Aminoxyls

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

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. 

Organolithium additions to 2-phenyl-3,3-dimethyl-3//-indole (37), followed by oxidation of indolines (38) with m-chloroperoxybenzoic acid, gave indolinic aminoxyls (39) in 20-30% yield. The organolithium addition does not occur when groups other than phenyl are present at C(2). Attempts to synthesize suitable precursors such as 1,2-dihydro-2-phenyl-2-alkylbenzothiazole, 1,2-dihydro-2-phenyl-2-alkylbenzoxazole, and l,2-dihydro-2-phenyl-2-alkyl-4//-3,l-benzoxazin-4-one for other new aminoxyls failed. 

The copolymerization of styrene and maleic anhydride was studied by the spintrapping technique using 2-methyl-2-nitrosopropane as a spin trap. Four types of ESR spectra were obtained, of which three corresponded to trapping of the growing polymer chain at a centre originating from the styrene part or from two centres originating from the maieic anhydride part. The fourth EPR spectrum may be due to a cyclic flve-membered aminoxyl or a six-membered 1,2-oxazine radical cation. ... 

Chiral non-racemic 0-(2-ketoalkyl) A-phenylhydroxylamines such as 115 (equation 84) can be prepared through catalytic enantioselective a-aminoxylation of carbonyl compounds catalyzed by proline. This reaction proceeds with a variety of ketones and aldehydes although it has been tried only with a nitrosobenzene component ... 

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... 

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