Cyclohexenone, excited state

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

If thermal motion on the Ti (or Si) surface leads to a quasi-equilibrium distribution of molecules between several minima, some of them are likely to provide a faster return to So than others and they will then drain the excited state population and determine which products will be formed. This is a straight-forward kinetic problem and it is clear that the process need not be dominated by the position of the lowest-energy accessible minimum in the excited hypersurface. Such minima may correspond to conformers, valence isomers, etc. Of course, it is well known that ground-state conformers may correspond to excited-state isomers, which are not in fast equilibrium. 65,72) Also, there is no reason why several separate minima in Si or Ti could not correspond to one minimum in So, and there is some evidence that this situation indeed occurs in certain polycyclic cyclohexenones. 73,74)... 

Enones in their triplet excited state also undergo [2 + 2] cycloaddition to thiophene. Cyclohexenone thus gives the adducts (296 8%) and (297 43%) with thiophene and 2,5-dimethylthiophene respectively (74JOC3063). 

Corey has made a study of the relative reactivities of various olefins to attack by excited cyclohexenone and has concluded that the excited state is an electrophilic reagent.420 This fact has led him to postulate formation of a charge-transfer complex between excited... 

The relative linearity of a Stern-Volmer plot of 1/< > versus 1/[A] for the photoreaction of cyclohexenone supports this mechanism and suggests that only one excited state is involved (30). The [2+2] addition of the triplet to a ground state molecule is not concerted, and there is a large solvent polarity effect on the regioselectivity for the head-to-head (HH) versus head-to-tail (HT) photodimers, with the more polar head-to-head isomer being favored in polar media. The polarity effect is attributed to the large difference in the dipole moments of the transition states leading to the products. 

Cyclohexenones with substituents at the S- and 6-positions tend to give the product which would be predicted by steric approach control to an excited state with a trigonal P-carbon (equations 34-36). As a final cautionary note, while these hypotheses are often useful for prediction of stereochemical results, the true source of the selectivity may involve the rates of cleavage of the intermediate diradicals to the ground state versus rates of cyclobutane ring closure. Insufficient data are currently available to explain these results completely. 

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. 

A review has discussed the photochemical cyclization of a variety of aryl- and heteroaryl-prop-2-enoic acids. Irradiation (A,>340nm) of the cyclohexenone (119) in argon purged benzene solution affords the cyclized derivatives (120) as a 3 1 mixture of diastereoisomers. The formation of the cyclopropyl group arises via the carbene (121) and insertion into a C-H bond of a neighbouring methyl group. This carbene is formed, presumably, from the biradical (122) which arises by addition of the alkene to the excited state of the enone. Further evidence for the carbene intermediate comes from a reaction in methanol when the diastereo-isomeric mixture of the ethers (123) and (124) is obtained." ... 

An interesting use of removable chiral auxiliaries in photocycloaddition reactions concerns imminium salts. With cyclic enones, the observed asymmetric induction does not result from an approach of the double bonds in parallel planes because of the triplet nature of the reactive excited state. In contrast, the corresponding imminium salts react through their singlet excited state, and an approach of the reactants in parallel planes is now required during the cycloaddition process. For chiral imminium salts 130 derived from a cyclohexenone and a pyrrolidine having a C2 axis of symmetry, the intramolecular [2 -I- 2] photocycloaddition process occurs with a de up to 82%. As expected, the stereochemis-... 

As mentioned in the previous section, Corey15 considered an oriented jt-complex between the photoexcited enone and the ground state alkene. In the case of cycloaddition of methoxy-ethylene or acrylonitrile to 2-cyclohexenone the preferred orientation is shown in the scheme. Assuming that the triplet excited state of the enone is n,7t in nature15, calculations of charge distribution indicate that is fairly negative relative to Ca consequently the excited enone has a polarization complementary to its ground state. The more favorable orientation of the k complex is that in which the dipole of the olefin is opposed to that of the excited enone. 

Cyclohexenones can add methanol in an acid-catalyzed reaction. This is thought to involve protonation of the tt-tt excited state. 

Photoreactivity of androstenedione 116 (Scheme 1.26) was demonstrated to be dramatically affected by the electric field of zeolite. The epimerization of 116 to yield 117 is the major reaction in isotropic solution such as hexane, methylene chloride, methanol and cyclohexane. The reduction of the cyclohexenone A-ring can be observed in only propanol, with 118 and 119 being given in 14% combined yield. However, irradiation of 116 in NaY afforded exclusively the reduction products in >85% yield, and no product due to the reaction at the D-ring was observed. This reactivity change observed for the androstenedione included in zeolite cavity was ascribed to the lowering of the n,n excited state of the A-ring due to the electric field in zeolite cavity. 

Both these cyclohexenones react with their triplet excited states through diradical intermediates. In both cases, the cyclohutanone products were formed by the homolytic cleavage of C(4)-C(5), followed by attack of diphenyl methyl radical... 

The addition of 4, 4-dimethyl-2-cyclohexenone to 1,1-dimethoxyethylene also involves triplet excited state. In this cycloaddition less stable, highly strained trans-addition product is predominantly formed in comparison to more stable cis-adduct ... 

Stern-Volmer slope for quenching by naphthalene of oxetane formation, a reaction attributed to states, is indeed different from that for quenching 29, 30, and 31. This leaves no doubt that in this system and, by extension, in other cyclohexenones the reactive excited state is the enone state. 

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