Cyclohexadienones, complex rearrangements

Elaborate mechanistic schemes have been suggested for the principal rearrangements of cyclohexenone, 2,5-cyclohexadienone, and bicyclo-hexenone systems induced by w - tt excitation which are compatible with the experimental data outlined above. In essence, these mechanisms are based on the common concept that the complicated structural changes are initiated in an electronically excited state. For the appreciably complex ketones considered, reaction initiation in a vibrationally excited ground state produced by adiabatic ir n demotion is expected to be readily suppressed in solution by collisional deactivation. It has been pointed out that by this general concept the rearrangements provide a decay path for electronically excited states which allows transfer of minimal amounts of enei to the environment in each step. 

Moderate yields of 2- and 4-cyclopropylanisoles are obtained by irradiation of the corresponding allylanisoles. The di-7c-methane rearrangement provides a general photochemical route to cyclopropane derivatives and its mechanism and peculiarities are still under intensive investigation. These are better discussed in detail in a review on photochemistry, as are the complex bond reorganizations observed on irradiation of cyclohexanones and cyclohexadienones. 

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