Ketone Radical Anions

Several aromatic ketyl radical anions such as those from acetophenone and henzophenone have been prepared in solution and their e.s.r. and 

The reduction in the hyperfine splitting is attributed to the participation of the structure (CH3)2CO- in the ketyl radical anion which allows delocalization of the unpaired electron to the oxygen atom (Symons, 1963). 

Attempts to trap other di-alkyl ketyl radicals from methyl-ethyl, di-ethyl, di-n-propyl and di-isopropyl ketones were unsuccessful. The observed spectrum was not that expected from the ketyl radical but in each case it could be identified as that of a radical formed by a hydrogen abstraction from the parent ketone. The structurally similar radicals, CH3—CH—CO—CHg and CH3—CH—CO—CHa—CHg, were formed from methyl-ethyl and di-ethyl ketone respectively and the radical (0113)2—C—CO—CH—(0113)2 was formed by abstraction of the tertiary hydrogen from di-isopropyl ketone. The reaction of di-n-propyl ketone was somewhat unexpected as the spectrum obtained was typical of that for an allyl radical rather than an alkyl radical. It is possible that the allyl radical was formed by abstraction of a hydrogen adjacent to the carboxyl group in a parent molecule followed by an intramolecular rearrangement. Thus 

The equivalent reaction for methyl-ethyl or di-ethyl ketones would be less favourable as the intramolecular rearrangement would involve the breaking of a primary CH-bond. 

Some comment may be made on the formation of these radicals. In all previous experiments we have not found any evidence that alkali metal atoms can abstract hydrogen directly from an aliphatic group and thus we consider that in every case the initial reaction is the formation 

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The radical pair generated by proton transfer from tertiary amine radical cations to a,p-unsaturated ketone radical anions (e.g., 71) couple in the p position, forming... 

The photoreduction of aryl ketones by amines generally occur via a charge transfer interaction between the triplet state of ketone and the amines, as shown in the following scheme. A ketone radical anion and an amine radical cation are formed in the intermediate stage. 

Early workers [103] detected benzilic acid formed during the reduction of benzophenone in dimethylformamide in the presence of carbon dioxide. The carbon dioxide radical anion system is known to have E" = —2.2V (vs. SCE) [104] and will thus not be formed in preference to the ketone radical anion. Reaction occurs through trapping of aromatic carbonyl radical anions by carbon dioxide, and this has been developed into a convenient synthesis of aryllactic acids. The modern technological process uses constant current conditions. On a small scale, a divided cell with mercury cathode has been used to obtain benzilic acids from substituted benzophenones and carbon dioxide in 70-90% yields [105] and to convert 4-isopropylacetophenone to the corresponding phenyllactic acid in 85% yield [106]. On a technical scale, these reactions are best carried out in an undivided cell using a lead cathode and a sacrificial aluminum anode with dimethylformamide as solvent... 

Dianions of aryl ketones in aprotic solutions protonate to become secondary alcohols whereas those of cc-p unsaturated ketones form the corresponding saturated ketone. Radical anions derived from a-jS unsaturated carbonyl compounds cause extensive polymerisation of the substrate. Dimerisation at the )8-positions occurs in some systems with the formation of diketones. " ... 

Type II initiators containing carbonyl groups can also undergo electron-transfer reactions, which lead to hydrogen abstraction after an intermediate excipkx (excited complex) has been formed between the diaryl ketone radical anion and the amine radical cation, as illustrated in Scheme 10.3. 

Analogous bonding occurs in several important classes of free radicals, including nitroxides, peroxy radicals, ketone radical anions, and superoxide, all of which also have a spin-bearing center adjacent to an atom bearing an unshared pair of electrons. 

The unique chemical behavior of KO2 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubiHty of KO2 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its appHcations in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzyHc methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

When saturated steroidal ketones are reduced in ammonia, an alcohol is usually present to act as a proton donor and high yields of steroidal alcohols are obtained. Under these conditions, reduction probably proceeds by protonation of the radical-anion (or ketyl) (61), which results from a one electron addition to the carbonyl group, followed by addition of a second electron and proton. Barton has proposed that reduction proceeds via protonation of the dianion (62) arising from addition of two electrons to the carbonyl group. This proposal implies that the ketyl (61) undergoes addition of a second electron in preference to undergoing protonation by the... 

For the mechanistic course of the reaction the diketone 5 is assumed to be an intermediate, since small amounts of 5 can sometimes be isolated as a minor product. It is likely that the sodium initially reacts with the ester 1 to give the radical anion species 3, which can dimerize to the dianion 4. By release of two alkoxides R 0 the diketone 5 is formed. Further reaction with sodium leads to the dianion 6, which yields the a-hydroxy ketone 2 upon aqueous workup ... 

Thus a single two-electron wave is observed and only one product, the alcohol, can be isolated. Finally, at high pH neither the ketone nor the radical anion are protonated by this basic medium and it is not until the dianion, formed by successive electron transfers, that protonation occurs. 

Stocker and Jenevein have suggested that in alkali the pinacols arise by combination of the neutral radical 81 with the radical anion (91) of the ketone ... 

In neutral, alkaline, and aprotic media the first step is a one-electron reduction of the neutral ketone to a radical anion. There is some disagreement i3o,i3i,i44) concerning whether the next step is dimerization of the radical anion or reaction between it and the starting material. These two possibilities may be illustrated for benzylidene ketones (132) as follows ... 

Conversely, electrolysis of ketones, (35), results in their cathodic reduction to radical anions (36), which dimerise to the dianions of pinacols (37) ... 

We have seen similar radical anions generated from ketones in pinacol reduction with sodium or magnesium (p. 218), and also from esters with sodium in the acyloin condensation (p.218). 

Biradicals have also been encountered as intermediates in the Mg reduction of ketones to pinacols (p. 218) and, as radical anions, in the acyloin condensation of esters (p. 218). The thermolysis of cyclopropane (131) to propene (132) at 500° is also believed to involve... 

Reaction of dpp-bian with Mg in THF for 30 min reflux gives complex 87 (Ar = 2,6-diisopropylphenyl) which undergoes oxidative addition via m-bond metathesis with PhC=CH to give the black alkynyl amido complex 88. The insertion reaction of 88 with Ph2CO in EtzO yields complex 89. Unexpectedly, hydrogen abstraction to give the radical anion occurs simultaneously with ketone insertion.268... 

The importance of radical ions and electron-transfer reactions has been pointed out in the preceding sections (see also p. 128). Thus, in linear hydrazide chemiluminescence (p. 103) or acridine aldehyde or ketone chemiluminescence, the excitation steps consist in an electron transfer from a donor of appropriate reduction potential to an acceptor in such a way that the electron first occupies the lowest antibonding orbital, as in the reaction of 9-anthranoyl peroxide 96 with naphthalene radical anion 97 142> ... 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

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