Propellane

[1][2] The concept was introduced in 1966 by D. Ginsburg [1][3] Propellanes with small cycles are highly strained and unstable, and are easily turned into polymers with interesting structures, such as staffanes.

The IUPAC nomenclature of the homologue series of all-carbon propellanes would be called tricyclo[x.y.z.01,(x+2)]alkane.

The resulting steric strain causes such compounds to be unstable and highly reactive.

The interbridgehead C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons.

[6] Studies by Sterling et al. suggest delocalisation effects onto the three-membered bridges relaxing Pauli-repulsion and thus stabilising the propellane core.

Anions and radicals add towards the interbridgehead bond resulting in bicyclo[1.1.1]pentyl-units.

In contrary, cations and metals decompose the tricyclic core towards monocyclic systems by opening of the bridged bonds forming exo-methylene cyclobutanes.

For the propellanes with small cycles (such as [1.1.1], [3.2.1], or 1,3-dihydroadamantane), this process is easily achieved, yielding either simple polymers or alternating copolymers.