Propellanes

Propellane: (from left to right) [1.1.1] Propellane, [2.2.2] Propellane, 1,3-Dihydroadamantane

Propellanes are a class of tricyclic hydrocarbon compounds. In their unsubstituted form, propellanes belong to the cycloalkanes , more precisely to the tricyclic alkanes.

Structure description

Propellanes are compounds that have a C - C axis from which three different rings emanate. These rings have at least 1 carbon atom. If you look along the CC axis, the molecule has a rotational symmetry of 120 °, which is reminiscent of a propeller.

stability

The stability of the molecule increases with the size of the three rings. In propellanes with a small ring size, the bond angles for the carbon atoms of the CC axis are unusually small, so that such molecules are under tension and are correspondingly unstable. The exception here is [1.1.1] -propellane, which should be extremely unstable due to the ring size of only 3 carbon atoms, but is much more stable than systems with larger rings such as. B. [1.2.2] propellane. [1.1.1] -Propellan is stable at room temperature and can even be purified by distillation. This can be explained by Walsh orbitals, with the help of which it can be shown that the orbital energies in this system are astonishingly low.

To illustrate: [1.1.1] -propellane contains three 3-carbon rings, so a total of 5 carbon atoms. The bond angles are 60 ° (as with cyclopropane). Since there are three small rings on the CC axis, the bonds of the C atoms on the CC axis are “turned inside out”: All bonds point to one side (so-called inverted carbon). On the other hand, the carbon is “bare-bonded”, so to speak, which is a special property of short-chain propellanes.

synthesis

[1.1.1] propellane was synthesized from bicyclo [1.1.1] pentane-1,3-dicarboxylic acid via the intermediate stage of a dibromo compound:

nomenclature

The numbers in front of the name [xyz] -Propellan indicate the number of carbons in the three bridges over the common CC edge. The bridges are listed in descending length of the bridges. Propellane is a common name for the somewhat more complicated IUPAC name ; [1.1.1] propellane would be officially referred to as tricyclo [1.1.1.0] pentane.

Individual evidence

1. ↑ J. Michl, GJ Radziszewski, JW Downing, KB Wiberg, FH Walker, RD Miller, Peter Kovacic, Mikolaj Jawdosiuk, V. Bonacic-Koutecky: Highly strained single and double bonds. In: Pure and Applied Chemistry. 55, 1983, doi: 10.1351 / pac198855020315 .
2. ↑ Thomas Laue, Andreas Plagens: Name and Keyword Reactions of Organic Chemistry , Vieweg + Teubner, 5th Edition, 2006, p. 192, ISBN 978-3-8351-0091-6 .