- Antiaromaticity
Antiaromatic
molecule s arecyclic systems containing alternating single anddouble bond s, where thepi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Therefore antiaromatic compounds are unstable and highly reactive; often antiaromatic compounds distort themselves out of planarity to resolve this instability. Antiaromatic compounds usually fail theHückel's rule ofaromatic ity.Examples of antiaromatic systems are
cyclobutadiene (A), the cyclopentadienyl cation (B) and the cyclopropenyl anion (C).Cyclooctatetraene is a 4n system but neither aromatic or antiaromatic because the molecule escapes a planar geometry.By adding or removing an electron pair via a
redox reaction, a π system can become aromatic and therefore more stable than the original non- or anti-aromatic compound, for instance thecyclooctatetraenide dianion . TheIUPAC criteria for antiaromaticity are as follows: [Compendium of Chemical Terminology , [http://goldbook.iupac.org/A00382.html antiaromatic compounds] , accessed 1 Feb 2007.]
# The molecule must have 4"n" π electrons where "n" is any integer.
# The molecule must be cyclic.
# The molecule must have a conjugated pi electron system.
# The molecule must be planar.However, most chemists agree on the definition based on empirical (or simulated) energetic observations.Fact|date=February 2007
It is observed that the energy difference between aromatic and antiaromatic compounds diminishes with increasing size ["A Thiadiazole-Fused N,N-Dihydroquinoxaline: Antiaromatic but Isolable" Shaobin Miao, Paul v. R. Schleyer, Judy I. Wu, Kenneth I. Hardcastle, and Uwe H. F. Bunz
Org. Lett. ; 2007; 9(6) pp 1073 - 1076; (Letter) DOI|10.1021/ol070013i] . For instance the 12-pi systemdiphenylene is an antiaromatic compound but stable and even commercially available. The low energy penalty for antiaromaticity is also demonstrated in certainpyrazine -dihydropyrazine pairs::
The compound on the left is a 14 pi-electron aromatic compound (NICS value -26.1 ppm) which can be reduced in a strongly
exothermic reaction to the 16 pi-electron antiaromatic compound (NICS +27.7 ppm) on the right [Reducing agent:Sodium hypophosphite , tips stands fortriisopropylsilyl ] . The dihydropyrazine slowly converts back to the pyrazine under the action ofoxygen . It shows that other electronic factors can overpower aromaticity.Antiaromaticity is also observed in a
chemical equilibrium between these twoporphyrin derivatives ["22-Hydroxybenziporphyrin: Switching of Antiaromaticity by Phenol-Keto Tautomerization Marcin Stpie", Lechosaw Latos-Grayski, and Ludmia SzterenbergJ. Org. Chem. ; 2007; 72(7) pp 2259 - 2270; (Article) DOI|10.1021/jo0623437] ::A regular porphyrin is an 18 electron aromatic compound (not counting two non-contributing double bonds) but on substituting apyrrole ring by a meta-phenylene ring aromaticity is lost due to lack ofconjugation . In this system the phenylene group is also aphenol and structure A is found to interconvert with 20 electron antiaromat B via phenol-keto tautomerism. Antiaromaticity is evident fromNMR spectroscopy with the inner NH protons shifting downfield by 10 ppm to 21 ppm. The NICS values compare +0.7 for A (non-aromatic) and +5 (antiaromatic) for B and otherin silico experimentation predicts that B is actually more stable than A.References
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