Cyclooctatetraene molecular orbitals

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

FIGURE 11 13 Frost s circle and the TT molecular orbitals of (a) square cyclobutadiene (b) ben zene and (c) planar cyclooctatetraene... 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

Fig. 2. Correlation of the molecular orbitals of cyclooctatetraene with those of pentalene...

Figure 2.28 Relative energies of benzene and cyclooctatetraene molecular orbitals from Frost circles...

The simple resonance theory fails to explain the singular lack of effectiveness of delocalization in cyclobutadiene and cyclooctatetraene, but we may turn to molecular orbitals for the solution. 

Another aspect of the chemistry of M(CO) fragments that the computed molecular orbital diagram could help to explain was the structure of the cyclooctatetraene complexes of iron and chromium tricarbonyl.10 Chromium tricarbonyl was shown to have three relatively low-lying vacant orbitals, with the right spatial characteristics to be able to accept electron donation from three of the double bonds of cyclooctatetraene, and accordingly adopts the r geometry shown in Figure 10.4. In iron... 

Energies of the pi molecular orbitals of cyclooctatetraene and the cycloheptatrienyl carbocation. Note that the electrons are not yet shown in the MOs in these diagrams. 

Of course, this isn t the molecular orbital energy level diagram for real cyclooctatetraene since COT is not planar but tub-shaped. 

Although Hiickel molecular orbital theory is not completely consistent in its application to nonalternant systems such as the pentalenyl dianion, it does implicate a certain degree of stabilization for this species. If the transannular bond introduces little or no perturbation, then the pentalenyl dianion is seen to be closely related to the cyclooctatetraene dianion. Katz and his co-workers successfully developed a... 

A molecule with An n electrons in the ring, with the molecular orbitals made up from An p orbitals, does not show this extra stabilisation. Molecules in this class that have been made include cyclobutadiene 1.13 (n = 1), the cyclopentadienyl cation 1.14, cyclooctatetraene 1.15 andpentalene 1.16 (n 2), [12]annulene 1.17 (n = 3) and [16]annulene 1.18 (n A). 

Andrew Streitwieser, Jr. and Ukich Muller-Westerhoff Bis(cyclooctatetraene)uranium(IV), in which tt-molecular orbitals share electrons with uranium f-orbitals... 

Ac = Acetyl acac = Acetylacetonate bda = Benzylidene-acetone BINOL = l,l -bi-2-naphthol Bn = Benzyl brsm = Yield based on recovered starting material Bu = Bntyl CAN = Ceric anunonium nitrate CBS = Corey/Bakshi/Shibata catalyst [(+) or (—)-(S)-2-methyl-oxazaborolidine] COD = Cyclo-l,5-octadiene COT = Cyclooctatetraene Cp = Cyclopentadienyl Cp = Penta-methylcyclopentadienyl Cy = Cyclohexyl DCC = Dicy-clohexylcarbodiimde DMF = Ai,A-dimethylformainide DMPU = l,3-dunethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidin-one DMSO = Dimethylsnlfoxide dppe = Diphenylphosp-hinoethane dr = Diastereomer ratio dppm = Diphenylphos-phinomethane E = Electrophile ee = Enantiomeric excess EHMO = Extended htickel molecular orbital Et =... 

Abbreviations ave, average cot, cyclooctatetraene Cp, tT -cyclopentadienyl DME, 1,2-dimethoxyetbane DMF, A, iV-dimethylformamide dppe, Ph2PCH2CH2PPIi2 ESR, electron spin resonance Fc, ferrocenyl HOMO, highest occupied molecular orbital LUMO, lowest unoccupied molecular orbital see, saturated calomel electrode TCNQ, 7,7,8,8-tetracyano-p-quinodimethane THF, tetrahydrofiiran. 

Use the inscribed polygon method to show the pattern of molecular orbitals in cyclooctatetraene. 

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