Cyclooctatetraene interconversion

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. 

Interconversions between syn- 21, anti- 21, and cyclooctatetraene 24 take place during irradiation and also lead to 22 and 23 as the major products. Valence isomers 21, 22, and 23 are all quantitatively converted into the more stable perfluoro(octamethylcyclooctatetraene) (24) upon heating at 300 C. 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

This would appear to be analogous to the cyclooctatetraene (COT)-bicy-clo[4.2.0]octa-2,4,7-triene interconversion (See Chapter 9, Section 2). The product, pentaene, is remarkably stable, dimerizing only slowly at 100°C. 

It has long been known that the interconversion of cyclooctatetraene (COT) and bycyclo[4.2.0]octa-2,4,7-triene (BCO) is rapid in both directions at relatively low temperatures (ca. 100°C) [19, 20] the product has a cis junction, so the disro-tatory pathway is evidently preferred. The conventional procedure would be to freeze one tt bond and regard the reaction as being an allowed disrotatory six-electron cyclization of the hexatriene moiety. One could, of course, freeze one pair of adjacent tt bonds instead, and produce the imns-joined product by a no less allowed conrotatory four-electron cyclization of the residual butadiene system. In view of the fact that adjacent double bonds of cyclooctatetraene (COT) are twisted away from each other in its stable tub conformation, the latter mode would seem to be the more facile, so the failure to observe the tran - o ned isomer has to be rationalized in terms of its lesser thermodynamic stability, rather than a symmetry-imposed potential barrier along the conrotatory pathway. 

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. 

PROBLEM 20.29 The thermally induced interconversion of cyclooctatetraene (1) and bicyclo[4.2.0]octa-2,4,7-triene (2) can be described with two different sets of arrows. Although these... 

Photo-induced bond switching in substituted cyclooctatetraenes has been reported Anet, EA.L. and Bock, L.A., Photochemical interconversion of cyclooctatetraene bond shift isomers, J. Am. Chem. Soc., 90, 7130, 1968. 

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