb in a region typical of a cyclic polyolefin (r 3.6 to 3.8) whilst the bridging methylene... 

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