Other Kinds of Selectivity in Pericyclic and Related Photochemical Reactions

3 Other Kinds of Selectivity in Pericyclic and Related Photochemical Reactions 

Clearly there is a strong electronic effect determining the regioselectivity, but accounting for it has not been straightforward. The frontier orbitals are ip3 and r/ 4 of hexadiene, modified by the presence of an X- or 

The two pathways, electrocyclic closure to the bicyclo[3.2.0]heptadiene and the [l,7]-shift of hydrogen, are in competition. This makes the experimental work more difficult, because the four possible isomers of the cycloheptatriene interconvert during the photolysis. Nevertheless the pattern of reactivity is clear, and the observation is that the better the donor substituent, the more it favours the formation of the bicyclo[3.2.0]heptadiene at the expense of the [l,7]-shift. This too is reasonable, since the hydride migration towards the donor-substituted atom 8.187 leads to a less stable cation, and ought to be slowed down by that feature. 

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