2.5- Cyclohexadienones, excited states rearrangements

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

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. 

The reactive excited state of dienone is n -> Ji triplet state. The excited cyclohexadienones undergo rearrangement reactions. The rearrangement has been extensively studied for 4, 4- diphenylcyclohexa-2, 5-dienone. 

The presence of other rings fused to the cyclohexadienone chromophore may lead to failure of the photochemical rearrangement, probably as a consequence of steric factors preventing P,P -bonding in the excited states. This is the case with dienones 71, 72 and 73. ... 

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