Semidione anion-radicals

Semidiones are vinylogues of the superoxide anion radical, GO—0 <-> 0—OG, in the same way that benzosemiquinones are phenylogues of the same radical anion. Semidiones may be obtained by a one-electron reduction of conjugated aliphatic or cycloaliphatic diketones [196]. 

Phthalic anhydride forms a persistent anion-radical on reduction, exemplified by the isobenzofuran-l,3-semidione (20). The ESR spectrum of 20 was observed by several groups at about the same time and analyzed with comparable results. The splittings indicated for 20 are those of... 

Section 15.4 contains anion radicals from nitro compounds. No subdivision has been made into compounds containg one, two or more nitro groups. In the case of a dianion that follows die monoanion directly. Section 15.5 contains tables of magnetic data obtained from anion radicals wifli carbonyl functionality and their sulphur analogs. The data have been divided into subsections consisting of esters and thioesters aldehydes, ketones and their thio analogs semidiones and acid anhydrides. These subsections have been furdier subdivided, for example the subsection Esters and thioesters has been subdivided into arylesters, fliioe-sters and oxocarbothioate and dithioate esters. 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

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. 

The acyloin condensation is closely related to the radical anion coupling forming pinacolate anions two ester radical anions couple to form a dianion, which readily loses two alkoxide ions. The resulting diketone then is reduced by sodium, first to a semidione radical anion, then to the dianion. Finally, aqueous work-up produces the acyloin. Acyloins are convenient precursors for the generation of semidione radical anions. ... 

Most interestingly, reduction of 9-cyclopropylanthracene and 1,4-dicyclopropyl-naphthalene led to the planar cyclopropyl conformations in 127 and 128 as opposed to the normally observed bisected cyclopropyl conformations as, for example, in the cyclopropylsemidione 129 . In all other cyclopropane derivatives previously studied except 127 and 128 , the excess charge density present in the relevant p orbital of the electrophore, is zero or positive. Even in semidione radical anion systems is calculated (HMO) to be positive +0.10). Thus it is reasonable to assume that the change in the conformational preference of cyclopropyl may be related to the change in sign of q. ... 

The autoxidation of ketones which contain a-methylene groups has been studied in basic solution (Russell et al., 1966a). The spectrum observed is that of the corresponding semidione radical-anion whose... 

There are also examples of rotational isomerism in which the rates of interconversion are slow enough for the spectra of the individual isomers to be observed e.g., the cis and trans isomers of the terephthalaldehyde radical-anion (Maki, 1961), acyclic semidiones (Russell and Stephens,... 

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 inference that semidione radicals are intermediates in diketone photolysis has been widely accepted for many years since it provides a satisfactory rationale for the results observed in many reactions. Recently, direct evidence for the presence of these radicals has been obtained from flash photolysis 13> of benzil in alcoholic solvents and esr studie of irradiated solutions of biacetyl 12,171,183 camphorquinone 9,i83)> and other diones 183>. The related radical anions, generated by chemical means, have been studied extensively 5,3i,i3 ) and reports of the radical cations have also appeared m.137)... 

UV-Photolysis of semidione 2 -24 forms still another apparently isomeric radical anion which also is destroyed by oxygen with the formation of The... 

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