Hydroperoxide ion

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Henbest and Jackson have rationalized these remote directive effects on the basis of the well-supported mechanism of alkaline epoxidations. The initial step in the reaction is the reversible addition of the hydroperoxide ion... 

Cyclopropenones can be oxidized according to several methods. When diphenyl cyclopropenone is treated with alkaline H202 desoxybenzoin is formed as the main product and is claimed to arise from primary addition of hydroperoxide ion to the Cl/C2 bond206. Treatment with KMn04 gave benzil67. ... 

Early research suggested that the key intermediate in the POCL reaction was 1,2-dioxetandione (structure II in Fig. 5) [2, 3], which is formed following the multistage nucleophilic attack by the hydroperoxide ion on one of the carbonyl carbons in the oxalate compound. This series of reactions is proposed to occur... 

In addition, other processes may occur such as the reaction of the hydroperoxide ion with the conductor to form metal-oxygen bonds which in turn may be reduced. The hydroperoxide ion may itself decompose to reform oxygen, etc. The potential of an oxygen electrode is invariably a mixed potential with a value of about 1.0 V on the standard hydrogen scale, at zero current drain. 

Peroxide and hydroperoxide ions. A patent disclosure h Barueoh and Payne101 has described addition of teri-bntylliydi- peroxide to ethylene oxide, propylene oxide, and isobutylene oxide in ether, in the presence of either basic or acidic catalysts. The <°rt-Imtylperoxide ion, like other nucleophiles, apparently prefers to uttar. c... 

Studies of the reaction between 2,4-dinitrophenyl benzenesulfonate and OH-, CN , and N3- in 20 mol% DMS0-H20 at 25 °C have shown that OH- attacks exclusively at the sulfonyl group, but the softer nucleophiles react - as shown in (40) - to give mixtures of products of S-0 and C-O bond fission, the fraction of the latter being 0.10 for CN- and 0.66 for N3-. 58 The rate of reaction of hydroperoxide ion with 4-nitrophenyl 4-toluenesulfonate was reported.43... 

Hydrogen peroxide is an ineffective oxidant in polar solvents at acidic or neutral pH, as the formation of a hydroperoxide ion is quite difficult [Eq. (33)]. When the pH is increased, the equilibrium that exists in neutral H202 solutions is shifted to the right [Eq. (34)]. 

The catalytic dismutation of superoxide is actually more complicated in E. coli [42] and B. thermophilus [43] Mn-SODs than that of either Cu or Fe proteins since it may involve an inactive form of the enzyme. The inactive form is believed [44] to contain a Mnm-side-on peroxo unit (of the type shown in Figure 29) formed within the hydrophobic environment of MnSOD, in the absence of H+, by the oxidative addition of the superoxide ion to the Mn11 center. When H+ ions are present, an active, end-on peroxo complex forms, yielding successively a bound hydroperoxide ion and free dihydrogen peroxide (cf. Figure 3). Thus, the key parameter that turns the reaction off or on may be the absence or presence of a H+ ion [44],... 

Superoxide Ion. The dominant characteristic of 02 in any medium is its ability to act as a strong Br nsted base via formation of HOO-,11 12 which reacts with itself or a second Oj (Scheme 9.1). Within water, superoxide ion is rapidly converted to dioxygen and hydroperoxide ion ... 

However, these reactions can actually be considered as electron transfer or oxygen atom transfer reactions, as in the case of the ozone reactions with the hydroxyl and hydroperoxide ions or with the nitrite ion, respectively ... 

On the other hand, the indirect type of ozonation is due to the reactions of free radical species, especially the hydroxyl radical, with the organic matter present in water. These free radicals come from reaction mechanisms of ozone decomposition in water that can be initiated by the hydroxyl ion or, to be more precise, by the hydroperoxide ion as shown in reactions (4) and (5). Ozone reacts very selectively through direct reactions with compounds with specific functional groups in their molecules. Examples are unsaturated and aromatic hydrocarbons with substituents such as hydroxyl, methyl, amine groups, etc. [45,46],... 

Figure 4 Scheme of ozone decomposition mechanism in water. P = promoter (e.g., ozone, methanol). S = scavenger or inhibitor (i.e., /-butanol, carbonate ion). I = initiators (e.g., hydroxyl ion and hydroperoxide ion). 

Hydrogen peroxide was discovered in 1818 by Tenard the molecular structure forms an oxygen bridge, with each oxygen bonded to one hydrogen atom. In water, it is a weak acid, which dissociates to yield the hydroperoxide ion, H02 ... 

As reported by Staehelin and Holgne [115], ozone reacts only with the ionic form of hydrogen peroxide, the hydroperoxide ion, H02 . These authors studied this reaction at different hydrogen peroxide concentrations... 

Reactions (58) and (61) constitute the main initiation reactions of the mechanism leading to the formation of hydroxyl radicals. The reaction between ozone and the hydroperoxide anion constitutes the faster mechanism as has been demonstrated in previous studies [119,120]. The reaction between ozone and the hydroxyl ion is negligible compared to the reaction between ozone and the hydroperoxide ion because the rate constants of these reactions differ by several orders of magnitude 70 M 1 s 1 for reaction (59)... 

Brillas E, Maestro A, Moratalla M, Casado J. Electrochemical extraction of oxygen from air via hydroperoxide ion. J Appl Electrochem 1997 27 83-92. 

Alcaide F, Brillas E, Cabot PL, Casado J. Electrogeneration of hydroperoxide ion using an alkaline fuel cell. J Electrochem Soc 1998 145 3444-3449. 

The normal synthetic pathway for hydroboration is reaction with an ambiphilic nucleophile of which the simplest example is hydroperoxide ion. This elicits a 1,2-migration of an alkyl group from boron to oxygen with concurrent loss of hydroxide ion. The step occurs with essentially complete retention of configuration. In similar vein, ambiphilic species with the structure NH2X may be used in amination, so that the overall reaction is an addition of ammonia to the alkene with the regio- and chemoselectivity driven by the hydroboration step. A majority of reactions of organoboranes can be rationalized in terms of these ionic mechanistic pathways, or closely related protocols (Scheme 2). 

The oxidation occurs by nucleophilic attack of the hydroperoxide ion on the empty orbital of the boron atom followed by a migration of the alkyl chain from boron to oxygen. Do not be alarmed by hydroxide ion as leaving group. It is, of course, a bad leaving group but a very weak bond—the 0-0 <5 bond—is being broken. Finally, hydroxide attacks the now neutral boron to cleave the B-O-alkyl bond and release the alcohol. 

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