Superphane
Encyclopedia
[2.2.2.2.2.2]Cyclophane or superphane is a 6-fold bridged cyclophane
with all arene positions in the benzene dimer taken up by ethylene spacers. The compound has been of some scientific interest as a model for testing aromaticity
and was first synthesised by Boekelheide in 1979. Superphane is the base compound for a large group of derivatives with structural variations. The analogs with 2 to 5 bridges are also known compounds. The benzene rings have been replaced by other aromatic units, such as those based on ferrocene
or stabilized cyclobutadiene
. Numerous derivatives are known with variations in the type and length of the bridging units.
to form o-xylylene
s, either directly or via benzocyclobutene
intermediates. Upon further pyrolysis, these each undergo electrocyclic ring-opening
to form o-xylylene
s. These structures were not isolated—they immediately react via [4+4] cycloaddition
reactions to form two adjacent bridges between the aromatic rings.
The process started from 2,4,5-trimethylbenzyl chloride 1, which was pyrolyzed at 700 °C to give benzocyclobutene 2 and further pyrolyzed to the cyclooctane
dimer 3. Rieche formylation
afforded 4 (after separation from other regioisomers), aldehyde reduction using sodium borohydride
gave diol 5, and then chlorination
using thionyl chloride
) gave dichloride 6. Another pyrolysis gave tetrabridged cyclophane 7, another formylation reaction
gave dialdehyde 8, another reduction/chlorination sequence gave dichloride 9, and a final pyrolysis gave superphane 10 as hard white crystals with melting point
325–327 °C.
Other synthetic routes were published by Hopf (1983) and another by Boekelheide (1984).
with the aromatic planes separated by 262 pm. The sp2-sp3 carbon carbon bonds are out of planarity with the benzene rings by 20°. The strain energy
is estimated at 20 kcal/mole. Proton NMR
shows just one peak at 2,98 ppm and carbon NMR two at 32 ppm and 144 ppm.
Cyclophane
A cyclophane is a hydrocarbon consisting of an aromatic unit and an aliphatic chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known...
with all arene positions in the benzene dimer taken up by ethylene spacers. The compound has been of some scientific interest as a model for testing aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
and was first synthesised by Boekelheide in 1979. Superphane is the base compound for a large group of derivatives with structural variations. The analogs with 2 to 5 bridges are also known compounds. The benzene rings have been replaced by other aromatic units, such as those based on ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
or stabilized cyclobutadiene
Cyclobutadiene
Cyclobutadiene is the smallest [n]-annulene , an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula 44 and a rectangular structure verified by infrared studies. This is in contrast to the square geometry predicted by simple Hückel...
. Numerous derivatives are known with variations in the type and length of the bridging units.
Synthesis
The first synthesis of superphane itself by Boekelheide involved forming pairs of bridging units. At each stage, two o-chloromethyl toluene structures are pyrolyzedPyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
to form o-xylylene
Xylylene
Xylylene comprises two isomeric organic compounds with the formula C6H42. These compounds are related to the corresponding quinones by replacement of the oxygen atoms by CH2 groups. ortho- and para-xylylene are best known, although neither is stable in solid or liquid form...
s, either directly or via benzocyclobutene
Benzocyclobutene
Benzocyclobutene is a benzene ring fused to a cyclobutane ring. It has chemical formula 88.BCB is frequently used to create photosensitive polymers. BCB-based polymer dielectrics may be spun on or applied to various substrates for use in Micro Electro-Mechanical Systems and microelectronics...
intermediates. Upon further pyrolysis, these each undergo electrocyclic ring-opening
Electrocyclic reaction
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement reaction where the net result is one pi bond being converted into one sigma bond or vice-versa...
to form o-xylylene
Xylylene
Xylylene comprises two isomeric organic compounds with the formula C6H42. These compounds are related to the corresponding quinones by replacement of the oxygen atoms by CH2 groups. ortho- and para-xylylene are best known, although neither is stable in solid or liquid form...
s. These structures were not isolated—they immediately react via [4+4] cycloaddition
Cycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...
reactions to form two adjacent bridges between the aromatic rings.
The process started from 2,4,5-trimethylbenzyl chloride 1, which was pyrolyzed at 700 °C to give benzocyclobutene 2 and further pyrolyzed to the cyclooctane
Cyclooctane
Cyclooctane is a cycloalkane with the molecular formula 8. It is a simple colourless hydrocarbon, but it is often a reference compound for saturated eight-membered ring compounds in general.- Conformation :...
dimer 3. Rieche formylation
Rieche formylation
Rieche formylation is a type of formylation reaction . The substrates are sterically hindered aromatic compounds - for example mesitylene - and the formylation reagent dichloromethyl methyl ether. The catalyst is titanium tetrachloride, the workup is acidic. The reaction is named after Alfred...
afforded 4 (after separation from other regioisomers), aldehyde reduction using sodium borohydride
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
gave diol 5, and then chlorination
Chlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...
using thionyl chloride
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...
) gave dichloride 6. Another pyrolysis gave tetrabridged cyclophane 7, another formylation reaction
Formylation reaction
A formylation reaction in organic chemistry is the catch-all name for any organic reaction in which an organic compound is functionalized with a formyl group .Aromatic formylation reactions via electrophilic aromatic substitution include:...
gave dialdehyde 8, another reduction/chlorination sequence gave dichloride 9, and a final pyrolysis gave superphane 10 as hard white crystals with melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
325–327 °C.
Other synthetic routes were published by Hopf (1983) and another by Boekelheide (1984).
Structure and properties
X-ray analysis shows D6h molecular symmetryMolecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
with the aromatic planes separated by 262 pm. The sp2-sp3 carbon carbon bonds are out of planarity with the benzene rings by 20°. The strain energy
Strain energy
In a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
is estimated at 20 kcal/mole. Proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
shows just one peak at 2,98 ppm and carbon NMR two at 32 ppm and 144 ppm.