Everything about Fenestrane totally explained
A
fenestrane in
organic chemistry is a type of
chemical compound with a central
quaternary carbon atom which serves as a common vertex for four
fused carbocycles . They can be regarded as
spiro compounds twice over. Because of their inherent
strain and instability fenestranes are of theoretical interest to chemists. The name proposed in 1972 by Vlasios Georgian and Martin Saltzman is derived from the
Latin word for
window:
Fenestra.
The smallest member of the family consisting of 4 fused
cyclopropane rings is
[3,3,3,3]fenestrane or
pyramidane a molecule related to
tetrahedrane with an extensive history on its own. In the next member 4
cyclobutane rings are fused to the archetypical window motif. It is called in its own
chemical nomenclature [4,4,4,4]fenestrane simply by counting the number of carbon atoms in each ring. The formal name for this compound is less catchy:
tetracyclo[3.3.1.03,9.07,9]nonane.
In an extreme case the central carbon atom ordinarily with a
tetrahedral molecular geometry gets completely flattened. In the
molecular orbital picture for
square planar methane two of a total of 3
sp2 hybridized carbon atomic orbitals form regular bonds with two of the hydrogen atoms as in a planar
alkene. The third sp2 orbital interacts in a
three-center two-electron bond with the two remaining hydrogen atoms utilizing only the hydrogen electrons. Two additional carbon
valence electrons are situated in a p-orbital perpendicular to the plane of the molecule. The four C-H bonds are equal because they
resonate.
In silico calculations show that it takes 95 to 250 kcal/mol (400 to 1,050 kJ/mol) for this process.
One of the highest strained fenestranes actually isolated is a [4,4,4,5]fenestrane with
bond angles through the central carbon atom of around 130° based on
X-ray diffraction. In this molecule the bonds extending from the central carbon atom are shortened with
bond lengths of 149
picometer while those at the perimeter are extended (159 pm.)
The first ever synthesized fenestrane is a [4,5,5,6]fenestrane :
Current research
In a recent effort a [4,5,5,5]fenestrane was synthesized with one carbon atom replaced by nitrogen because
aza compounds and their salts are more likely to form crystalline compounds suitable for x-ray analysis :
In the
borane salt the N-C-C angle is now 126°.
One study describes an unusual 8π
disrotatory - 6π
conrotatory electrocyclic cascade reaction aiming to minimise the number of steps required to synthesise a fenestrane. :
Further Information
Get more info on 'Fenestrane'.
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