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 propellane means the member of the family whose rings have x, y, and zcarbons, not counting the two bridgeheads; or x + 2, y + 2, and z + 2 carbons, counting them. The chemical formula is therefore C2+x+y+zH2. 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, propellane is the same substance as propellane. Therefore, it is customary to sort the indices in decreasing order, x ≥ y ≥ z.
General properties
Strain
In the propellanes with small cycles, such as 1.1.1-Propellane|propellane or 2.2.2-Propellane|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 to yield less-strained bicyclic or even monocyclic hydrocarbons. Surprisingly, the most strained member is far more stable than the other small ring members.
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, this process is easily achieved, yielding either simple polymers or alternating copolymers. For example, propellane yields spontaneously an interesting rigid polymer called staffane; and propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units alternate with 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 propellanes for any n ≥ 3.
Propellane, C6H8, CAS number . This compound was detected by infrared spectroscopy at 30 K but has not been isolated as a stable molecule at room temperature. It is believed to polymerize above 50 K. The bonds of the shared carbons have an inverted tetrahedral geometry; the compound's strain energy was estimated as 106 kcal/mol.
Propellane, C7H10, CAS number . Obtained gas-phase dehalogenation with alkali metal atoms. Stable only in frozen gas matrix below 50 K; oligomerizes or polymerizes at higher temperatures. The strain energy released by breaking the axial bond was estimated as 75 kcal/mol.
Propellane, C7H10, CAS number. Isolable.
Propellane or tricyclooctane, C8H12, CAS number . Isolable. Has inverted tetrahedral geometry at the shared carbons. Estimated strain energy of 60 kcal/mol. Remarkably resistant to thermolysis; polymerizes in diphenyl ether solution with halflife of about 20 hours at 195 °C. It reacts spontaneously with oxygen at room temperature to give a copolymer with –O–O– bridges.
Propellane, C8H12, CAS number Isolable.
2.2.2-Propellane|Propellane or tricyclooctane, C8H12, CAS number . This propellane is unstable, too, due to the three cyclobutane-like rings and the highly distorted bond angles at the axial carbons. Its strain energy is estimated to be 93 kcal/mol.
Propellane, C11H18, CAS number. It is a stable solid that melts at 130 °C. It was synthesized in 1978 by Robert W. Weber and James M. Cook who developed a general synthetic route for all propellanes, with n ≥ 3:
Propellane, C12H20, CAS number . A stable solid that melts at 100–101 °C.
Propellane, C14H24, CAS number . An oily liquid that boils at 275–277 °C.
Propellane, C18H32, CAS number . A stable solid that sublimes at 33–34 °C.
Propellane derivatives
1,3-Dehydroadamantane, C10H14 Pincock and Torupka, 1969). This compound is formally derived from adamantane by removing two hydrogens and adding an internal bond. It can be viewed as propellane, with an extra methylene bridge between its two larger "propeller blades". It is unstable and reactive and can be polymerized.
Dichrocephone B, a sesquiterpenoid with a propellane core was isolated in 2008 from dichrocephala benthamii. It was first synthesized in 2018 using a general strategy for the synthesis of carbocyclic propellanes from 1,3-cycloalkanediones.
