Fenestrane
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 with an extensive history on its own. In the next member, 4 cyclobutane rings are fused, forming 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 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.
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