Propellane
In organic chemistry, propellane is any member of a class of polycyclic hydrocarbons, whose carbon skeleton consists of three rings of carbon atoms sharing a common carbon–carbon covalent bond. The name derives from a supposed resemblance of the molecule to a propeller: namely, the rings would be the propeller's blades, and the shared C–C bond would be its axis. The concept was introduced in 1966 by D. Ginsburg Propellanes with small cycles are highly strained and unstable, and are easily turned into polymers with interesting structures, such as staffanes. Partly for these reasons, they have been  the object of much research. In the literature, the bond shared by the three cycles is usually called the "bridge"; the shared carbon atoms are then the "bridgeheads". The notation means the member of the family whose rings have "x", "y", and "z" carbons, not counting the two bridgeheads; or "x" + 2, "y" + 2, and "z" + 2 carbons, counting them. The chemical formula is therefore . The minimum value for "x", "y", and "z" is 1, meaning a 3-carbon ring. There is no structural ordering between the rings, so, for example, [1.3.2]propellane is the same substance as  [3.2.1]propellane. Therefore, it is customary to sort the indices in decreasing order, . General properties. Strain. In the propellanes with small cycles, such as [1.1.1]propellane or [2.2.2]propellane, the two carbons at the ends of the axial bond will be highly strained, and their bonds may even assume an inverted tetrahedral geometry. The resulting steric strain causes such compounds to be unstable and highly reactive. The axial C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons. Surprisingly, the most strained member [1.1.1] is far more stable than the other small ring members ([2.1.1], [2.2.1 ], [2.2.2], [3.2.1], [3.1.1], and [4.1.1]). Polymerization. In principle, any propellane can be polymerized by breaking the axial C–C bond to yield a radical with two active centers, and then joining these radicals in a linear chain. 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. For example, [1.1.1]propellane yields spontaneously an interesting rigid polymer called staffane; and [3.2.1]propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units [–CH–] alternate with [–O–O–] groups. Synthesis. The  smaller-cycle propellanes are difficult to synthesize because of their strain. Larger members are more easily obtained. Weber and Cook described in 1978 a general method which should yield ["n".3.3]propellanes for any "n" ≥ 3.