2,5-Semidiones

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. 

Reductants such as zinc or sodium dithionite generate the semidione from diketones. Electrolytic reduction can also be used. Enolates can reduce diones to semidiones by electron transfer. 

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 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. 

Coumaran-2,3-semidione was prepared by treatment of eoumaran-2,3-dione with the enolate anion of propiophenone in DMSO (70JA2762). 

Diacyl-3,4-dihydroxythiophenes have been formulated as dienol tautomers (77T191). The same holds for 2,5-diaryl- and 5-aryl-2-carboxy derivatives (91JHC1449). ESR investigations of 6-halogeno-benzo[h] thiophene-2,3-semidiones (Scheme 13) have been reported (81JOC751). 

The coupling reaction of arenediazonium ions with semidione radicals (12.84, obtainable by reduction of 1,2-diketones, 12.83) is also included here in the discussion of 1,3-dicarbonyl compounds, although it is a coupling with a nucleophilic radical and does not strictly belong in this context. The reaction (Scheme 12-43) was... 

The one-electron reduction of 3,4,5-trimethoxyphenyl glyoxal with potassium tert-butoxide in DMSO gives rise mainly to the ctT-semidione, whereas on electrolysis in dimethylformamide, in the presence of tetraethylammonium perchlorate as the carrier electrolyte, the main product is the trans isomer (Sundaresan and Wallwork 1972 Scheme 3.47). 

The ESR spectrum of the sodium salt of dibenzoyl ketyl in THF shows splitting due to Na, whereas in the ESR spectrum of the sodium salt of the ketyl of 2,2,6,6-tetramethylhexane-3,4-dione in THE, there is no splitting whatever due to Na (Luckhurst and Orgel 1963). Although dibenzoyl ketyl can give rise to the contact (or penetrate) ion pair, the formation of such a type of species from the 3,4-dione is difficult, owing to steric hindrance. This semidione exists in the form of a free ion (or an ion pair separated by solvent molecules) compare the two structures given in Scheme 3.48. 

The formation of the bottom structure in Scheme 3.51 seems surprising. The related semidione from l,4-dimesitylbutatetra-l,2,3,4-one (dimesityl tetraketone) forms ion pairs with Na, Cs, and Ba + but do not exhibit any chemical reactions (Bock et al. 1990). 

The advent of ESR opened several classes of moderately stable radical ions to scrutiny, namely, ketyls, semidiones, semiquinones, and those of aromatic sys-... 

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. ... 

Isatin, JV-methylisatin, and JV-hydroxyisatin can be reduced to semidiones by treatment with the enolate anion of propiophenone in dimethylsulfoxide.258 Nitroxides are prepared from iV-hydroxyisatin by treatment with lead dioxide in dioxan, while another nitroxide was formed spontaneously from iV-hydroxyisatin in basic dimethyl sulfoxide solution in the presence of oxygen.256... 

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). 

As calculations (Calle et al. 1992) show, the preferential approach of an alkaline cation to a 1,2-semidione is indeed in the middle of the two oxygen atoms. A cation-semid-... 

In all the cases considered, stabilization of the cis isomers of semidiones is observed when the cation is present. As a rule, for the nonchelated semidione the trans isomer is more stable than the cis isomer. However, the interaction with cations produces the opposite effect, and the cis isomer appears to be more stable than the trans form. Carbonyl compounds capture an electron and are converted into ketyls, which contain a negatively charged oxygen atom. This atom is a particularly powerful proton acceptor. Therefore, a proton, which steps forward as a cation, can provide the contour closure for ion-pair formation. 

Stable paramagnetic binuclear complexes [Ru(bipy)2]2(p-L) 3+ with L being N,0 N, 0 -coordinating 4,7-phenantroline-5,6-semidione and P,0 P, 0 -coordi-nating 2,5-6z,s(diphenylphosphino)-/ -benzosemiquinone were reported [232], o-Semiquinone phdo - complex is formed as follows (5.41) ... 

Other heterocyclic semiquinones which have received attention are 120, for which Evans and co-workers have studied the variation of ESR parameters with solvent composition,386 and 121, whose hyperfine splittings have been determined by an Indian group.387 They have also measured the spectra of the 2,3-diphenyl analog and a comparable semiquinone from isoquinoline.387 The heterocyclic semidione 122 has been obtained by Htinig and Piitter as a stage in a suite of redox reactions.388... 

We illustrate the sometimes conflicting results with several experiments involving the diketone benzil, originally prepared by Liebig and Wohler. Since neither the quadrivalence of carbon nor its proper atomic weight had been established, the compound was described as the radical benzoyl with a composition of C28H10O2. Laurent treated the new compound with potassium tartrate and observed a rose colored solution [34]. It is plausible that this reaction leads to the semidione in analogy to the formation of semiquinones under similar conditions. 

Most of the alcohol radicals have hyperfine coupling constants slightly dependent on both temperatures and solvents. Ayscough and McClung (220) have also studied in detail the effect of temperature variation on proton hyperfine splitting of the biacetyl semidione radical in the range 250-400 K. Livingston and Zeldes (156) observed... 

Irradiation of l-(o-ethylphenyl)propane-l,2-dione (144) in the presence of sulfur dioxide gave two interconverting sulfones 145 and 146 in 75 % yield. This indicates the occurrence of rapid tautomerism in semidione radicals such as 140 and 141 and emphasizes the point that product structures do not necessarily provide information on the initial hydrogen abstraction step (cf. also the photo reduction of unsymmetrical benzils, 21>). Thus, granting the assumption of facile isomerization... 

Reactions of this type have been referred to in connection with intramolecular cyclizations of acyclic diones. They are exceedingly common with all types of diketones (excepting cyclobutane derivatives and bridged cyclohexenediones) since almost any C—H bond is susceptible to attack by triplet diketone. The products are semidione radical 164) and substrate derived radical. 

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... 

Detailed studies of H-atom abstraction reactions of cyclopropyl diketones U1), 2,2,5,5-tetramethyltetrahydrofuran-3-4-dione 130) cycloalkanediones57) and substituted benzils 121 , among others, have appeared. The cyclopropyl diketones reacted normally via semidione radicals without ring enlargement. 

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