Esters 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 characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

Anodic oxidation of cyclic enol esters with /1-hydrogens leads to allyl radicals, which then lose acyl radical to form a, /1-unsaturated ketones. When the electrolysis is performed in an undivided cell, these are converted by the cathode into enolate anion radicals, which then couple to form /1-dimers (Scheme 66)164. 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

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. 

For example, a solution of acrylamide and thiol ester (1.5 equiv) in THF was added dropwise to a 0.1 M solution of Sml2 cooled to -78 °C. After the mixture was stirred for 1 h, followed by oxidation of the excess Sml2 with oxygen, y-keto-amide was formed in 49% yield. That decarbonylation (as shown in Eq. 29) is not observed implies that the reacting species is not a free acyl radical. It is proposed that electron transfer into the carbonyl group could generate a ketyl radical anion equivalent to the structure 44 coordinating to samarium(III) (Scheme 10). 

Macrocyclic bis-thionolactones have been prepared with LR. These were converted by reduction with sodium naphthalenide into the radical anions, which gave bicyclic systems through radical dimerization and subsequent methylation (eq 14). This method was successfully applied by Nicolaou et al. in the total synthesis of hemibrevetoxin B. One of the crucial steps of the synthesis was the preparation of the bis-thiono ester (11), which was achieved by using LR together with tetramethylthiourea in xylene at 175 °CP... 

Carboxylic AcidPerivatives. Detailed investigations into the electrolytic reduction of carboxylic acid derivatives (esters, anhydrides, amides) in nonaqueous solutions and the procedures for producing the respective radical anions have been described in the report by iDyasov and his co-workers f39]. They obtained radical anions from esters of aromatic carboxylic acids (benzoates, phtha-lates, isophthalates) and from phthalic anhydride and analyzed their EPR spectra. The production of radical anions of acrylates and methacrylates by electrochemical generation and the effect of proton donors on their stability was also described [11]. 

The perben/oate ester decomposes under copped I) bromide catalysis into a hen/oate anion and a terr-hutoxy radical. This radical abstracts a hydrogen atom at an allylic position... 

Hydrostannation can also be catalyzed by palladium compounds. Other tin hydrides may react by a polar rather than a radical mechanism. A series of PVC stabilisers (the Estertins) have been developed by the AKZO company, and which are prepared by the reaction of HCl with metallic tin or SnCla, to give the solvated chlorotin hydrides H2SnCl2 and HSnCls, respectively, which add, in a heterolytic Michael fashion to acrylic or propargylic esters [e.g. Equations (1.7) and (1.8)] the chlorides are then converted by anion exchange into the thioglycollates. 

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. 

Acrylic adhesives are based on the esters of acrylic and substituted acrylic acids and can be categorized into two main groups the acrylates and methacrylates (I), which cure by free radical-catalysed addition polymerization, and the cyanoacrylates (II), which cure by anion-catalysed addition polymerization. The term acrylic adhesives is normally restricted to include the former, while the latter is usually considered separately (see Cyanoacrylate adhesives) this categorization is adhered to here. 

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