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 C2+x+y+zH2(x+y+z). The minimum value for x, y, and z is 1, meaning a 3-carbon ring.
1.1.1-Propellane
[1.1.1]Propellane is an organic compound, the simplest member of the propellane family. It is a hydrocarbon with formula C5H6 or C2(=CH2)3. The molecular structure consists of three rings of three carbon atoms each, sharing one C-C bond.
[1.1.1]Propellane is a highly strained molecule. The bonds of the two central carbon atoms have an inverted tetrahedral geometry, and the length of the central bond is 160 pm. The strength of that bond is disputed; estimates vary from 59–65 kcal/mol to no strength at all. The energy of the biradical state (without the central bond and unfilled central carbons) is calculated to be 80 kcal/mol higher. The compound is highly unstable, and at 114 °C it will spontaneously isomerize to 3-methylidenecyclobutene with a half-life of 5 minutes. Its strain energy is estimated to be 102 kcal/mol (427 kJ/mol).
The type of bonding in this molecule has been explained in terms of charge-shift bonding
Synthesis
[1.1.1]Propellane was first synthesized by K. Wiberg and F. Walker in 1982,. according to the following schema:
2.2.2-Propellane
[2.2.2]Propellane, formally tricyclo[2.2.2.01,4]octane is an organic compound, a member of the propellane family. It is a hydrocarbon with formula C8H12, or C2(=C2H4)3. Its molecule has three rings with four carbon atoms each, sharing one C-C bond.
This compound is unstable (although not as much as [1.1.1]propellane). The bond angles on the shared carbons are considerably strained: three of them are close to 90 degrees, the other three to 120 degrees. The strain energy is estimated to be 93 kcal/mol (390 kJ/mol).
Synthesis
[2.2.2]Propellane was first synthesized in 1973 by the group of Philip Eaton (who had earlier obtained cubane), according to the following scheme:
The synthesis begins with photochemical [2+2]cycloaddition of ethylene on the cyclohexene derivative 1 to produce the bicyclic compound 2, followed by elimination reaction with potassium t-butoxide of acetic acid to cyclobutene 3, followed by another cycloaddition with ethylene to 4. This compound is converted to the diazo ketone 5 by deprotonation (using acetic acid and sodium methoxide) and reaction with tosyl azide. The ketone then undergoes Wolff rearrangement to ketene 6. Ozonolysis forms the ketone 7, another diazotation yields the ketene 8, which undergoes Wolff rearrangement again to the ketene 9. Reaction with dimethylamine affords the [2.2.2]propellane backbone with a dimethylamide substituent 10.
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