Radical anions of carbonyl compounds

Radical-anions of carbonyl compounds are basic and undergo protonation at the oxygen centre generating highly reactive radical species. 

In many cases, the reactions of carbonyl compounds are interpreted in terms of the reactivity of the triplet carbonyl compound. However, the work on [123] in which a fluorescent excited charge-transfer complex was detected, and the finding that some amine radical cations react with the radical anions of carbonyl compounds to produce exciplex fluorescence (Zachariasse, 1974) shows that, although intersystem crossing in carbonyl compounds is usually highly efficient, they may participate in excited singlet-state reactions. 

Kubo, Maruyama and Takuwa reported the photoaddition of allylic radicals generated from allyUc silane and tin compounds to the radical anions of carbonyl compounds (Scheme 45) [137-139]. 

Finally, radical anions of carbonyl-containing compounds are often produced unexpectedly in reactions involving nucleophiles or easily oxidized free radicals. In a seminal 1964 report, Russell, Janzen and Strom noted that ketyl anions could be detected by ESR... 

The radical anions of carbonyl groups can also be generated via PET from activated alkenes, e.g. allylic silanes or stannanes. Triplet excited aromatic ketones, a-dicarbonyls and Michael systems are suitable substrates for oxidizing allylic Group 14 organometallic compounds with subsequent formation of homoallylic alcohols or S-allylated ketones (Scheme 32) [120-122]. 

Addition of an electron to the LUMO of the carbonyl group to form a radical anion is the first step in the reduction process. Radical anions can be characterized in aprotic solvents by electron spin resonance (esr) spectroscopy. Those derived from unconjugated carbonyl compounds are highly reactive and can only be detected in a matrix at low temperatures [3]. Decay is rapid because the excess carbonyl compound acts as a proton donor toward the basic oxygen center in the radical anion. Aromatic carbonyl compounds give less reactive radical anions in which the free electron is delocalized over the whole... 

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. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Anion radical species formed by electroreduction of carbonyl compounds show interesting reactivities. In some cases, the... 

Uv-absoiption of carbonyl compound radical-anions and values of pK, for their conjugate acids... 

Values of 1554 cm for the carbonyl stretching frequency in tlie radical-anion of benzophenone [16] and of 1558 cm for the radical-anion of di-ter/.-butyl ketone [17] suggest that in general Vco will be found at lower frequencies for the radical-anion than for the parent carbonyl compound. 

The reduction of carbonyl compounds to form pinacol dimers can be accomplished photochemically, electrochemically or with chemical reducing agents. When conducted under acidic conditions or in protic solvents, pinacols are likely produced by coupling of two neutral ketyl radicals (vs radical anions). The electrochemical reduction is especially complicated in terms of the role of the electrode surface, counterion and solvent, and an excellent review has appeared on the subject32. 

Another example concerning the reduction of carbonyl compounds also relates to the salt effect theme. Shaefer and Peters (1980), Simon Peters (1981,1982,1983,1984), Rudzki et al. (1985), and Goodman and Peters (1986) described photoreductions of aromatic ketones by amines. In this case, the addition of excess NaC104 results in considerable retardation, even prevention, of final product formation. The two fundamental steps in this photoreduction consist of rapid electron transfer from the amine to the photoactivated ketone (in its triplet state), followed by the slow transfer of proton from the amine cation radical to the carbonyl anion radical ... 

Studies on the reduction of carbonyl compounds using low-valent metallic reagents, including Sml2, have led to an understanding of the mechanism involved. Single electron transfer to the aldehyde or ketone generates a metal ketyl radical anion 3 that can form dimeric or polymeric ion pairs 4 (Scheme 4.4).10... 

Electrolysis of carbonyl compounds provides pinacols, alcohols or hydrocarbons, depending on the conditions, such as pH, the nature of the electrode, and its potential. Fundamental studies have been carried out on the mechanisms of hydrocarbon formation using acetone as a substrate. Although several electrodes, such as Cd, Pt, Pb or Zn, are recommended, carbonyl compounds, including aryl and alkyl derivatives, require strong aqueous acidic media for reduction to the hydrocarbons. The mechanism of the electrolytic reduction is probably similar to that of Clemmensen reduction, which starts from anion radical formation by one-electron transfer, as indicated in Scheme 3. The difference is that electrolytic reduction takes place in an electric double layer, rather than on the surface of the zinc metal. 

Two thiolate radical anions can couple to give the dithiolate precursor, 5-49, of the second product. This is analogous to the well-documented coupling of ketyls (radical anions derived from one-electron reduction of carbonyl compounds), which results in the formation of pinacols (1,2-diols). 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

Electrochemical modulated infrared (ir) spectroscopy has been used to obtain in situ spectra of the radical anion of benzophenone [5], There is a red shift of the vibrational modes of the benzene rings and considerable loosening of the carbonyl bond. Carbonyl compound radical anions are protonated on the oxygen center. The conjugate acids of alkanones and alkanals have pKg values [6] in the range 11-12, while those from conjugated aromatic carbonyl compounds [7] are in the range 8-10. 

An oxidation using a nickel hydroxide electrode is shown in 15.8.433 Electrochemistry is also a way to produce radicals and anions. The hydrodimerization of acrylonitrile to adiponitrile just mentioned may involve the coupling of free radicals. The coupling of carbonyl compounds, such as p lolualdehyde, to form pinacols with up to 100% selectivity, by way of free radicals, can be done electrically.434 Anions can also be formed electrochemically and used in situ, as in example (15.9).435... 

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