Anion radicals from carbonyl compounds

The pathway in Scheme 3 relates mainly to alkenes activated by keto or aldehyde groups, for reduction in hydroxylic solvents. Under these conditions, radical anions derived from carbonyl compounds are protonated at oxygen, and the resulting enolic radical, HI, is more difficult to reduce than the starting compound. Consequently, fast dimerization of the enol radicals may compete with further reduction. For other substrate types, especially in aprotic solvents containing added acids, proton transfer is to carbon... 

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. 

The anion BH, formed in Eq. (3), is thermodynamically a stronger base than B and will react with another molecule of substrate, as in Eq. (4). Each PB will therefore consume a total of two protons (and two electrons). An exception to the fast removal of BH by further reduction is sometimes found for radical anion oxygen bases derived from carbonyl compounds, as discussed in Sec. III.B.2. Radical anion EGBs are usually derived from aromatic systems such as aromatic hydrocarbons, A-heteroaromatic systems or azo-arenes. An example was given in Scheme 3 [3]. Radical anion EGBs are normally pro-... 

Quantitative determination of proton-transfer rates to radical anion EGBs derived from carbonyl compounds (Sec. III.B.2) is problematic using voltammetric methods, since the protonated EGB is not undergoing further reduction at the potential of radical anion formation (cf. Sec. III.B.2). this may lead to reversibility of the proton transfer. 

The distinguishing feature of this mechanism is the second step, in which an electron is transferred from the organometallic reagent to the carbonyl compound to give the radical anion of the carbonyl compound. Subsequent collapse of the ion pair gives the same product as is formed in the normal mechanism. The electron transfer mechanism would be expected to be favored by structural features that stabilize the radical anion. Aryl ketones and diones fulfill this requirement, and much evidence for the electron transfer mechanism has been accumulated for such ketones. In several cases, it is possible to observe the intermediate radical anion by EPR spectroscopy. ... 

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. 

Radical anions resulting from cathodic reductions of molecules react with electrophilic centers. As an example (Scheme 8), the reduction of compounds in which a double bond is not conjugated with a carbonyl group, involves an intramolecular coupling reaction of radical anion with alkene [12]. 

Radical anion EGBs derived from aromatic carbonyl compounds are expected to be relatively weak bases but since the radical anions undergo dimerization, the more basic dimer dianions may be active as EGBs for substrates with pK values in the range 20 to 23. Aromatic carbonyl compounds have primarily been used as PBs in catalytic reactions in which the PB also functions as an electrophile (cf. Sect. 14.9.2). 

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... 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

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. 

Radical-anions can be characterised by esr-spectroscopy. Those derived from simple carbonyl compounds such as acetone or 3-methylbutan-2-one are highly reactive and can only be detected in a glassy matrix prepared by the alternate d o-sition of layers of sodium and the ketone at 77 K [4], lire radical-anion from the... 

At low pH values, dimerization must involve the combination of two neutral carbon radicals since the same ( ) / meso ratio is obtained as from the photochemical reaction of the carbonyl compound in methanol [26], a process which also involves neutral radicals. The switch in isomer ratio to that characteristic of alkaline media occurs in the region of pH close to the value of pKj for the neutral radical. Dimerization then occurs in a fast reaction between the radical-anion and the neutral radical. In strongly alkaline solutions where the pH pK the major reactive species formed at the potential of the first reduction wave is the radical-anion. Reaction between two radical-anions is relatively slow due to coulorobic repulsion so that dimerization in strongly alkaline solution still occurs by reaction... 

Radical anions derived from 2,5-diformylthieno[3,2-6]thiophene (39) as well as (40) were studied in connection with the conformational analysis of heteroaromatic carbonyl compounds. Different ESR signals were given by distinct rotational isomers. Information on interconversion of the rotamers could not be obtained since the radicals were unstable at the temperatures necessary for interconversion. On the other hand, ketyl radicals derived from ketones (41) and (42) are relatively more stable at the temperatures needed for the study of the conformational mobility in these systems. The ESR spectra of the bis-thienothienyl ketyls from (41) and (42) at room temperature show that the unpaired electron is coupled to three pairs of equivalent protons. Both spectra exhibited a certain amount of asymmetry, which was enhanced by lowering the temperature. At -10°C the highfield part of the spectra split into new lines arising from two species which have similar hyperfine splitting, but different g factors. These have been identified as the rotational isomers of the radicals. The two preferred conformations are cis-trans and trans-trans. An examination... 

Cathodic coupling of ketones with 64 takes place regioselectively at the /3-position to the silyl substituent to give 65 in good yields (equation 44). A radical anionic intermediate derived from the carbonyl compound is proposed107. 

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 samarium-lV-bromosuccinimide combination reductively dimerizes carbonyl compounds.163 This pinacol-type coupling gives diols in 60-80% yield, with some diastereoselectivity the by-product from simple reduction (i.e. alcohol) is typically 5-10%. The conditions suggest a single electron transfer to give carbonyl radical anion, which then self-couples. Even congested ketones such as benzophenone and fluorenone worked well. 

Cleavage of the S-C(2) bond occurs when thiochroman is treated with the radical anion 4,4 -di-/-butylbiphenylide (LDBB). The resulting ring-opened dianion 317 has good synthetric potential. For example, the alcohols arising from reaction with carbonyl compounds can be cyclized under acidic conditions to benzothiepines (Scheme 56) < 1995TL4459>. 

Cathodic coupling proceeds via radicals or radical anions, which are reductively generated from suitable substrates, in general electrophiles, e.g., carbonyl compounds and activated olefins. These intermediates either dimerize (Eq. (182)) or add to activated double bonds to yield 1,4-radical anions, which are subsequently reduced to hydrodimers (Eq. (183) ). 

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