2.5- Cyclohexadienones, excited states mechanism

Still another example of bond order control of photochemical reactivity is found in the photochemistry of 2,5-cyclohexadienones. Here, strong evidence suggests that the n-7t excited triplet is the species responsible for the rearrangement6,12,17 23). It is seen that the critical step is P, (3-bonding in the excited state. The reaction mechanism is depicted in Scheme 2. 

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 effectiveness of the UV absorbers may not be completely explained in terms of tautomerism. Among other mechanisms that have been proposed are energy transfer, involving an excited triplet state of the polymer (B-79MI11506), and peroxy radical scavenging, involving the formation of stable cyclohexadienones (78MI11502). 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,w excitation. Thus, there is little doubt in this case that it is the ir,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cyclohexadienones have likewise ruled out the... 

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