1,4-Semidiones derived from

Semidiones Derived from (CH)6 Hydrocarbons. The possibility of reaction 8 was investigated by the electrolytic reduction of the stable dione 11 (12). 

A number of attempts have failed to produce the 1 2-semidione derived from cyclooctetraene (Jj ) equation 9 Treatment of the appropriate a-benzoyloxy... 

The EPR spectra of semidione radical anions can provide information on the spin density at the individual atoms. "The semidione derived from butane-2,3-dione, for example, has a spin density of 0.22 at each oxygen and 0.23 at each carbonyl carbon. The small amount of remaining spin density is associated with the methyl groups. This extensive delocalization is consistent with the resonance picture of the semidione radical anion. 

Semidiones Derived from (CH)ip Hydrocarbons. Treatment of or lb withoxygen in basic DMSO produce the extremely stable semiquinone (12-14),... 

Perhaps the most important conclusion to be reached from this study of the isomeric semidione derived from (CH)io hydrocarbons, is that in this case the radical anions are themselves thermally stable at 25° in respect to valence isomerization. On the other hand apparently the diones rapidly equilibrate. It begins to appear that when the following three oxidation states are considered, that one or the other of... 

The radicals that are formed from the enolate in this process are rapidly destroyed so that only the stable semidione species remains detectable for EPR study. Semidiones can also be generated oxidatively from ketones by reaction with oxygen in the presence of base. The diketone is presumably generated oxidatively and then reduced to the semidione via reduction by the enolate derived from the original ketone. 

The radicals derived from ethylene glycol and related compounds react with H202 in an alkaline medium (pH > 7.5), to give101 semidione (11) and carboxyalkyl radicals (12). 

Rapid hydrogen transfer occurs between semidione radicals and ground state diketones. This w as illustrated by irradiation of a mixture of camphorquinone (174, A-max 470 nm, Ex = 51 kcal/mole) and tetralindione 68 (Amax 388 nm, ET = 54 kcal/ mole) in 2-propanol at various wavelengths 141). The exclusive product, no matter what the wavelength, was reduced 68 indicating that the equilibrium below lies to the right. Ketyl radicals derived from monoketones, such as benzophenone, transfer... 

G. A. Russell, C. L. Myers, P. Bruni, F. A. Neugebauer, and R. Blankespoor, Semidiones. X. Semidione radical anions derived from indan-2.3-dione, coumaran-2.3-dione, thianaphthalenequi-none, isatin, and IV-hydroxy isatin. Nitroxide radicals derived from isatin, dioxindole, oxindole, and other indole derivatives, /. Am. Chem. Soc. 92, 2762-2768 (1970). 

The semidione radical derived from open-chain diones may, like the diones themselves, assume various conformations. It has been assumed that the preferred conformation is "s-trans but results appear to vary with the method used for generating radicals. The conformational factor might play a role in determining the relative importance of subsequent reactions. 

The inital H-abstraction step results in formation of a pair of radicals in a solvent cage. To the extent that one of the partners escapes from the cage, encounters between like radicals will be possible and dimeric products may be formed. Thus dimers of the semidione radical, of the radical derived from the donor, or both are sometimes observed. 

With the exception of reactions involving alcohols as hydrogen donors, the major process observed upon irradiation of diones in the presence of H-donors is formation of 1 1 adducts derived from coupling of the pair of radicals generated in the H-abstraction step. The ambident character of the semidione radical allows such coupling to occur at either of two... 

The radicals derived from the enolate are unstable and couple or undergo other modes of decomposition, so that only the stable semidione remains for study by EPR techniques. Because of the sensitivity of EPR methods, the amount of dione converted to semidione does not have to be high to permit structural information to be obtained. 

The diketone is presumably generated oxidatively and then reduced to the semidione via reduction by the enolate derived from the original ketone. 

---