Bridged [14]Annulenes with a Phenanthrene‐Perimeter: syn‐1,6:7,12‐Bismethano[14]annulene

Abstract

The idea of transforming a classical arene, by “bridging,” into a [4n+2]annulene has been further extended to the phenanthrenes. The spectroscopic data and structural parameters for the title compound 1 reveal it to be a borderline case, with respect to its π‐electron structure, for an aromatic molecule.

Structural Aspects of Aromaticity

“…The syn 115 and anti 116 conformers of bridged annulene[14] derivatives differ much in aromaticity. Application of HOMA and its components to the perimeter bond lengths allowed us to look at it in a more subtle way.…”

Cycloproparenes

“…The syn-bismethano [14]annulene 135, prepared in 1986 by the Vogel group, 123 has all of the necessary features for bis-addition of an alkynyl dienophile that would provide the essential progenitor for 137. However, while the authors found that addition of DCA does take place, it is only to the bay region "diene" to give 136, and not across the bridge centers as is required for the ultimate preparation of 137.…”

“…Here it should be noted that the ring openings, e.g., with PhSH and Bu 3 SnH, give aryl-substituted product as major product with an orientation precisely the opposite to that of classical thermal chemistry. Despite these observations, Okazaki and co-workers had earlier shown that the addition of iodine and thiocyanogen to 2 under photochemical conditions leads to 1,6-disubstituted cycloheptatrienes in good yields such that the reaction has provided for much use of 2 in synthesis. ,, In these reactions radical addition is thought to take place across the bridge bond to give a 1,6-disubstituted norcaradiene that opens to the preferred valence bond isomer, but smaller amounts (to ca. 30%) of α, o- disubstituted toluenes are still formed.…”

Combining Aromaticity and Strain with Tetsuo Nozoe—A Passage of Time with Friends

“…By that time in 1989, we and others had attempted the synthesis of the benzenoid nucleus strained by double cyclopropa ring fusions as illustrated by 10–13 . Biscyclopropa–fused aromatics such as 10 had been prepared relatively easily, but the efforts to impose markedly more strain by fusing two three‐membered rings onto one naphthenoid ring failed.…”