1,6-Methano(10)annulene

1,6-Methano[10]annulene (also known as 1,6-methanonaphthalene or homonaphthalene) is an aromatic hydrocarbon with chemical formula C$11$H$10$. It was the first stable aromatic compound based on the cyclodecapentaene system to be discovered.

Preparation
The classical organic synthesis of this compound starts with Birch reduction of naphthalene to isotetralin, which adds dichlorocarbene (prepared in situ from chloroform and potassium tert-butoxide) to form a transannular cyclopropane ring. A second reduction then removes the chloride substituents. Finally, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation removes the central transannular bond and dehydrogenates unto aromaticity.

Aromaticity
It is analogous to cyclodecapentaene ([10]annulene), but with two hydrogen atoms replaced by a transannular methylene bridge (--). Consequently, it obeys Hückel's rule (n = 2) and despite the distortion from planarity introduced by the methylene bridge, the compound is aromatic. In fact, when prepared by Vogel in the 1960s it was the first stable aromatic cyclodecapentaene to be discovered. Its aromaticity is confirmed by two main pieces of evidence. Firstly, the similarity in carbon-carbon bond lengths as measured by x-ray crystallography is inconsistent with alternating single and double bonds. The actual structure is better considered as a pair of resonance hybrids (like the Kekulé structures of benzene) rather than as having alternating single and double bonds.

Secondly, its $1$H NMR spectrum displays influence of the diamagnetic ring current which is characteristic of aromatic compounds. The peripheral protons around the ring are deshielded while the methylene bridge nuclei are strongly shielded.

Its resonance energy is smaller than that of naphthalene.

Possible applications
Homonaphthalene has been used in the production of conductive polymers.