Prismane photochemical formation

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

Pyridines show a number of thermal and photochemical transformations which are analogous to the valence tautomerization of benzene, i.e. formation of Dewar benzene, prismane and benzvalene. 

The Photochemical synthesis of alkyl substituted benzene takes advantage of various pathways to provide one or more of the valence bond isomers or a positional isomer of the starting material is illustrated for 1,2,4-tiibutyl benzene, shown below. The formation of the 1,3,5-isomer results from cycloreversion of the prismane depicted or arises via the benzvalene to deld the least sterically congested product. By an analogus sequence o-xylene can gives nv and p-isomer. 

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