Superoxide, nucleophile

A more direct procedure for the displacement step is the use of a very powerful nucleophilic superoxide radical ion where an alkyl halide or sulfonate is directly converted into the corresponding alcohol (Scheme 1). Elimination is an important side reaction in certain cases. ... 

Fe(II), is assumed to be converted to a nucleophilic superoxide moiety and attack the ring at sites of low electron density. The carbon adjacent to the oxygen-bearing carbon is chosen as the position to be attacked by the oxygen species, but further discussion is necessary for the site of dioxygen attack on the aromatic ring. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

Both the basic and nucleophilic reactions within the cage are totally inhibited by the presence of one equivalent of Na + PFg in THF. Double ion exchange between the two ion pairs is favored which removes superoxide from the cage Eq. (10) ... 

Niehaus, W.J., Jr (1978). A proposed role of superoxide anion as a biological nucleophile in the deesterification of phospholipid. Bioorganic Chemistry, 7, 77-84. 

Two alternatives to conventional acid/base hydrolyses for cleaving esters are Sn2 displacement of the carboxylate group by reactive nucleophiles and nucleophilic attack at the carbonyl carbon. In this latter context we investigated the reaction of S-b-MM with potassium trimethylsilanolate, a so-called potassium superoxide equivalent (15). One advantage that this reagent has over potassium... 

Displacement by cyanide works particularly well, and many other nucleophilic substitution reactions are enhanced by PTC. Most monovalent anions can be transferred, including alkoxides, phenoxides, thiocyanates, nitrates, nitrites, superoxides and all of the halides. Divalent anions are usually too hydrophilic to be transferred into the organic phase. 

It has been shown that when nucleophilic aromatic photo-substitution reactions are carried out in non-deoxygenated solutions of aprotic solvents, such as DMSO and acetonitrile, destructive superoxide anions may be formed from aromatic radical anions. Such solvents are best avoided. There has been a review of mechanistic aspects of photo-substitutions of the cyano group in aromatic compounds. ... 

As noted earlier, the superoxide ion reacts with effective electron acceptors. It is a one-electron reductant of moderate strength. However, the superoxide ion can act as a nucleophile if a substrate has a decreased electron affinity. For instance, alkyl halides react with the ion O2 RCH2OO -h HaE. This reaction initiates the next ones RCH2OO ... 

Hence, nucleophilic reactions of the superoxide ion are typical. This ion can be compared with the thiophenoxide and thiocyanate ions with respect to nucleophilicity. The cause of such high nucleophilicity lies in a so-called a-effect In 0—0 ion, an attacking site (O ) adjoins directly to a site (O ) with a significant electronegativity. This effect usually confers special activity to nucleophiles. The effect can be additionally enhanced by including the 0—0 group in sulfenate. 

To summarize, the superoxide ion has high basicity and nucleophilicity. It can react as a reducer and cannot serve directly as an oxidant. 

Cathodic reduction of oxygen is the most convenient method of production of the superoxide radical-anion,. The properties of this important species have been well reviewed and key references to the extensive work on the electrochemistry of oxygen are contained therein. Of immediate significance is the large cathodic shift in E° for the 0 /0 couple which accompanies a change from aqueous to aprotic solvent (e.g. DMF, DMSO, and MeCN) this is interpreted in terms of relatively weak solvation in aprotic media which enhances the nucleophilicity of the superoxide anion. However, in the presence of acids the chemistry of superoxide is dominated by the disproportionation shown in equation 1. 

If a tetraalkylammonium salt is used as supporting electrolyte, this process is either reversible or quasi-reversible and occurs at around -0.8 V vs aqueous SCE in various aprotic solvents and with various electrode materials (Hg, Pt, GC). If a Bmisted acid is added to the solution, the first step is converted to a two-electron process 0 produced in the first step is protonated to form 02H, which is more reducible than 02. Thus, 02H is further reduced to 02H at the potential of the first step. According to detailed polarographic studies in H20-DMS0 mixtures, about 30% v/v water is needed to convert the one-electron process to the two-electron process [41]. A metal ion, M+, interacts with 02 to fonn an ion-pair M+-02 (often insoluble) and shifts the half-wave potential of the first wave in a positive direction [42]. Electrogenerated superoxide 02 can act either as a nucleophile or as an electron donor and has been used in organic syntheses [43],... 

The most important chemical function of Zn2+ in enzymes is probably that of a Lewis acid providing a concentrated center of positive charge at a nucleophilic site on the substrate/ This role for Zn2+ is discussed for carboxypeptidases (Fig.12-16) and thermolysin, alkaline phosphatase (Fig. 12-23),h RNA polymerases, DNA polymerases, carbonic anhydrase (Fig. 13-1),1 class II aldolases (Fig. 13-7), some alcohol dehydrogenases (Fig. 15-5), and superoxide dismutases (Fig.16-22). Zinc ions in enzymes can often be replaced by Mn2+, Co2+, and other ions with substantial retention of catalytic activity/ ... 

The reaction of alkyl isothiocyanates, RNCS, with diphenylphosphinic hydrazide (338) in benzene has been reported.308 The bis(diethylamino)[(methylthio)thiocarbon-yl]carbenium salts (339 X = I or BF4) display ambident reactivity and can react either at carbenium carbon (hard nucleophiles) or at the thiocarbonyl sulfur atom (soft nucleophiles).309 Electrochemically generated superoxide reacts with dithioic S,S -diesters (dicarbothiolates) (340 Ar = C5H3N or C6H4) to give the monocarboxylate anions in 100% yield before giving the dicarboxylate anions.310... 

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