Isophorone, photodimerization

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

The unusual increase in regioselectivity with a decrease in pressure is not present for the photodimerization of a similar enone, isophorone, in SCF CO2 (31). The difference for the two enones is a result of differences in their phase diagrams and differences in the phase region in which the experiments were performed. As shown in Figure 8, the critical point for the cyclohexenone-ethane mixture is close to that of pure ethane. Hence the compressibility is large. In contrast, the... 

Additions to Cyclohexenones and Related Systems - A re-investigation of the photodimerization of isophorone (37) has been reported. The study examined the influence of solvent and of the concentration of the enone. Some of the results and the yields of dimers obtained are shown in Scheme 4. From this detailed study the authors suggest that supramolecular structures are involved in the dimerization. These apparently take part even at low concentrations of enone. The photocycloaddition of enones such as (39) to buckminsterfullerene (Ceo) has been studied. The outcome of the addition is the formation of low yields of furanylfullerenes. This addition occurs to the exclusion of de Mayo type of addition. Photocycloaddition of the cyanocyclohexenone derivative (40) to alkenes has been reported. ... 

Aqueous medium containing micelles have a great influence on the photodimerization of 3-n-butylcyclopentenone (106) and isophorone (109) (Scheme 5.35). Reversed orientations in aqueous micellar medium, with respect to organic solvent, were also observed in the photodimerization of 2-substituted naphthalenes, 2-pyridones ° (a [4 + 4] cycloaddition), and in the photocycloaddition reaction of 1-heptenylacetate with 3-n-butyl-cyclopenten-2-one (106) (Scheme 5.36). 

Hrnjez et al. evaluated the product distribution of the photodimerization of isophorone in supercritical fluoroform and CO2 (177). The reaction typically produces a mixture of various regioisomers and stereoisomers. Relative yields of the regioisomers are fluid density dependent in the polar fluid fluoroform but exhibit little or no change with fluid density in CO2. On the other hand, relative yields of the stereoisomers are affected by changes in the fluid density in both fluoroform and CO2. The results were discussed in terms of solvation and various degrees of solvent reorganization required for the various products. (See Scheme 5.)... 

BJ Hrnjez, AJ Mehta, MA Fox, KP Johnston. Photodimerization of isophorone in supercritical trifluoromethane and carbon dioxide. J Am Chem Soc 111 2662, 1989. 

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