Carbonyl anion radical

Other examples concern the interaction between iron carbonyles and potassium alkylthiolates that is accompanied by disproportionation. The anion radicals Fe2(CO)g, Fe3(CO)fi, Fe4(CO)i3, and Fe(CO)2 are formed (Belousov et al. 1987). The interaction of iron carbonyls Fe(CO)5, Fe2(CO)9, and Fe3(CO)i2 with (CH3)3NO occurs according to a one-electron redox-disproportionation scheme, giving rise to iron carbonyl anion radicals Fe2(CO)s Fe3(CO)n, l e3(CO)i2, and Fe4(CO)ii (Belousov Belousova 1999). 

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 ... 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

With substances that give up an electron more readily than aromatic hydrocarbons, such as potassium, nickel carbonyl, cyanide ion, or iodide ion, complete transfer of an electron occurs and the TCNE anion radical is formed (11). Potassium iodide is a particulady usefiil reagent for this purpose, and merely dissolving potassium iodide in an acetonitrile solution of TCNE causes the potassium salt of the anion radical to precipitate as bronze-colored crystals. 

Spectroscopic evidence (44,45) has been adduced for the formation of electron-gain centres upon y-irradiation of the binuclear carbonyls Mn2(C0)lo and Re2(C0)lo. A study (45) of a single crystal of irradiated Mn2(CO)10 has shown that the radical anion contains two equivalent 55Mn nuclei whose hyperfine tensors lie 118 apart. This has led to the suggestion that the anion radical contains a bridging CO and that its correct formulation is Mn2(C0)9 . The observation of a bridged Mn2(C0)9 species in u.v.-photolyzed material lends some support to this hypothesis (46). 

Ketones - in sharp contrast to enols - are electron acceptors in a wide variety of organic transformations that occur at the carbonyl carbon. For example, the familiar dark-blue benzophenone anion radical is produced via one-electron reduction of benzophenone with sodium in anhydrous THF (equation 21). 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

Indirect electrosynthesis of reactive formyl transition metal compounds involves an initial step of reduction of metal carbonyls to radicals followed by transfer of a hydrogen atom from trialkyltin hydrides190. Electroreduction of metal carbonyls yields products of dimerization and loss of CO from the radical anion. Electroreduction in the presence of R3SnH yields the formylmetalcarbonyls ... 

The preparation and ESR measurements of two tetranuclear carbonyl cluster radical anions have been reported . [Ir4(CO)12] , (g = 2.002) 208 and [Fe4(C0)4Cp4], ... 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

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

A somewhat more complex mechanism takes place with other H-atom donors, such as primary and secondary alcohols, either added to the liquid ammonia solution or used as the solvent (Andrieux et al., 1987). Instead of being totally reduced, the hydroxyalkyl radical, resulting from the H-atom abstraction from the alcohol, partly deprotonates, generating the anion radical of the parent carbonyl compound. The latter is then generated by... 

Therefore, the N(9) radical should be more stable than the N(6) one. That is why both radicals coexist in the system and both N(9) and N(6) deprotonations take place. In the case of the guanine cation-radical, the presence of the carbonyl group in the pyridazine ring brings about two additional effects Deprotonation infringes on this ring exclusively, and double deprotonation leads to the formation of a distonic anion-radical. Scheme 1.25 depicts the differences mentioned. Adhikary et al. (2006) substantiated it experimentally (ESR and UV) and theoretically (B3LYP). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

All of the examples considered in this section up to now, have carbonyl (chelating) groups. Alkali salts of anion-radical without chelating groups should also be considered, to complete the picture. 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

In the case of the free 9-acetylanthracene anion-radical, the spin density on the carbonyl group is lower than that in the para position (position 10) by a factor of 5. The formation of the tail-to-tail dimer should be expected. Actually, preparative reduction of 9-acetylanthracene in DMF against the background of a tetrabutylammonium salt results in the tail-to-tail dimer with the yield of 70%. Addition of a lithium salt, however, decreases the dimer yield to 45% (Guftyai et al. 1987b, Mendkovich et al. 1991). 

During transformation from the neutral molecules to the corresponding anion-radicals, the rate of the fragment rotation, relative to one another, decreases. This also results in the nonequivalence of the meta and ortho protons. Thus, 3-acetylpyridine gives two different anion-radicals as conformational isomers (Cottrell and Rieger 1967). In the case of 3-benzoylpyridine anion-radical, the phenyl group rotates freely about the carbonyl center, whereas the rotation of the pyridyl group is limited. The ESR spectrum shows that the spin density in the phenyl ortho positions is half of that... 

As a rule, if the unpaired electron density in the anion-radical is redistributed, the rotation barrier decreases. Thus, the barrier of the phenyl rotation in the benzaldehyde anion-radical is equal to 92 kJ mol", whereas in the 4-nitrobenzaldehyde anion-radical, the barrier decreases to 35 kJ mor (Branca and Gamba 1983). Ion-pair formation enforces the reflux of the unpaired electron from the carbonyl center to the nitro group. Being enriched with spin density, the nitro group coordinates the alkali metal cation and fixes the unpaired electron to a greater degree. The electron moves away from the rotation center. The rotation barrier decreases. The effect was revealed for the anion-radical of 4-nitrobenzophenone and its ionic pairs with lithium, sodium, potassium, and cesium (Branca and Gamba 1983 Scheme 6.19). 

Reduction potentials for carbonyl compound - radical-anion systems in dimethyl-... 

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

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