Structure and properties of 9,10,11,12,13,14-hexahydro-9,10[1′,4′]-benzenoanthracene and 9,10,11,12,13,14,15,16-octahydro-9,10[1′,4′]-benzenoanthracene

• The structure of the anthracene–cyclohexadiene photocycloadduct is confirmed.
• This adduct is disordered with respect to the cyclohexadiene-derived moiety.
• Two interannular bonds are longer than 1.59 Å, as reproduced by theory.
• Hydrogenation of the adduct increases the symmetry and removes the disorder.
• The interannular bonds in the hydrogenated derivative also are over 1.59 Å.

The title compounds were prepared by 4πs + 4πs photochemical cycloaddition between anthracene and 1,3-cyclohexadiene, followed by catalytic hydrogenation. The results confirm the structure of the initial cycloadduct, 9,10,11,12,13,14-hexahydro-9,10[1′,4′]-benzenoanthracene (1), which in the crystal exhibits positional disorder about the cyclohexadiene-derived fragment that is only partially resolved. The origin of the disorder is considered in light of the crystallographic packing interactions and compared with intermolecular contacts in the hydrogenated derivative, 9,10,11,12,13,14,15,16-octahydro-9,10[1′,4′]-benzenoanthracene (2), which is not disordered. The asymmetric unit of 2 contains one-half of a molecule situated about a two-fold symmetry axis. Both structures contain relatively long interannular bonds between the bridgehead carbons of the anthracene- and cyclohexadiene-derived fragments, in agreement with ab initio calculations and considered in terms of bond strain in the carbon framework of these compounds.

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