1.6- Diketones photoisomerizations

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

In hydrocarbon solvents, /3-diketones are predominantly (> 90%) enolized and these solutions have been subjected to flash photolysis, which causes photoisomerization to the diketo form9). Reversion of the diketone to the more stable cis enol was then followed by UV spectroscopy and pseudo first order rate constants at room temperature were in the range 14-68 x 10-3 s-1 (e.g. AA = 23 x 10-3 s 1) with half-lives of several hours. The same research also reveals an alternative transformation on irradiation. In this the cis enol form is converted by rotation about a C-C or C=C bond into one of the possible trans enol isomers. These may then go on to the diketo form but mainly they revert very rapidly to the cis enol with rate constants of 0.1 to 70 s 1 (e.g. AA = 0.27 s-l)9). 

This actinometric procedure was utilized to determine the photochemical quantum yield for the solid-state photoisomerization of the Diels-Alder adduct (I) (formed from 2,5-dimethylbenzoquinone and cyclopentadiene) to the pentacyclic diketone (II), i.e.. 

Mechanistic photochemistry of metal )8-diketonate complexes has received increasing attention. Near-ultraviolet irradiation of diketonate complexes of Mn ", Fe , Co ", No" and Cu" eventuates in one-electron reduction by the ligand of the metal center whereas Cr " complexes photo-isomerize. Irradiation of franj-Rh(CF3COCHCOMe)3 appears to lead to two photoactive excited states which may have significant radical character. Photoisomerization to the cis form occurs in inert solvents whereas decomposition takes place in the presence of potential hydrogen atom donors such as alcohols. ... 

Reactions of 1,3-Diketones. - A tuneable laser has been used to study the photoisomerization of acetylacetone in a nitrogen matrix at 10 Organero and Douhal have reported a temperature effect on non-radiative decay... 

Polyakov et al. have investigated the effect of the electron donor triphenylamine on the photoisomerization reactions of a,p-unsaturated diketones. CIDNP has also been applied in mechanistic studies of free radical polymerization reactions . ... 

In contrast, irradiation of 22c (R = CH, R = CH3) produced bicycHc diketone 25c and alkyHdene phthaKde 26c together with cyclobutanol 23c and the unsaturated alcohol 24c. Compound 26 is the result of secondary photoisomerization of 25. The formation of 25 can be explained by a cyclopropyl-carbinyl rearrangement of ketyl radical VII followed by 1,5-biradical-recombination. Such an inner cyclopropane bond fission may be favored by the better stabilization by R (= CH3) of the new radical center. As found in 22d, replacement of R = CH3 by a Ph group simplified the reaction outcome to provide only 25. 

---