In aprotic media

As indicated in the general scheme below, butatrienes are the first products from base-induced 1,4-elinination of hydrogen and a suitable leaving group. The butatriene in general very readily undergoes isomerization into enynes, if sufficiently "acidic" protons are available (see Chapter 11 in Ref. 3a). In aprotic media cumulenic ethers are fixed as their lithio derivatives if an excess of alkyllithium is applied... 

It has been found50 that such a multielectron step does not exist with 58, which exhibits a classical two-electron scission. In general, allylic sulphones (59) without an unsaturated system in a suitable position are not reducible. Thus, they do not exhibit a cathodic step in protic solutions. However, in aprotic media the isomerization may be base catalyzed, since small amounts of electrogenerated bases from electroactive impurities, even at low concentration, may contribute to start the isomerization. Figure 10 shows the behaviour of t-butyl allylic sulphone which is readily transformed in the absence of proton donor. On the other hand, 60 is not isomerized but exhibits a specific step (Figure 10, curve a) at very negative potentials. 

Cyclisation of o-hydroxyphenyl ethynyl ketones under basic conditions is known to produce benzopyran-4-ones and benzofuranones by 6-endo-dig and 5-exo-dig processes, respectively. However, both cyclisations are reversible in aprotic media thereby generating anions, of which that derived from the pyranone is rapidly and irreversibly protonated and hence selective formation of the chromone results <96T9427>. 

Le Mest Y, Inisan C, Laouenan A, L Her M, Talarmin J, El Khalifa M, Saillard J-Y. 1997. Reactivity toward dioxygen of dicobalt face-to-face diporphyrins in aprotic media. Experimental and theoretical aspects. Possible mechanistic implication in the reduction of dioxygen. J Am Chem Soc 119 6095. 

Electrochemical methods have been used extensively to elucidate the mechanism of reduction of tetrazolium salts. In aprotic media, the first step is a reversible one-electron reduction to the radical 154 as confirmed by ESR spectroscopy.256,266 As shown in Scheme 26, this radical can then disproportionate to the tetrazolium salt and the formazan anion (166) or take up another electron to the formazan dianion (167). The formation of the dianion through a direct reduction or through the intermediate tetrazolyl anion (168) has also been proposed.272-28 1,294 In aqueous solutions, where protonation/deprotonation equilibria contribute to the complexity of the reduction process, the reduction potentials are pH dependent and a one-electron wave is seldom observed. 

The reaction is complicated in aprotic media by polymerization of the olefin at the electrode 132> apparently because anions such as 134 or 136 can initiate anionic polymerization of the activated olefin. Steric hindrance about the double bond can retard polymerization yields of hydrodimer from 132 in di-methylformamide as a function of the size of R are R = hydrogen or methyl, 0%, R % n-propyl, 25% R % i-propyl, 65% R % /-butyl, 95%, 32). Saturation of the double bond to produce, e.g., 136 from 132, is a side reaction in neutral... 

In aprotic media, the chemiluminescent oxidation of luminol requires only oxygen and a strong base2 4. 

This is further accentuated by the surprising results of the reaction between aziridine and diphenyl cyclopropenone which was elucidated by Dehmlow224. In aprotic media two molecules of aziridine react with a cyclopropenone moiety eliminating ethylene and forming enamino amide 527, whereas in protic media one molecule of aziridine reacts with the exclusive formation of the aziridide 326 ... 

E. H. White and coworkers 2> have proposed a mechanism of luminol (Lum) chemiluminescence in aprotic media in which the luminol dianion (Lum2<-)) is a key intermediate ... 

Reaction (1) is a reversible process, and it can be a source of superoxide if only its equilibrium is shifted to the right. The estimation of the equilibrium constant for this reaction in aqueous solution is impossible because the reduction potential of water-insoluble ubiquinone in water is of course undetectable. However, Reaction (1) occurs in the mitochondrial membrane and therefore, the data for the aqueous solutions are irrelevant for the measurement of its equilibrium. Some time back we studied Reaction (1) in aprotic media and found out that Ki is about 0.4 [23]. As the ubiquinone concentration in mitochondria is very high (it is about... 

In a thorough study by Balia and coworkers, it was shown that in slightly acidic aqueous solution (25 °C, /jl = 1.0 KN03), the mechanism is very similar to that followed in aprotic media (36). Parallel measurement of the 02 consumption and quinone formation confirmed the following stoichiometry up to 35% conversion ... 

It was suggested that initiation proceeds via the same dimer that was proposed in aprotic media (34). Furthermore, the reverse step of this reaction was considered to be very slow. The Cu02 intermediate is formed in a reversible step in this model, which is in agreement with the results reported for autoxidation of Cu(T) (17). It should be added that Shtamm et al. assumed that this reaction is irreversible (40). 

A typical example is the synthesis of oxalic acid. Electrochemical synthesis of oxalic acid by reduction of C02 in aprotic media with a Zn sacrificial anode was brought to pilot scale by the Dechema Institute some years ago (1981) [177]... 

A charge relay system (Blow, 1976) exists in a number of hydrolytic enzymes. In such systems, an aspartate carboxylate group buried in a hydrophobic microenvironment activates a seryl hydroxyl group through hydrogen bonding. Thus, it is interesting to study the effect of carboxylate ions on other nucleophiles in aprotic media. 

Benzenecarhodithioesters and carbo-thio-S-esters were shown to yield diphenylacetylene by the cathodic reduction in aprotic media. Thus, the formation of diphenylacetylene involves [202] two molecules of a substrate. The cathodic reactivity of thioamides involving a similar alkylation of the C=S group in the presence of primary alkyl halides was reviewed [199-201]. 

It should be mentioned that in aprotic media redox potential for the reduction of superoxide to peroxide, E 02 /0 ), is significantly catodically shifted, so that it is even more negative that the redox potential for the oxidation of superoxide to dioxygen. This is exactly the reason why the superoxide is stabilized in aprotic solvents, whereas peroxide is extremely unstable under such conditions. However, coordination of superoxide to the metal center induces effect similar to that caused by protonation, and the Oj /0 redox potential shifts anodically. Thus, upon binding to a metal cation, superoxide can be reduced in aprotic media, as well. 

The physical properties of many macrocyclic polyethers and their salt complexes have been already described. - Dibenzo-18-crown-6 polyether is useful for the preparation of sharpmelting salt complexes. Dicyclohexyl-18-crown-6 polyether has the convenient property of solubilizing sodium and potassium salts in aprotic solvents, as exemplified by the formation of a toluene solution of the potassium hydroxide complex (Note 13). Crystals of potassium permanganate, potassium Lbutoxide, and potassium palladium(II) tetrachloride (PdClj + KCl) can be made to dissolve in liquid aromatic hydrocarbons merely by adding dicyclohexyl-18-crown-6 polyether. The solubilizing power of the saturated macrocyclic polyethers permits ionic reactions to occur in aprotic media. It is expected that this [)ropcrty will find practical use in catalysis, enhancement of... 

The localization of electron density on heteroatoms is seen during the nitration of anilines which are A-nitrated in aprotic media. " Anilines can give abnormal o p-isomer ratios resulting from a process of A-nitration followed by rearrangement to the ring nitrated product, a process which often occurs in situ in the strongly acidic medium of mixed acid (Section 4.5). 

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