Cubane

Cubane (C₈H₈) is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon molecule. It is one of the Platonic hydrocarbons. Cubane is a solid crystalline substance. The cubane molecule was first synthesized in 1964 by Dr. Philip Eaton, a professor of chemistry at the University of Chicago.^[1] Before its synthesis, researchers believed that cubic carbon-based molecules could only exist in theory. It was believed that cubane would be impossible to synthesize because the unusually sharp 90-degree bonding angle of the carbon atoms would be too highly strained and hence unstable. Surprisingly, once formed, cubane is actually quite kinetically stable due to a lack of readily available decomposition paths.

Cubane and its derivative compounds have many important properties. The 90-degree bonding angle of the carbon atoms in cubane means that the bonds are highly strained. Therefore, cubane compounds are highly reactive, which in principle may make them useful as high-density, high-energy fuels and explosives. Cubane also has the highest density of any hydrocarbon, further contributing to its ability to store large amounts of energy. Researchers are looking into using cubane and similarly synthesized cubic molecules in medicine and nanotechnology.

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Synthesis

The original 1964 cubane organic synthesis is a classic and starts from 2-cyclopentenone (compound 1.1 in scheme 1)^[1][2]:

Reaction with N-bromosuccinimide in tetrachloromethane places an allylic bromine atom in 1.2 and further bromination with bromine in pentane - methylene chloride gives the tribromide 1.3. Two equivalents of hydrogen bromide are eliminated from this compound with diethylamine in diethyl ether to bromocyclopentadienone 1.4

In the second part (scheme 2), the spontaneous Diels-Alder dimerization of 2.1 to 2.2 is akin the dimerization of cyclopentadiene to dicyclopentadiene. For the next steps to succeed only the endo isomer should form which it does because the bromine atoms on their approach take up positions as far away from each other and the carbonyl group as possible. In this way the like-dipole interactions are minimized in the transition state for this reaction step. Both carbonyl groups are protected as acetals with ethylene glycol and p-toluenesulfonic acid in benzene and then one of them is selectively deprotected with aqueous hydrochloric acid to 2.3

In the next step endo isomer 2.3 with both alkene groups in close proximity forms the cage-like isomer 2.4 in a photochemical [2+2] cycloaddition. The bromoketone group is converted to ring-contracted carboxylic acid 2.5 in a Favorskii rearrangement with potassium hydroxide. Next the thermal decarboxylation takes place through the acid chloride (with thionyl chloride) and the tert-butyl perester 2.6 (with t-butyl hydroperoxide and pyridine) to 2.7. then the acetal is once more removed in 2.8, another Favorskii rearrangement gives 2.9 and finally another decarboxylation 2.10 and 2.11.

Inorganic cubes and related derivatives

The cube motif occurs outside of the area of organic chemistry. A prevalent non-organic cube are the [Fe₄-S₄] clusters found pervasively iron-sulfur proteins. Such species contain sulfur and Fe at alternating corners. Alternatively such inorganic cube clusters can often be viewed as interpenetrated S₄ and Fe₄ tetrahedra. Many organometallic compounds adopt cube structures, examples being (CpFe)₄(CO)₄, (Cp*Ru)₄Cl₄, and (Ph₃PAg)₄I₄,

See also

• Octanitrocubane (explosive)
• Heptanitrocubane (explosive)
• dodecahedrane

References

1. ^ ^{a b} Cubane Philip E. Eaton and Thomas W. Cole J. Am. Chem. Soc.; 1964; 86(15) pp 3157 - 3158; doi:10.1021/ja01069a041.
2. ^ The Cubane System Philip E. Eaton and Thomas W. Cole J. Am. Chem. Soc.; 1964; 86(5) pp 962 - 964; doi:10.1021/ja01059a072