Cyclooctatetraene photochemical isomerization

Cyclooctatetraene (COT) —> Semibullvalene (SB) Photorearrangement. Irradiation of COT yields semibullvalene [97], in spite of the fact that this photochemical reaction is forbidden by orbital conservation mles. The Longuet-Higgins loop for this system actually predicts that this should happen, although the reaction is phase preserving. (Fig. 42). This is another example of type C loop (Fig. 11). Only six of the eight electrons re-pair as COT transforms to SB. The reaction is made possible by the fact that COT valence isomerization, a phase-inverting reaction (four electron-pair Hiickel system), takes place simultaneously. One expects to produce in the reaction a COT isomer, that can be detected solely by proper substitution. 

The primary ortho adducts formed from benzene derivatives and acetylenes are derivatives of bicyclo[4.2.0]octa-2,4,7-triene. These products usually are not isolated but isomerize during the irradiation to cyclooctatetraenes [58,59,68,72], From combinations of hexafluorobenzene and pentafluoroalkoxybenzenes with various disubstituted acetylenes, however, the isolation of relative stable primary ortho adducts has been reported [65-67], In Scheme 46, the products of the photochemical reaction of hexafluorobenzene with but-2-yne are shown [67],... 

Cyclooctatetraenes are one of the most interesting and well studied non-benzenoid (CH) families that provide a considerable variety of isomeric strained compounds.This section deals with the photochemical modification of cyclooctapyrimidines. 

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