Everything about Cubane totally explained
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Cubane (C
8H
8) 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. 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 for example
octanitrocubane and
heptanitrocubane. 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.
Synthesis
The original 1964 cubane
organic synthesis is a classic and starts from
2-cyclopentenone (compound
1.1 in
scheme 1):
»
Reaction with
N-bromosuccinimide in
Carbon tetrachloride 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. Prevalent non-organic cubes are the [Fe
4-S
4] 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 S
4 and Fe
4 tetrahedra. Many organometallic compounds adopt cube structures, examples being (
CpFe)
4(CO)
4, (
Cp*Ru)
4Cl
4, and (
Ph3PAg)
4I
4,
Reactions
Cuneane may be produced from cubane by
metal-ion-catalyzed σ-bond rearrangement.
Further Information
Get more info on 'Cubane'.
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