Anionic reductive processes

Anionic/oxidative reaction sequences have been developed in addition to the domino anionic/reductive processes. For example, with regard to the synthesis of novel diaryl heterocycles as COX-2 inhibitors [500], including rofecoxib (Vioxx) 2-972 [501] (which has recently been withdrawn from the market) or the pyrrolin-2-one derivative 2-973 [494], Pal and coworkers reported on a so-far unique domino aldol condensation/oxidation sequence (Scheme 2.218) [503]. 

In the case of LiC104, there is some spectroscopic evidence of anion reduction below 1.5 V [39], The stable surface species which may precipitate due to the Cl()4 reduction is, among others, Li20. We have no spectroscopic evidence for precipitation of stable LiC10x (x = 1-3) or for LiCl onto noble metals at potentials above those of Li bulk deposition. In any event, the above salt anion reduction processes do not dominate the overall surface film formation on nonactive electrodes at low potentials in most aprotic solvents. Thus, both anions can be considered as only moderately reactive. The onset potential for the reduction of the anions from the third group is about 2 V (Li/Li+). This is clearly demonstrated in Figures 18 and 19, which show FTIR spectra measured in situ from... 

LiAsFg, LiPFe, and LiC(S02CF3)3 solutions (23a-c, respectively)/ " The fluorine peaks in Figures 23a-c and the arsenic, phosphorous, and sulfur peaks in Figures 23a, b, c, respectively, clearly demonstrate the strong involvement of the salt anions reduction processes in the surface film formation on Li. 

The red [SSNO] anion (9.2) (2max 448 nm) is produced by the reaction of an ionic nitrite with elemental sulfur or a polysulfide in acetone, DME or DMSO. ° The formation of 9.2 probably proceeds via an intermediate such as the [S2NO2] anion. This process is thought to occur in the gunpowder reaction, which also entails the reaction of potassium nitrite (produced by reduction of potassium nitrate with charcoal) and sulfur. 

The polarographic method is applicable to the determination of inorganic anions such as bromate, iodate, dichromate, vanadate, etc. Hydrogen ions are involved in many of these reduction processes, and the supporting electrolyte must therefore be adequately buffered. 

In the pyroaurite structure the brucite layers are cationic. However, on oxidation the resultant brucite layers in y - NiOOH are anionic. To preserve electroneutrality, cations and anions are exchanged in the intercalated layer during the oxidation-reduction process. This is illustrated in Fig. 4. In the case of Mn-substituted materials, some Mn can be reduced to Mn(II). This neutralizes the charge in the brucite layer this part of the structure reverts to the P - Ni(OH)2 structure and the intercalated water and anions are expelled from the lattice. With this there is a concomitant irreversible contraction of the interlayer spacing from 7.80 to 4.65A [72]. 

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. 

Sonochemical reduction processes of Pt(IV) ions in the presence of anionic, cationic or non-ionic surfactants was investigated by Mizukoshi et al. [38]. During the processes, the color of the aqueous solution containing H2PtCl6 and surfactants... 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

Anionic domino processes are the most often encountered domino reactions in the chemical literature. The well-known Robinson annulation, double Michael reaction, Pictet-Spengler cyclization, reductive amination, etc., all fall into this category. The primary step in this process is the attack of either an anion (e. g., a carban-ion, an enolate, or an alkoxide) or a pseudo anion as an uncharged nucleophile (e. g., an amine, or an alcohol) onto an electrophilic center. A bond formation takes place with the creation of a new real or pseudo-anionic functionality, which can undergo further transformations. The sequence can then be terminated either by the addition of a proton or by the elimination of an X group. 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

Oxidative or Reductive/Anionic/Anionic Domino Processes... 

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