Tetrahedrane

Tetrahedrane is a hypothetical platonic hydrocarbon with chemical formula and a tetrahedral structure. The molecule would be subject to considerable angle strain and has not been synthesized . However, a number of derivatives have been prepared. In a more general sense, the term ''tetrahedranes'' is used to describe a class of molecules and ions with related structure, e.g. white phosphorus.

C₄ tetrahedranes

Tetrahedrane () is one of the possible platonic hydrocarbons and has the IUPAC name tricyclo .1.0.0^2,4utane.

Unsubstituted tetrahedrane remains elusive, although predicted kinetically stable. One strategy that has been explored (but thus far failed) is reaction of propene with atomic carbon.

Contrariwise, several organic compounds with the tetrahedrane core are known. All have multiply bulky substituents, ''tert''-butyl (''t''-Bu) or larger. Locking a tetrahedrane molecule inside a fullerene has only been attempted ''in silico''.

All known syntheses have relied on rearrangement from another unstable moiety. In Maier's original synthesis, photochemical cheletropic decarbonylation converts a cyclopentadienone to the tetrahedrane. In a later synthesis, irradiation directly converted a cyclobutadiene to tetrahedrane. And more recently, single-electron oxidation can induce a radical chain isomerization with the same effect.

Tetrahedrane with small substituents would have a variety of interesting properties. Due to its bond strain and stoichiometry, tetranitrotetrahedrane has potential as a high-performance energetic material (explosive).

Calculations suggest that tetrahedrane's molecular strain reduces if slightly-flexible diyne spacers separate the vertices.

Tetra-''tert''-butyltetrahedrane

In 1978, Günther Maier first prepared tetra-''tert''-butyl-tetrahedrane, with a deceptively short and simple synthesis that required "astonishing persistence and experimental skill". "The relatively straightforward scheme shown ..conceals both the limited availability of the starting material and the enormous amount of work required in establishing the proper conditions for each step." In Maier's own account, it took several years of careful observation and optimization to develop the correct conditions for the reactions. For instance, the synthesis of tetrakis(''t-''butyl)cyclopentadienone from the tris(''t''-butyl)bromocyclopentadienone (itself synthesized with much difficulty) required over 50 attempts before working conditions could be found.

Maier began with cycloaddition of an alkyne to ''t''-Bu substituted maleic anhydride. Rearrangement and decarboxylation gave a corset-stabilized cyclopentadienone. To add the fourth ''t''-Bu group, Maier brominated the only labile hydrogen to give an electrophile that coupled directly to tert-butyllithium. Photochemical cheletropic decarbonylation then gave the target.
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Heating tetra-''tert''-butyltetrahedrane gives tetra-''tert''-butyl cyclobutadiene. The reversibility of this rearrangement proved key to developing a more scalable synthesis. In the last step, photolysis of a cyclopropenyl-substituted diazomethane affords the desired product through a tetrakis(''tert''-butyl)cyclobutadiene intermediate:
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Trimethylsilyl tetrahedranes

Tetrakis(trimethylsilyl)tetrahedrane can be prepared by treatment of the cyclobutadiene precursor with tris(pentafluorophenyl)borane and is far more stable than the ''tert''-butyl analogue. The silicon–carbon bond is longer than a carbon–carbon bond, and therefore the corset effect is reduced. Whereas the ''tert''-butyl tetrahedrane melts at 135  °C concomitant with rearrangement to the cyclobutadiene, tetrakis(trimethylsilyl)tetrahedrane, which melts at 202 Â°C, is stable up to 300 Â°C, at which point it cracks to bis(trimethylsilyl)acetylene.

The tetrahedrane skeleton is made up of banana bonds, and hence the carbon atoms are high in s-orbital character. From NMR, sp- hybridization can be deduced, normally reserved for triple bonds. As a consequence the bond lengths are unusually short with 152 picometers.

Reaction with methyllithium with tetrakis(trimethylsilyl)tetrahedrane yields tetrahedranyllithium. The lithium compound can then couple to electrophiles, even relatively small ones.

A bis(tetrahedrane) has also been reported. The connecting bond is even shorter with 143.6 pm. An ordinary carbon–carbon bond has a length of 154 pm.

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Tetrahedranes with non-carbon core atoms

The tetrahedrane motif occurs broadly in chemistry. White phosphorus (P₄) and yellow arsenic (As₄) naturally form tetrahedrane-like clusters. There are a wide variety of synthetic pnictogen-substituted tetrahedranes, and metallatetrahedranes with a single metal (or phosphorus atom) capping a cyclopropyl trianion also exist. Several metal carbonyl clusters are referred to as tetrahedranes, e.g. tetrarhodium dodecacarbonyl.
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Silicon also can be induced to form a tetrahedral core, but heavier adamantogens tend to form cubane-like clusters.

Tetrasilatetrahedrane

In tetrasilatetrahedrane features a core of four silicon atoms. The standard silicon–silicon bond is much longer (235 pm) and the cage is again enveloped by a total of 16 trimethylsilyl groups, which confer stability. The silatetrahedrane can be reduced with potassium graphite to the tetrasilatetrahedranide potassium derivative. In this compound one of the silicon atoms of the cage has lost a silyl substituent and carries a negative charge. The potassium cation can be sequestered by a crown ether, and in the resulting complex potassium and the silyl anion are separated by a distance of 885 pm. One of the Si^−–Si bonds is now 272 pm and the tetravalent silicon atom of that bond has an inverted tetrahedral geometry. Furthermore, the four cage silicon atoms are equivalent on the NMR timescale due to migrations of the silyl substituents over the cage.

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The dimerization reaction observed for the carbon tetrahedrane compound is also attempted for a tetrasilatetrahedrane. In this tetrahedrane the cage is protected by four so-called supersilyl groups in which a silicon atom has 3 ''tert''-butyl substituents. The dimer does not materialize but a reaction with iodine in benzene followed by reaction with the tri-''tert''-butylsilaanion results in the formation of an eight-membered silicon cluster compound which can be described as a dumbbell (length 229 pm and with inversion of tetrahedral geometry) sandwiched between two almost-parallel rings.

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See also

* Dodecahedrane
* Prismane
* Prismane

References

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Cycloalkanes
Cluster chemistry
Hypothetical chemical compounds
Tricyclic compounds
Tetrahedra