Radicals aminoxyl

Other [AuCIL] complexes with monophosphines are those derived of o-carborane (427),2506 pyr-anoside group (428),2507 a-dimethylamino(3)ferrocenophan-2-yl,2508 calix[4]arene derived phosphites,25 or aminoxyl radical-substituted phosphines (429),2510 for which ferromagnetic interactions have been observed. 

A spin trap is a diamagnetic compound that reacts with a radical by addition of the radical functionality typically to a double bond in the trap, thus forming a new radical that is more stable (better, less unstable) than the original radical. By far the most common class of spin traps are nitrone compounds that, upon addition of the primary radical, produce a stable aminoxyl radical (Figure 10.1). The compound DMPO is the paradigmatic spin trap it is readily available, widely used, and its EPR spectra are relatively easy to interpret. Some of its radical adducts have unpractically short lifetimes. 

In atmospheric chemistry, reactions between pure nitric oxide and a range of activated alkenes have been examined. However, no addition products were observed." Only in the presence of NO2 was addition to give jS-nitroalkyl radicals, followed by trapping to /3-nitronitroso compounds, observed. These final products can also trap other radicals to give aminoxyl radicals. 

Furthermore, the rearrangement of ( )-A-allylhydroxylamines into ( )-0-allylhydrox-ylamines at room temperature was reported . An aminoxyl radical was detected by EPR and its participation in a Meisenheimer [2,3]-rearrangement was proposed. 

The recent TUPAC Compendium of Chemical Terminology—The Gold Book recommends that the name of compounds having the structure R2N—O" R2N +—0 is more appropriately that of aminoxyl radicals . The synonymous terms nitroxyl radical or nitroxide are accordingly not desirable, even though quite popular in various fields of science and technology. This chapter follows a previous chapter of the series and, for this historical reason, retains the old terminology of the compounds in the title, but this use will be discontinued from now on in the text. 

Besides this hydrogen atom transfer (HAT) route, the aminoxyl radical may also take part in oxidation procedures where, through a preliminary monoelectronic step, it is converted into an oxoammonium ion (R2N=0+), or variations of this route. Examples of... 

II. PHYSICOCHEMICAL PROPERTIES OF THE AMINOXYL RADICALS A. General Features... 

It is appropriate to summarize here relevant data of both the aminoxyl radicals and their precursors that are useful for the ensuing discussion. Hydroxylamine (H2NOH) is the precursor of the archetypal aminoxyl radical (HaNO ). Upon one-electron oxidation of H2NOH by a suitable oxidant, e.g. Ce, the formation of H2NO (plus one proton) has been documented by the first step being the formation of the transient radical... 

SCHEME 2. Possible pathways for the generation of aminoxyl radicals... 

H2NO is a typical reactive intermediate endowed with short hfetime, aminoxyl radicals deriving from secondary amines may instead be stable whenever complying with the general structure 1 (R H), in which there are no hydrogens attached to the a-carbon atoms. 

TEMPO (2,2,6,6-tetramethylpiperidine-Af-oxyl) and its cognates (4-OH, 4-oxo, 4-OMe substituted derivatives TMIO etc.) belong to a group of stericaUy hindered aminoxyl radicals (Chart 1) and, in view of the long hfetimes, are said to be persistent and... 

The Fremy s salt (nitrosodisulfonate), whose properties as a selective oxidant have been reviewed , and bis(trifluoromethyl)aminoxyl radical, (CF3)2N—O" which has been... 

In general, dialkyl aminoxyl radicals show an intense absorption band in the 220-260 nm range, and a yellow colour due to a weaker absorption band in the 410-450 nm region. a-Electron-withdrawing or a-conjugating groups shift X ax towards 290-300 nm, and the weaker absorption band towards 480-510 nm °. Factors affecting... 

A few hints about methods of generation of non-persistent aminoxyl radicals, which will be quoted a few times later in the chapter, are now given. The dichotomy of generation of an aminoxyl radical from the parent hydroxylamine has already been outlined in Scheme 2. 

SCHEME 5. Generation of an aminoxyl radical by H-abstraction, ensuing laser flash photolysis of an appropriate peroxide. Redrawn with permission from Reference 134. Copyright (2005) American Chemical Society... 

There is an obvious expectation that the larger the BDEno-h value, the more efficiently the aminoxyl radical will abstract H-atom from a substrate (cf. Scheme 1). We will see in Section III how this expectation finds reasonable experimental support, and how the BDE data of Tables 1 and 2 provide a guideline to foresee H-abstraction proficiency. However, before proceeding to Section III, more important information needs to be added. 

Persistent or moderately stable aminoxyl radicals (1, in Scheme 6) lend themselves to one-electron oxidation to yield an oxoammonium ion (4) at a redox potential that depends on the structure of the hydroxylamine precursor Calculation methods have... 

SCHEME 6. Oxidation of an alcohol by the oxoammonium ion 4 derived from an aminoxyl radical 1... 

III. REACTIVITY STUDIES A. H-abstraction by Persistent Aminoxyl Radicals... 

As a rule of thumb, oue would expect that the more stable the aminoxyl radical, the lower its overall reactivity. This matches the above reported expectation (cf. Scheme 1) that the lower the BDEno-h value, the less efficiently the aminoxyl radical wiU abstract H-atom from a substrate. These two statements are indeed corroborated by a few literature examples. TEMF 0 (Chart 1), a persistent aminoxyl radical , upon H-abstraction forms one of the weakest NO—H bonds among those reported in Table l . It is no wonder... 

The reactivity of TEMPO, and of other aminoxyl radicals, in the scavenging of carbon-or sulfur-centred radicals (equation... 

Table 1 shows that iV-hydroxyphthalimide (HPI) presents features almost opposite to those of TEMPO. In fact, the dissociation energy of the NO—H bond in HPI (88 kcal moR ) , accessible through EPR measurements" , is much higher than that of the corresponding NO—H bond in the hydroxylamine moiety of TEMPO (viz. TEMPOH, 69 kcalmoR ) . TEMPO is a persistent aminoxyl radical , whereas the aminoxyl radical of HPI, dubbed PINO (phthaUmide-N-qxyl)", is not stable but endowed with a half-life of ca 8000 s in AcOH solution at 25 °C . By using an oxidant such as a Pb(OAc)4 , or a Co(III) salt " " , HPI can be oxidized into PINO, and the H-abstraction reactivity of the latter subsequently exploited in the catalytic oxidation of appropriate H-donor substrates (RH) (Scheme 7). 

SCHEME 7. The hydrogen atom transfer (HAT) pathway of an aminoxyl radical... 

More recently, the soundness of the electrochemical approach for the generation of PINO from HPI has been confirmed by using cyclic voltammetry at a rotating disk electrode. Anodic oxidation had been also employed for the generation of the aminoxyl radical from hydroxamic acids. ... 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

Additional clues confirm the radical nature of the reaction of BTNO with RH substrates. A Hammett correlation, obtained on plotting log A h for reaction of BTNO with p-substimted benzyl alcohols, gave a p value of —0.55 vs. a+. This small value, which is reasonable for a radical reaction, compares well with the p values ranging from —0.54 to —0.70 and obtained vs. the same substrates with the aminoxyl radicals generated from X-aryl-substituted HPIs (Table 5). In all cases better Hammett correlations were obtained vs. the values. Hence, a uniform pattern of selectivity emerges among these electrophilic >N—O" species in H-abstraction reactions. 

It will be useful to evaluate and compare the reactivity of additional aminoxyl radicals in the H-abstraction reaction in future investigations. 

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