Semibullvalenes, photoreactions

Fig, 28. H CIDNP spectra 90 MHz) observed during the photoreaction of chloranil with semibullvalene (0.02 M each in acetonitrile-d3) during UV irradiation [391]... 

The di-TT-methane rearrangement was discovered and conceptually developed by H. E. Zimmerman (University of Wisconsin), one of the earliest examples being the photoisomerization of barrelene (1) into semibullvalene (2) (equation 2). Through intensive subsequent work, this photoreaction was shown to be quite general for 1,4-dienes. In view of the major contributions by the original author, the rearrangement is also referred to as the Zimmerman rearrangement. 

The valence isomerization of cyclooctatetraene to semibullvalene developed by H. E. Zimmerman may be rationalized in terms of an intramolecular cycloaddition. Direct formation of C-C bonds between C1-C5 and C4-C6 of a hexatriene unit seems to be the most probable reaction pathway. It should be noted that this photoreaction belongs to the rare examples of a gas phase photolysis useful for synthetic purposes. 

Figure 6.5 Phase-rule loops for photoreactions of COT. (a) The COT o semibullvalen conversion. All reactions in the loop constructed from the two Kekule forms of COT and semibullvalene are of Htickel type from the number of pairs of electrons that exchange spin, it is seen that the loop is ip and encloses a conical intersection, (b) Loop for the COT tricyclo[3.3.0.0 ]octa-3,7-diene conversion...

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