Cyclooctatetraene structural isomers

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

From the cyclooctatriene - bicyclooctadiene equilibrium, it is possible to trap the structure containing the four-membered ring with dienophiles, e.g. maleic anhydride, including those cases where the bicyclic isomer is only present in small amounts. For example, the dimer of cyclooctatetraene 7 on reaction with maleic anhydride gives the 1 1 adduct 8 in refluxing benzene and the. n 7-tricyclo[4.2.0.02,5]oclane derivative 9 in refluxing toluene.62... 

Interestingly, treating (>/4-cyclooctatetraene)Fe(CO)3 with acetyl chloride under Friedel-Crafts reaction conditions yielded unexpectedly222-223 the (>/2,>/3-8-e.x0-acetyl bicy-clo[3.2.1]octadienylium)Fe(CO)3 cation complex, presumably by rearrangement of the intermediate bicyclo[5.1.0]octadienylium isomer (Scheme 8). The structure of the rearranged cation was confirmed from the X-ray crystal structure and from the typical 1,3-cr.ji-allylic products obtained upon nucleophilic reaction with LiAlD4 and NaCN. The nucleophilic reaction of the more bulky iodide occurs, however, on the metal. 

Intramolecular photoaddition has been observed in nonconjugated olefins. Thus irradiation of Formula 410 gives the cage structure of Formula 411 (181). Two isomers of the hexachlorocyclopentadiene-cyclooctatetraene adduct give analogous cage structures on irradiation... 

In some additional studies concerning cyclooctatetraene-iron carbonyl chemistry, Keller, Emerson, and Pettit 145) have isolated two additional isomers having the composition (C8H8)Fe2(CO)6, for which Structures XXXVII and XXXVIII have been proposed on the basis of infrared, NMR, and Mossbauer measurements. 

The spectrum of cyclooctatetraene (COT) contains a single peak, but differe at spectra arc observed for 1,2-dimethyl cyclooctatetraene and 2,3-dimethylcycIo octatetraene, indicating that these molecules are isomers rather than resonance structures. Use SpartanBuild to build and minimize COT and the two dimethyl derivatives. Examine their structures, and explain the data. How many pea does the spectrum of each dimethylcyclooctatetraene have ... 

A third product of the iron carbonyl-cyclooctatriene reaction is a red complex of formula CgHioFe2(CO)g, which was initially formulated as C9Hi2Fe2(CO)e (61). This is derived from the 1,3,5-isomer and probably has a bis(7r-allyl) structure like the corresponding complexes with cyclo-heptatriene (Section II, A) and cyclooctatetraene (Section III, B) (46, 90). 

Sol 14. (a) The c/i-orientation of hydrogens in the product indicates that cyclization involves disrotatory mode. Under thermal conditions, cyclo-octatetraene preferentially undergoes a hrr-electron disrotatory electrocyclization forming only the cw-isomer. It should be noted that cyclooctatetraene does not undergo the thermally allowed 4TT-electron conrotatory electrocyclization as the frani-fused bicyclic structure is highly strained. 

However, under photochemical conditions, cyclooctatetraene preferentially undergoes a 4TT-electron disrotatory electrocyclization forming only the c/i-isomer. Cyclooctatetraene does not undergo the photochemically allowed hrr-electron conrotatory electrocyclization, which generates the less stable frani-fused bicyclic structure. 

These are interconversions of (CH) molecules, in which no hydrogens move, only skeletal rearrangements occur. This may seem a fairly narrow class, but it is remarkably rich. For example, cyclooctatetraene is a (CH) molecule. However, there are at least 21 such structures, cubane being another example. Typically, a large number of pericyclic reactions can be envisioned that would interconvert all the (CH)s valence isomers. For an example of such an analysis, see the article by L. R. Smith. 

Figure 3.46. Structural and conformational isomers of cyclooctatetraene derivatives.

Other equilibria which have been studied by NMR spectroscopy include those of oxepine-benzene oxide isomerization (Figure 3.45), structural and conformational isomers of cyclooctatetraene derivatives (Figure 3.46), car-bocations, and organometallic compounds. 

Compound (XLI) does not undergo a Diels-Alder reaction with maleic anhydride but recently a 1 1 adduct with tetracyanoethylene has been obtained (30, 61). Schrauzer and Eichler (61) have found that two isomeric products, presumed to be Fe(CO)3 complexes of cyclooctatetraene dimers, are obtained when (XLI) is irradiated in the presence of cyclooctatetraene. As mentioned in Section III, A, 2, these isomers, upon irradiation with Fe(CO)5, eliminate benzene to form the same binuclear complex CioHtoFe2 (CO)g. The structures of these complexes have not been rigorously established. 

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