Everything about Pyramidal Alkene totally explained
Pyramidal alkenes are
alkenes in which the two
carbon atoms making up the
double bond are not
coplanar with their four
substituents . This deformation from a
trigonal planar geometry to a
tetrahedral molecular geometry is the result of
angle strain induced in the molecule due to geometric constraints. Pyramidal alkenes only exist in the laboratory but are of interest because much can be learned from them about the nature of
chemical bonding .
In
cycloheptene (
1.1) the
cis isomer is an ordinary unstrained molecule but the heptane ring is too small to accommodate a trans configured alkene group resulting in strain and twisting of the double bond. The p-orbital misalignment is minimized by a degree of
pyramidalization. In the related
anti-bredt molecules it isn't pyrimidalization but twisting that dominates.
Pyramidalized cage alkenes also exist where symmetrical bending of the substituents predominates without p-orbital misalignment.
The
pyramidalization angle (
b) is defined as the angle between the plane defined by one of the doubly bonded carbons and its two substituents and the extension of the double bond and is calculated as:
»
the
butterfly bending angle or
folding angle (
c) is defined as the angle between two planes and can be obtained by averaging the two
torsional angles R1C:::CR3 and R2C:::CR4.
In alkenes
1.2 and
1.3 these angles are determined with
x-ray crystallography as respectively 32.4°/22.7° and 27.3°/35.6°. Although stable, these alkenes are very reactive compared to ordinary alkenes. They are liable to dimerization to
cyclobutadiene compounds or react with oxygen to
epoxides.
The compound
tetradehydrodianthracene also with a 35° pyramidalization angle is synthesized in a
photochemical cycloaddition of
bromoanthracene followed by
elimination of
hydrogen bromide
This compound is very reactive in
Diels-Alder reactions due to
through-space interactions between the two alken groups. This enhanced reactivity enabled in turn the synthesis of the first-ever
Möbius aromat.
In one study the strained alkene
3.4 was synthesized with the highest pyramidalizion angles yet, 33.5° and 34.3°. This compound is the double
Diels-Alder adduct of the diiodide-
cyclophane 3.1 and
anthracene 3.3 by reaction in presence of
potassium tert-butoxide in
refluxing dibutyl ether through a di-
aryne intermediate
3.2. This is a stable compound but will slowly react with oxygen to an
epoxide when left standing as a
chloroform solution.
In one study , isolation of a pyramidal alkene isn't even possible in by
matrix isolation at extremely low temperatures unless stabilized by
metal coordination:
A reaction of the di-
iodide 4.1 in fig. 4 with
sodium amalgam in the presence of
ethylenebis(triphenylphosphine)platinum(0) doesn't give the intermediate
alkene 4.2 but the platinum stabilized
4.3. The sigma bond in this compound is destroyed in reaction with
ethanol.
Further Information
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