A fenestrane in
organic chemistryis a type of chemical compoundwith a central quaternary carbonatom which serves as a common vertex for four fused carbocycles ["Fenestranes and the flattening of tetrahedral carbon" Bhaskar Rao Venepalli and William C. Agosta Chem. Rev.; 1987; 87(2) pp 399 - 410; DOI|10.1021/cr00078a007] . 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 "Syntheses directed toward saturated “flat” carbon" Vlasios Georgian Martin Saltzman Tetrahedron LettersVolume 13, Issue 42 , 1972, Pages 4315-4317 doi|10.1016/S0040-4039(01)94304-7] is derived from the Latinword for window: "Fenestra".
The smallest member of the family consisting of 4 fused
cyclopropanerings is [3,3,3,3] fenestrane or pyramidane a molecule related to tetrahedranewith an extensive history on its own. In the next member 4 cyclobutanerings 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 geometrygets completely flattened. In the molecular orbitalpicture for square planar methanetwo 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 bondwith 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 silicocalculations 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 picometerwhile those at the perimeter are extended (159 pm.)
The first ever synthesized fenestrane is a [4,5,5,6] fenestrane [The first step in this reaction sequence is an adaptation of the
Stork enamine alkylationreacting cyclopentanonewith 3-bromo-1-butene through an iminederivative with pyrrolidineand forming a magnesium salt with ethyl magnesium bromide. The next step is a regular Stork enamine reaction followed by an aldol condensationforming the cyclohexenonering. The final step is a photolytic [2+2] cycloaddition. ] :
In a recent effort a [4,5,5,5] fenestrane was synthesized with one carbon atom replaced by nitrogen because
azacompounds and their salts are more likely to form crystalline compounds suitable for x-ray analysis ["Synthesis, X-ray Crystallography, and Computational Analysis of 1-Azafenestranes" Scott E. Denmark, Justin I. Montgomery, and Laurenz A. Kramps J. Am. Chem. Soc.; 2006; 128(35) pp 11620 - 11630; (Article) DOI|10.1021/ja0632759] [In step 1 the alkyl halide"1-iodo-3-butene" 1 is converted to a cyanozinc cuprate 2 (by transmetalationof the organozinciodide with copper cyanide) which reacts in the next step with "1-nitro-cyclopentene" 3 in a nucleophilic additionwhereby the nitronate4 is captured by phenylselenenyl bromideto the selenium intermediate 5. Hydrogen peroxide oxidationof 5 yields the nitroalkene 6 as a mixture of syn and anti isomers. A [4+2] cycloadditionwith n-butyl- enol etherin presence of trimethylaluminiumgives the nitronate7 and a second [3+2] cycloaddition by heating in presence of potassium carbonategives the nitroso acetal8. Hydrogenationwith Raney nickelgives the diol9 which on a double Mitsunobu reaction(with an amine proton donor) gives the azafenestrane 10 as the boranesalt.] :
boranesalt the N-C-C angle is now 126°.
One study describes an unusual 8π
disrotatory- 6π conrotatory electrocyclic cascade reactionaiming to minimise the number of steps required to synthesise a fenestrane. ["Synthesis of [22.214.171.124] Fenestradienes and [126.96.36.199] Fenestrenes Based on an 8π−6π-Cyclization-Oxidation Cascade" Hulot, C.; Blond, G.; Suffert, J. J. Am. Chem. Soc.; (Communication); 2008; 130(15); 5046-5047. DOI|10.1021/ja800691c ] [Reagents: P-2 Ni (Ni(OAc)2.4H2O) / hydrogengas. Reaction initiated by organic reductionof alkyneto alkene] :
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