Superoxide anion radical, nucleophilic

Under some circumstances, such as in the presence of added nucleophiles and protons, coordinated dioxygen is displaced as the superoxide anion radical, O2, leaving the metal center oxidized by one electron and unreactive to dioxygen ... 

V. S. Bryantsev, V. Giordani, W. Walker, M. Blanco, S. Zecevic, Predicting solvent stability in aprotic electrolyte Li-air batteries nucleophilic substitution by the superoxide anion radical (02( -)) , The Journal of Physical Chemistry A, 115, 2399-12409, 2011. 

A. Mechanism. Alkyl halides undoubtedly represent the most well-tested functional group for nucleophilic reactivity. That superoxide ion reacts with alkyl haldies by an S 2 mechanism has been demonstrated. Dietz, et al., (J) y observed a relative reactivity which fell in the series n-BuBr > sec-BuBr > -BuBr > jt-BuBr for variation of alkyl group structure and in the series n -BuBr > n-BuOTs > n-BuCl for variation of leaving group. The former order is consistent with a Sj 2 reaction mechanism and the latter order suggests that superoxide anion radical is a strong nucleophile. 

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. ... 

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. 

The properties of superoxide anion have been well reviewed [45,46]. In water it is a relatively weak nucleophile, whereas in aprotic solvents, such as those commonly used for electrolysis, it is a powerful nucleophile. Its basic properties stem from the driving force of the disproportionation [45,46] depicted as Eq. (5), similar to the disproportionation discussed for organic radical anions in Sec. II.B. Thus, although superoxide anion itself is a relatively weak Bronsted base— pA a(H02) 12 in DMF [46]—the overall equilibrium... 

The first reduction step provides superoxide anion, which was shown to react with added silanone precursors as is seen fiom the decrease up to a total disappearance of the oxidation peak of O2. Using the ratio of these signals, /p(02 ) tp(02), and the kinetic treatment proposed in [10] for reversible electron transfer followed by an irreversible reaction of ion-radicals and modifying it to pseudo-first order reactions, we determined absolute rate constants of nucleophilic addition of electrogenerated superoxide anion on several silanone precursors (Scheme 5). 

Nucleophilic-addition reactions. The most common addition reaction is to a carbonyl group without an adequate leaving group. Examples include the reaction of HO" and Oi - with CO2 and quinones.a common feature of these reactions is the formation of an adduct that is sufficiently stable to be isolated or characterized (see Scheme 8-6). The same orange-colored species results from the reaction of solid tetramethylammonium superoxide with gaseous CO2 and with neat CCl4,34 and is believed to be an anion radical, OOC(O)O. 28... 

The raised energy of the HOMO also provides an explanation with an SET mechanism, since it allows an electron to be transferred more easily to the LUMO of the electrophile, and the radical pair then couple, as usual. A single electron removed from one of the two pairs will leave behind a stabilised radical, and so the rate constant of a reaction of an a-effect nucleophile ought to be more sensitive to the ionisation potential (LUMO energy) of the electrophile than the rate constant with a normal lone-pair nucleophile.277 This proved to be the case in the rates of N-methylation of a series of N-phenylhydroxylamines compared with the rates in some comparable anilines.278 Further support for an SET mechanism is provided by superoxide anions, RO2 , which are exceptionally powerful nucleophiles benefiting simultaneously from an a-effect and from the availability of an unpaired electron.279... 

Based on its pKs value under physiological conditions, this activated oxygen species occurs as an anion with its radical character suppressed. It acts as a nucleophilic reagent (e. g. it promotes phospholipid hydrolysis within the membranes) under such conditions, but is not directly able to abstract an H-atom and to initiate lipid peroxidation. The free radical activity of the superoxide anion appears only in acidic media, wherein the perhydroxy radical form (HOp prevails. Some reactions of (HOp are presented in Table 3.30. of is comparatively inactive (Table 3.30). As shown in Reaction 3.69, it dismutates at a rate that is dependent on the pH, e.g., pH 7 k = 5.1051 mol ... 

Aromatic nucleophilic substitution by superoxide occurs by a mechanism different from that encountered in aliphatic nucleophilic substitution reactions of this ion. Thus, reaction of enriched potassium superoxide with l-bromo-2,4-dinitro-benzene catalyzed by dicyclohexyl-18-crown-6 in benzene saturated with unlabeled oxygen results in 2,4-dinitrophenol almost devoid of label. The loss of label in this reaction rules out a direct displacement mechanism. This result is consistent with electron transfer from superoxide anion to the arene to form an intermediate aromatic anion radical which reacts with oxygen (from all sources) to yield phenol. This mechanism is formulated in equation 8.12. Examples of this reaction are presented in Table 8.7. 

A similar study of the photooxidation of some spiropyrans and spironaphthox-azines indicates that the spiro and open forms of these dyes are singlet oxygen quenchers and that the colored form does not act as a sensitizer. A mechanism is proposed that involves the formation of a superoxide radical anion by photoinduced electron transfer to oxygen from a merocyanine form of the dye, followed by nucleophilic attack of the radical anion on the radical cation of the dye.174... 

The carboxyperoxyl radical anion thus produced should be similar in reactivity to the hydroperoxyl radical, HO. The nucleophilic activity of the superoxide ion towards carbonyl groups in acid chlorides, esters and ketones is well documented The reaction between superoxide ion and the Py-Py" cation radical, which leads to destruction of the latter, would seem more likely to mitigate the long-term effects of the Py-Py rather than promote damage to components of the cell d . The occurence of Rh(bipy) -mediated photoreduction of alkenes with NADH models and... 

The nucleophilicity of superoxide toward different alkyl halides was estimated from kinetic measurements [255]. In particular substitution kinetics toward alkyl halides were compared to electron transfer rates with the same halides. Thus, the reactivity of aromatic radical anion A (a donor having the some potential as O2") was compared to that of superoxide. The influence of sub/ ET on the difference in self-exchange reorganization energy A(0) between O2/O2 and X/A was discussed. 

Methionine. The reaction of superoxide radical anions (02 with sulfide radical cation-nucleophile complexes might represent an efficient sulfoxide-forming process in peptides and proteins containing methionine under conditions where significant amounts of sulfide radical cation complexes and superoxide are formed simultaneously. The rate constant for the reaction of 02 with the (S.-. N)+ complex was found to be ca. 3-fold slower as compared to that ofthe reaction with the (S.-.Sf complex. This drop in reactivity may, in part, reflect the lower probability of 62 to encounter S-atom in the (S.-.N) complex as... 

The reactivity of O2 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution (Chapter 7).23-25,45 These and subsequent kinetic studies confirm that the reaction order is primary>secondary >tertiary and I>Br>Cl >F for alkyl halides, and that the attack by O2-- results in inversion of configuration (Sn2). Superoxide ion also reacts with CCl4,25,26 Br(CH2)2Br,46 C6Cl6, 2,48 and esters -Sl in aprotic media. The reactions are via nucleophilic attack by O2-- on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxy anions, water suppresses the nucleophilicity of 02 - (hydration energy, 100 kcal)52 and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO- and ROO ). 

In contrast to the case where aniline is used as the nucleophile, the benzamide reaction can be improved by utilizing dioxygen in the reaction mixture since 11 is resistent to autoxidation. Under aerobic conditions the nitrobenzene radical anion is readily trapped by O2 generating superoxide and nitrobenzene (Figure 10) (11). This reaction pathway inhibits the formation of azoxybenzene by diverting the electron transfer cascade and ultimately utilizing dioxygen as the terminal oxidant. Thus, under aerobic reaction conditions 12 is the only observed reaction product. 

---