Propellane

Some propellanes. From left to right: [1.1.1]propellane, [2.2.2]propellane, and 1,3-dehydroadamantane (a methylene-bridged derivative of [1.3.3]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.

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 [x.y.z]propellane 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 C_2+x+y+zH_2(x+y+z). 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, x ≥ y ≥ z.

General properties

Data can be found in Osmont et al., Energy and Fuels, Vol. 22, pp. 2241–2257 (2008).

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]).^[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 spontaneoulsy an interesting rigid polymer called staffane;^[2] and [3.2.1]propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units [-C₈H₁₂-] alternate with [-O-O-] groups.^[3]

Synthesis

The smaller-cycle propellanes are difficult to synthesize because of their strain. Larger members are more easily obtained. R. Weber and J. Cook described in 1978 a general method which should yield [n.3.3]propellanes for any n greater than 2.^[4]

Examples

True propellanes

• [1.1.1]propellane, C₅H₆ (K. Wiberg and F. Walker, 1982).^[5] It is a highly strained molecule: the two central carbons have an inverted tetrahedron geometry, and each of the three cycles is the notoriously strained cyclopropane ring. The length of the central bond is only 160 pm. It is an unstable product that undergoes thermal isomerization to 3-methylenecylobutene at 114 °C,and spontaneously reacts with acetic acid to form a methylenecyclobutane ester.^[2]

• [2.1.1]propellane, C₆H₈ (K. Wiberg, F. Walker, W. Pratt, and J. Michl, ????). This compound was detected by infrared spectroscopy at 30 K but has not been isolated as a stable molecule at room temperature (as of 2003). 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.^[6]

• [2.2.1]propellane, C₇H₁₀ (F Walker, K. Wiberg, and J. Michl, 1982). Obtained gas-phase dehalogenation with alkali metal atoms. Stable only in frozen gas matrix below 50K; oligomerizes or polymerizes at higher temperatures. The strain energy released by breaking the axial bond was estimated as 75 kcal/mol.^[7]

• [3.1.1]propellane, C₇H₁₀. Isolable.^[1][8][9]

• [3.2.1]propellane or tricyclo[3.2.1.0^1,5]octane, C₈H₁₂ (K. Wiberg and G. Burgmaier, 1969). 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°. It reacts spontaneously with oxygen at room temperature to give a copolymer with -O-O- bridges.^{[10][11][3][12][13]}

• [4.1.1]propellane, C₈H₁₂. (D. Hamon, V. Trennery, 1981) Isolable.^{[1][14][15][16]}

• [2.2.2]propellane or tricyclo[2.2.2.0^1,4]octane, C₈H₁₂ (P. Eaton and G. Temme, 1973).^[13][17] This propellane is unstable, too, due to the three cyclobutane-like rings and the highly distorted bond angles (three of them nearly 90°, the other three nearly 120°) at the axial carbons. Its strain energy is estimated to be 93 kcal/mol (390 kJ/mol).

• [3.3.3]propellane, C₁₁H₁₈ (R. Weber and J. Cook, 1978). It is a stable solid that melts at 130 °C.^[4]

• [4.3.3]propellane, C₁₂H₂₀ (R. Weber and J. Cook, 1978). A stable solid that melts at 100–101°C.^[4]

• [6.3.3]propellane, C₁₄H₂₄ (R. Weber and J. Cook, 1978). An oily liquid that boils at 275–277°C.^[4]

• [10.3.3]propellane, C₁₈H₃₂ (S. Yang and J. Cook, 1976). A stable solid that sublimes at 33–34°C.^[18]

Propellane derivatives

• 1,3-dehydroadamantane, C₁₀H₁₄ (R. Pincock and E.Torupka, 1969).^[19] This compound is formally derived from adamantane by removing two hydrogens and adding an internal bond. It can be viewed as [1.3.3]propellane (whose axis would be the new bond), with an extra methylene bridge between its two larger "propeller blades". It is unstable and reactive and can be polymerized.

See also

• cycloalkane

References