Aldehydes into radical anions

Dissolving metals initially convert aldehydes, ketones, and esters into radical anions. Subsequently, proton donors may react with the latter, which leads to neutral radicals. This mode of reaction is used, for example, in the drying of THF or ether with potassium in the presence of the indicator benzophenone. Potassium and benzophenone react to give the deep-blue potassium ketyl radical anion A (Figure 14.45). Water then protonates ketyl A to the hydroxylated radical B as long as traces of water remain. Further potassium reduces B via another electron transfer to the hydroxysubstituted organopotassium compound C. C immediately tautomerizes to the potassium alkox-ide D. Once all the water has been consumed, no newly formed ketyl A can be pro-tonated so that its blue color indicates that drying is complete. 

The electrocarboxylation of aldehydes and ketones leads to the corresponding a-hydroxycarboxylic acids that can easily be converted into carboxylic acids via a hydrogenation reaction [7]. It has been reported that the electrocarboxylation of aromatic ketones occurs through the reaction of C02 onto the activated carbon atom of the carbonyl group of the ketyl radical anion generated upon electron transfer to the ketone [7]. Otherwise, the aforementioned intermediate is likely to be a resonance hybrid (see Equation 12.23), and its electrophilic reaction with C02 may take place both at the carbon or the oxygen atom [42, 43]. 

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. 

The enzyme cytochrome P450 metabolizes tertiary amines in the liver by a one-electron transfer mechanism. An electron is transferred from N to a heme group featuring an Fe=0 bond to give an aminium radical cation and the radical anion [Fe]-0-. An a-hydrogen atom is then abstracted by the oxygen-based radical to give an iminium ion. Flydrolysis of the iminium ion affords a secondary amine and an aldehyde, both of which are further oxidized into water-soluble compounds and then excreted. The oxidation of tertiary amines to secondary amines can also be executed in the laboratory. 

The initiation of the zinc-mediated reaction could then be attributed to the formation of an allyhc radical anion of the type [CH2=CH-CH2-X] on the metal surface. This radical surface could then react with the carbonyl group to give an alkoxide radical, which could add an electron and form the alcohols (Einhom and Luche, 1987). This process takes place with an increase of the pH value of the reaction mixture (Sjohohn et al, 1994). Likewise, Wurtz coupling products were detected with cinnamyl chloride, which was supposed to stem from an allylic radical anion intermediate. Moreover, in one experiment without aldehyde, Sjdholm et al. (1994) observed that all cinnamyl chloride was transformed into dicinnamyl. 

Methylated aromatic heterocycles (HetCHj) form cation-radicals that are typical n acids and expel a proton. Methylene radicals are formed. These radicals give rise to the corresponding carbocations if an oxidant was taken in excess. Nucleophiles attack the ions, completing the reaction. If water is the reaction medium (the hydroxyl anion is a nucleophile), an alcohol is formed. The alcohol rapidly transforms into an aldehyde on the action of the same oxidant. 

At this point a common method of conversion of tertiary aliphatic nitro compounds into nitromethyl derivatives and, further, into aldehydes deserves mention (Komblum Erickson 1981). According to this method, the reagent NaCH2N02 is used. To prepare this reagent, sodium hydride reacts with nitromethane. Then a tertiary aliphatic nitro compound is introduced into the solution formed. Several organic solvents were probed. The reaction considered proceeds most effectively in DMSO. Komblum and Erickson (1981) attributed this feature to small amounts of NaCH2SOCH3 (sodium dimsyl) produced in DMSO at the expense of its reaction with sodium hydride. Sodium dimsyl acts as a powerful one-electron reducer that induces the following chain anion radical process ... 

Photochemical dehydrofragmentation has also been observed by Whitten et al. [16, 17] in PET reactions of aminoalcohols. The reaction is restricted to the geminate pair and the complimentary roles of reduced acceptor and oxidized donor facilitate chemical reaction in competition with back ET. The rapid fragmentation is dependent on the acceptor anion-radical induced deprotonation of the donor cation radical in the contact ion-pair and is strongly dependent on the structure of A [16]. The chemical transformation converts the aminoalcohol into the free amine, aldehyde and reduced electron acceptor. The efficiency of the PET induced fragmentation is affected by the stereochemistry of the aminoalcohol as well as the solvent [18]. Both the thioindigo (TI) and dicyanoanthracene (DCA) sensitized reactions are more efficient in nonpolar solvents such as benzene and... 

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

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. 

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