Oxidation cationic centers

All studied model compounds can distinctly be divided into three groups (Table VII). The first group is composed of substances in which the sulfur, selenium or cyclopentadienyl anion acts as an anionic center. They exist only in open betaine forms, and their PES do not contain local minima corresponding to cyclic isomers. The second group contains compounds with arsonium cationic and oxide anionic centers and silicon and germanium betaines with arsonium and amide centers. They exist as cyclic isomers and their PES have no local minima corresponding to the open forms. Finally, the third group consists of six studied compounds with phosphonium cationic and oxide or amide anionic centers and arsonium-imide betaine. Their PES have minima for both cyclic and open forms separated by low barriers. 

The cation pool method is based on the irreversible oxidative generation of organic cations. In the first step, the cation precursor is oxidized via an electrochemical method. An organic cation thus generated is accumulated in the solution in the absence of a nucleophile that we want to introduce onto the cationic carbon. Counter anions which are normally considered to be very weak nucleophiles are used to avoid the nucleophilic attack on the cationic center. In order to avoid thermal decomposition of the cation, electrolysis should be carried out at low temperatures such as -78 °C. After electrolysis is complete, the nucleophile is then added to obtain the desired product. The use of a carbon nucleophile results the direct carbon-carbon bond formation. 

The anodic oxidation of compound 35 resulted in the formation of dication 36 (Scheme 19). 13C NMR indicated that two cationic centers are equivalent. The addition of allyltrimethylsilane to the solution gave rise to the formation of diallylated compound 37 in 80% yield. 

The oxidation of8-f-hutyl-l-(2-pyridyl)-2-naphthol illustrates the reaction between a produced cationic center and a tertiary amine (Scheme 29) [40]. The produced pyridinium salt reacts in a basic medium with loss of isobutylene. 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

Cr(CO)6 undergoes a primary diffusion-controlled one-electron oxidation near -El.5 V versus SCE in MeCN or CF3 COOH containing [NBu4][Bp4] to give the unstable paramagnetic Cr 17-electron radical cation, [1, 2] whereas Mo(CO)6 and W(CO)6 both undergo irreversible multielectron oxidation at a similar potential under the same conditions [2]. This can be attributed to the ease of attack by nucleophiles on the larger Mo and W cation centers - the tendency of these metals... 

Armstrong D, Sun Q, Schuler RH (1996) Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals comparisons with phenoxyl radicals. J Phys Chem 100 9892-9899 Asmus K-D (1979) Stabilization of oxidized sulfur centers in organic sulfides. Radical cations and odd-electron sulfur-sulfur bonds. Acc Chem Res 12 436-442 Asmus K-D (1990a) Sulfur-centered free radicals. Methods Enzymol 186 168-180 Asmus K-D (1990b) Sulfur-centered three-electron bonded radical species. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 155-172... 

K.-D. Asmus, Acc. Chem. Res., 12, 436 (1979). Stabilization of Oxidized Sulfur Centers in Organic Sulfides. Radical Cations and Odd-Electron Sulfur-Sulfur Bonds. 

Important oxidation processes involving the metal centers can occur at the surfaces of transition metal oxides (e.g, -Cr203) upon dissociative oxygen adsorption (Scheme 3). In some cases the (nondissociative) adsorption of oxygen can lead to the formation of superoxide ( )2 or peroxide 02 species with simultaneous oxidation of surface metal cation centers. 

Three types of solvent or solute delocalization have now been examined, as summarized in Table III for three different adsorbent types (four, if we distinguish Cig-deactivated silica from silica). The theoretical requirements on the configuration and density of adsorption sites were discussed earlier (Section II,B) for a given type of localization/delocalization to be possible. In each case the nature of adsorption sites is fairly well understood for the four adsorbents of Table III, as disucssed in Ref. / and 17 and shown in Fig. 14. Thus, in the case of alumina, surface hydroxyls do not function as adsorption sites. Although surface oxide atoms are capable of interacting with acidic adsorbate molecules (see below), in most cases the adsorbate will interact with a cationic center (either aluminum atom or lattice defect) in the next layer. As a result, we can say that in most cases adsorption sites on alumina are buried within the surface, rather than being exposed for covalent site-adsorbate interaction. These sites are also rigidly positioned within the surface. Finally, the... 

The reaction has been used to synthesize libraries of benzonaphthyridines 196, in high diastereoselectivity, from the cycloaddition of 1,4-dihydrop3Tidines with imines formed from aldehydes and anilines. When cyclic enol ethers were used as dienophUes, mixtures of diastereomers 197 were obtained. These compounds were oxidized to the corresponding quinolines 198 and were further transformed to the quinolinium salts 199 as shown in Scheme 36 [76]. Compounds 196 and 198 were tested for their ability to inhibit human propyl oligopeptidase (POP) and were found to have modest potencies. Much better results were obtained when the quinoline nitrogen was methylated to provide adducts 199. The cationic center improved the inhibitory activity of these compounds (Fig. 23). 

Low-temperature adsorption of CO is widely used as an acid site probe. In Figure 3.3 the spectra of CO over different solids are compared. The CO stretching band may shift strongly up when it bonds as terminal carbonyl species on highly acidic cationic centers, and can shift down significantly when it bridges over two or three metal atoms over an extended metal surface. In Table 3.11 the position of some sensitive bands over oxide surfaces is reported, allowing the measurement of their Lewis acidity. 

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