Medium aprotic

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... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Consequently, in conducting aprotic media, most of the diaryl sulphones follow the scheme written below for diphenyl sulphone ... 

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. 

Majetich and Hicks <96SL649> have reported on the epoxidation of isolated olefins (e.g., 61) using a combination of 30% aqueous hydrogen peroxide, a carbodiimide (e.g., DCC), and a mildly acidic or basic catalyst. This method works best in hydroxylic solvents and not at all in polar aprotic media. Type and ratios of reagents are substrate dependent, and steric demand about the alkene generally results in decreased yields. 

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. 

In neutral, alkaline, and aprotic media the first step is a one-electron reduction of the neutral ketone to a radical anion. There is some disagreement i3o,i3i,i44) concerning whether the next step is dimerization of the radical anion or reaction between it and the starting material. These two possibilities may be illustrated for benzylidene ketones (132) as follows ... 

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... 

The ability of flavonoids (quercetin and rutin) to react with superoxide has been shown in both aqueous and aprotic media [59,94]. Then, the inhibitory activity of flavonoids in various enzymatic and nonenzymatic superoxide-producing systems has been studied. It was found that flavonoids may inhibit superoxide production by xanthine oxidase by both the scavenging of superoxide and the inhibition of enzyme activity, with the ratio of these two mechanisms depending on the structures of flavonoids (Table 29.4). As seen from Table 29.4, the data obtained by different authors may significantly differ. For example, in recent work [107] it was found that rutin was ineffective in the inhibition of xanthine oxidase that contradicts the previous results [108,109], The origins of such big differences are unknown. 

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). 

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