2.5- Cyclohexadienones, excited states

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. 

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. 

This bond order control is seen even in Huckel17,18) and SCF 23) calculations on the 3,5-cyclohexadienone system. Although there are definite quantitative and other advantages to the use of SCF and SCF-CI calculations in organic chemistry, Huckel theory very frequently provides a reward in simplicity and in allowing one to understand the basic factors giving rise to some phenomenon. In the present instance, reference to Fig. 5 12-n.18,20 anows ready understanding of the n-rc P,P-bond order and its contrast with the n-n excited state. 

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. 

Dimethyl-2,4-cyclohexadienones 102 undergo photoisomerization to 103 by cleavage of the 1,6-bond from their n,Jt singlet excited states [59]. 

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. ... 

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... 

---