- Pyramidal alkene
Pyramidal alkenes are
alkene s in which the twocarbon atom s making up thedouble bond are notcoplanar with their foursubstituent s Ref|. This deformation from atrigonal planar geometry to atetrahedral molecular geometry is the result ofangle 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 ofchemical bond ing ["Chemistry of pyramidalized alkenes" Santiago Vázquez, and Pelayo Camps Tetrahedron Volume 61, Issue 22, 30 May 2005, Pages 5147-5208 DOI|10.1016/j.tet.2005.03.055] .In
cycloheptene (1.1) thecis 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 ofpyramidalization . In the relatedanti-bredt molecule s it is not 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 angle s 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 tocyclobutadiene compounds or react with oxygen toepoxide s.The compound "tetradehydrodianthracene" also with a 35° pyramidalization angle is synthesized in a
photochemical cycloaddition of bromoanthracene followed by elimination ofhydrogen bromide This compound is very reactive in
Diels-Alder reaction s 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 ["A Highly Pyramidalized Cage Alkene Formed via the Double Diels-Alder Cycloaddition of syn-4,5,13,14-Bis(dehydro)octafluoroparacyclophane to Anthracene" Dolbier, W. R., Jr.; Zhai, Y.-A.; Battiste, M. A.; Abboud, K. A.; Ghiviriga, I.;
J. Org. Chem. ; (Article); 2005; 70(25); 10336-10341. DOI: 10.1021/jo051488v [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/jo051488v Abstract] ] the strained alkene 3.4 was synthesized with the highest pyramidalizion angles yet, 33.5° and 34.3°. This compound is the doubleDiels-Alder adduct of the diiodide-cyclophane 3.1 andanthracene 3.3 by reaction in presence ofpotassium tert-butoxide inreflux ing dibutyl ether through a di-aryne intermediate 3.2. This is a stable compound but will slowly react with oxygen to anepoxide when left standing as achloroform solution.In one study ["Evidence for the Formation of the (Ph3P)2Pt Complex of 3,7-Dimethyltricyclo [3.3.0.03,7] oct-1(5)-ene, the Most Highly Pyramidalized Alkene in a Homologous Series. Isolation and X-ray Structure of the Product of the Ethanol Addition to the Complex" Fanitsa A. Theophanous, Anastasios J. Tasiopoulos, Athanassios Nicolaides, Xin Zhou, William T. G. Johnson, and Weston Thatcher Borden
Org. Lett. ; 2006; 8(14) pp 3001 - 3004; (Letter) DOI|10.1021/ol060994j] , isolation of a pyramidal alkene is not 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 withsodium amalgam in the presence ofethylenebis(triphenylphosphine)platinum(0) does not give the intermediatealkene "4.2" but the platinum stabilized "4.3". The sigma bond in this compound is destroyed in reaction withethanol .References
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