Prismane

Prismane is a polycyclic hydrocarbon with the chemical formula C6H6. It is an isomer of benzene, but it is far less stable than benzene. The carbon atoms of the prismane molecule are arranged in the shape of a six-atom triangular prism. The systematic name is tetracyclo[2.2.0.02,6.03,5]hexane. Albert Ladenburg proposed this structure for the compound now known as benzene. The compound was not synthesized in the laboratory until 1973.

History
The discussion about the structure of benzene in the mid 18th century yielded several structures for the formula C6H6, which was accessible by combustion analysis. The first, proposed by Kekulé in 1867, later proved to be the right one. This structure inspired several others to come up with concurring structures; for example, Ladenburg with prismane, Dewar with Dewar benzene, and Koerner and Claus with claus benzene. Some of these structures could be synthesized in the following years. Prismane, like the other proposed structures for benzene, is still often cited in the literature, because it is part of the historical struggle toward understanding the mesomeric structures and resonance of benzene. Some computational chemists still research the differences among the possible isomers of C6H6.

Properties
The deviation of the carbon-carbon bond angle from 109° in a tetrahedron to 60° in a triangle leads to a high ring strain, similar to that of cyclopropane. Due to this ring strain, the bonds have a low bond energy and break at a low activation energy, which makes synthesis of the molecule difficult. The molecule in which all six hydrogens are substituted by methyl groups (hexamethylprismane) has a higher stability and was synthesized by rearrangement reactions in 1966.

Synthesis


The synthesis starts from benzvalene (1) and 4-phenyltriazolidone, which is a strong dienophile. The reaction is a stepwise Diels-Alder like reaction, forming a carbocation as intermediate. The adduct (2) is then hydrolyzed under basic conditions and afterwards transformed into a copper(II) chloride derivative with acidic copper(II) chloride. Neutralized with a strong base, the azo compound (3) could be crystallized with 65% yield. The last step is a photolysis of the azo compound. This photolysis leads to a biradical which forms prismane (4) and nitrogen with a yield of less than 10%.

1H-NMR
1H-NMR (CCl4): δ = 7.72 ppm

13C-NMR
13C-NMR(C6D6): δ = 30.6 ppm

Mass-spectroscopy
EI-MS (70 eV): m/z (%) = 78 (100) [M+], 77 (20) [M+ - H], 52 (22), 39 (13).