Cyclooctatetraene. and

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

The same thought occurred to early chemists However the complete absence of natu rally occurring compounds based on cyclobutadiene and cyclooctatetraene contrasted starkly with the abundance of compounds containing a benzene unit Attempts to syn thesize cyclobutadiene and cyclooctatetraene met with failure and reinforced the grow mg conviction that these compounds would prove to be quite unlike benzene if m fact they could be isolated at all... 

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... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

In the next section we 11 explore Huckel s rule for values of n greater than 1 to see how it can be extended beyond cyclobutadiene benzene and cyclooctatetraene... 

The pattern of orbital energies in Figure 11.13 provides a convincing explanation for why benzene is aromatic while square cyclobutadiene and planar- cyclooctatetraene are not. We start by counting tt electrons cyclobutadiene has four, benzene six, and cyclooctatetraene has eight. These tt electrons are assigned to MOs in accordance with the usual rules—lowest energy orbitals first, a maximum of two electrons per orbital. 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

A warning was given that the 5 molar solution in ether used as a solvent for Diels-Alder reactions would lead to explosions [1], Such a reaction of dimethyl acetylenedicarboxylate and cyclooctatetraene in this solvent exploded very violently on heating. The cyclooctatetraene was blamed, with no supporting evidence [2], It would appear desirable to find the detonability limits of the mixture with ether before any attempt is made to scale up. A safe alternative to lithium perchlorate/ether as a solvent for Diels-Alder reactions is proposed [3],... 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

In addition to cycloheptatriene, cyclooctatriene and cyclooctatetraene systems can be used as six-carbon components in the [6 + 2]-cycloaddition with 7t-systems (Scheme 36). Interestingly, in these cases, dienes react exclusively... 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

Cyclooctadienes, Cyclooctatrienes and Cyclooctatetraene. As mentioned in Section III.E, the 70 eV El mass spectra of the isomeric cyclooctadienes are strikingly different87. Not surprisingly, the three possible stereoisomeric 1,5-cyclooctadienes give similar spectra, the product ions C4H6+ of the apparent [4 + 4] cycloreversion, i.e. loss of... 

C. Deprotonation of Bicyclo[3.2.1]alkadiene, Some Other Cycloalkadienes and Cyclooctatetraene Bishomoaromaticity and Transannular Cyclization... 

Bis(trifluoromethyl)thioketene (42) is sufficiently stable to handle. It readily adds to 2,3-dimethyl-1,3-butadiene to yield 43 (equation 28)28, l,2,4,7-(tetrakis)methylenecyclooc-tane gives 44 (equation 29)29 and cyclooctatetraene affords 46 via the valence isomer 45 (equation 30)30. 

The triazolinedione adds to cycloheptatriene and cyclooctatetraene to yield the valence-isomeric adducts 265 and 266, respectively (equations 143 and 144)136. 

Di(thiocyano)thiophene also presents a different behavior at a mercury-dropping electrode and in the case of treatment by dipotassium salt of cyclooctatetraene dianion THF is the same solvent for both cases (Todres et al. 1979). The reaction between di(thiocyano) derivative and CjHgKj taken in equimolecular amounts leads to the formation of potassium salt of 2-mercapto-5-thiocyanothiophene (potassium mercaptide), potassium cyanide, and cyclooctatetraene (see Scheme 2.17). Potassium mer-... 

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

The compound HBNH is expected to be thermodynamically unstable toward oligomerization. The cyclodimer, (HBNH)2, isoelectronic with cyclobutadiene, is found to be more stable by 63.3 kcal/mol, and the cyclotrimer, (HBNHlj, by 193.8 kcal/mol than 2 or 3 mol of HBNH, respectively (23). The oligomerization energy is due to an increase in the number of (7-bonds at the expense of the relatively weak Tt-bonds. Note that the situation is the same for acetylene. Oligomers like cyclobutadiene, benzene, benzvalene, and cyclooctatetraene are... 

The synthesis of bis(rj8-cyclooctatetraene)uranium(IV) (uranocene)J from uranium tetrachloride and (cyclooctatetraene)dipotassium was first published in 1968.1 The method reported here is a modification of that procedure and is suitable for a large variety of cyclooctatefraene complexes.2-4 BisO 8-cyclo-octatetraene)uranium(IV) has also been prepared by the reaction of uranium tetrafluoride with (cyclooctatetraene)magnesium in the absence of solvent.5 Direct reaction of finely divided uranium metal with cyclooctatetraene vapors at 150° also gives some uranocene.5 However, both methods give low yields. 

From these quenching experiments there is no evidence that during the quenching process a triplet cyclooctatetraene is formed. Unpublished results of interactions between rose bengal triplet and cyclooctatetraene at very low temperatures,113,114 however, seem to indicate the production of a very short-lived cyclooctatetraene triplet. 

D. A. Hrovat, J. H. Hammons, C. D. Stevenson, and W. T. Borden, Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-dg and Cyclooctatetraene-dg, J. Am. Chem. Soc. 1997,119, 9523. B3LYP/6-31+G calculations on the title compounds and on the radical anions formed from them show that the very large difference between the equilibrium isotope effects, found by Stevenson, is due to an inverse isotope effect on the planarization of the COT ring. This explanation was subsequently confirmed by KIE measurements, carried out by C. D. Stevenson, E. C. Brown, D. A. Hrovat, and W. T. Borden, Isotope Effects on the Ring Inversion of Cyclooctatetraene, J. Am. Chem. Soc. 1998, 120, 8864. 

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

VEELS studies have been made of the 1,3- and 1,5-cyclooctadienes and cyclooctatetraene adsorbed on Pt(lll) (260, 276). The spectra recorded at 170 K are consistent with rf interactions of the hydrocarbon with the surface (with spectra distinguishable from those of the 1,3- and 1,5-dienes themselves) involving losses of C = C groups. For cyclooctatetraene the... 

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