2,2,6,6-tetramethylpiperidine A-oxyl

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

U sing TR ESR, a model reaction between the free radicals of Pis and stable nitroxyl radicals of 2,2,6,6-tetramethylpiperidine-A-oxyl (TEMPO) family was studied. We will abbreviate the TEMPO fragment further as N. Nitroxyl biradicals (N-O-N), had radical termini in proximity to each other (see Scheme 12.12). 

Vinyl Compounds. Photorefractive polymers can be prepared by living radical polymerization. 4-Vinyl-triphenylamine can be polymerized by a conventional radical catalyst or a 2,2,6,6-tetramethylpiperidine-A-oxyl (TEMPO) like catalyst, such as A-(a-methyl-benzyloxy)-2,2,6,6-tetramethylpiperidine [167]. 

Originally, an ionic mechanism was postulated, which is suggestive, as a strong base is used to initiate the polymerization reaction. However, some experimental results do not support the concept of an ionic mechanism. Instead it is believed that the reaction is initiated by a radical process in which diradicals are formed. The radical mechanism is supported by the finding that 2,2,6,6-tetramethylpiperidine-A-oxyl acts as a scavenger [21]. 

A spin label is a stable paramagnetic molecule that contains an atom or group of atoms with an unpaired electron spin that can be bonded to another molecule. In this way, one can detect molecules that otherwise would not give an ESR spectrum. Most spin labels are nitroxide compounds. For example, 2,2,6,6-tetramethylpiperidine-A-oxyl (known as TEMPO) is a common spin label with a characteristic three-line ESR spectrum, seen in Figure 3.87. 

In the 1990s the groups of Rizzardo and Georges reported a stable free radical polymerization process (SFRP) allowing the preparation of polystyrene with a narrow polydispersity. In the presence of stable free radicals, such as the mainly used 2,2,6,6-tetramethylpiperidine-A-oxyl (TEMPO), macromolecules based on styrene and styrene derivatives with well defined structures were synthesized [263,264],... 

Nitroxide-labeled Ru(bpy)2 -type complexes can be used as EPR probes for the study of oligonucleotides. A ligand phen-T has been prepared that has a TEMPO (2,2,6,6-tetramethylpiperidine-A -oxyl) group chemically bound to its periphery (Fig. 5.29). Even though TEMPO is known to quench the triplet excited state of Ru(bpy)2-type complexes, no intramolecular quenching occurs in the tethered complexes [Ru(phen)2(phen-T)] and [Ru(bpy)2(phen-T)] . These complexes have been used in a combination of EPR and luminescence techniques to study their binding with DNA. 

Another chemically more interesting spin labeled B12 derivative involves coordinate attachment of the nitroxyl function to the cobalt atom of a cobinamide. Fig. 22 shows a reaction in which an alkyl cobin-amide is mixed with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. The nitroxide displaces water from the 6th coordination position very slowly and therefore this reaction is usually allowed to proceed for a few days with a large excess of nitroxide. From the properties of the coordinated nitroxide derivative discussed below, it is certain that the cobalt is coordinated by the N—O functional group. An analogous compound to that shown in Fig. 22 can be made with a similar nitroxide in which the 4-hydroxyl-group is missing. In this case the N—O-function is the only basic site on the molecule and therefore must be the position of attachment to the cobalt 119). 

Fig. 25. Electron spin resonance spectra of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl aquocobinamide before and after treatment with CN (a) spectrum of aquo derivative, (b) Expanded view of center line before addition of CN-. (c) Spectrum of liberated nitroxide. (d) Expanded view of centerline after CN- treatment showing additional proton hyperfine...

The second synthesis of crystalline 43 was reported by Mori as summarized in Scheme 62 [93]. The building block (4.R,5S)-A was prepared by an enzymatic process, while another building block C was synthesized via Sharpless asymmetric epoxidation. Coupling of A with C gave D, which was cyclized under Op-polzer s conditions to give crystalline E. When E was oxidized with Dess-Martin periodinane or tetra(n-propyl)ammonium perruthenate or Jones chromic acid, crystalline 43 was obtained. Swern oxidation or oxidation with 2,2,6,6-tetramethylpiperidin-1 -oxyl of E afforded only oily materials. Accordingly, oxidation of E to 43 must be executed extremely carefully. A synthesis of oily 43 was reported by Gil [94]. 

FIGURE 10.3 The structure of TEMPO. The compound 2,2,6,6-tetramethylpiperidine-l-oxyl is a stable radical. 

Figure 5.9 TEMPO DE is obtained by electrodeposition of a thin layer of orga-nosilica doped with TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) upon application of — 1.1 V (v.v. Ag/AgCl) for 15 min to a solution of suitable organosilanes (left). The electrocatalytic film thereby obtained selectively converts benzyl alcohol dissolved in 0.2 M NaHC03 (right). 

In other cases, organic modification of the sol gel cages markedly protects the entrapped molecular dopant from degradation by external reactants, as shown for instance by the entrapment of the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). This is a highly active catalyst which in the NaOCl oxidation of alcohols to carbonyls in a CH2CI2-H20 biphasic system becomes highly stabilized upon sol gel entrapment in an ORMOSIL matrix it progressively loses it activity when entrapped at the external surface of commercial silica.25... 

This blend was obtained by polymers mixture extrusion and extraction with the azeotropic mixture of hexane/ethanol, and modifying the obtained polymer surface by coupling of 4-isocyanato butanoic acid methyl ester (as a spacer molecule) to PVA blend, saponification of methyl ester groups and coupling of 4-amino-TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) [229],... 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

A double mediatory system consisting of A-oxoammonium salts and active bromine species, generated from 2,2,6,6-tetramethylpiperidine-1 -oxyl derivatives... 

The formed acyl radicals are reactive towards efficient radical trapping reagents such as 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO), diphenyl diselenide and diphenyl disulphide, and A-f-butyl-a-phenylnitrone giving the respective adducts. ... 

Reaction of the Ru macrocyclic complex [RuLCy (L= 1,5,9,13-tetramethyl-l,5,9,13-tetraaza-cyclohexadecane) with N02 results in a disproportionation of the initial [Ru °LCl(N02)], the final products being traTO-[Ru L(0)Cl]" " and [Ru L(OH)(NO)] " "." The reaction between [Ru(OEP)Me] (H2OEP = octaethylporphyrin) and 2,2,6,6,-tetramethylpiperidine-l-oxyl (TEMPO) produces [Ru(OEP)CO]. There is clear evidence that the CO ligand is derived from the axially bound CH3 group, making this reaction an important example of CH3 to CO transformation." ... 

Three nitroxide derivatives of Ru2 species have been reported. The first contains the [Ru2(//-02C Bu)4L2] ion where L = 2,2,6,6-tetramethylpiperidine-l-oxyl. A relatively large antiferromagnetic coupling is observed between the Ru2 " core and the nitroxide radical. The second example is [Ru2(//-02C Bu)4L ] where L = 2-phenyl-4,4,5,5-tetramethyM,5-dihydro-lH-imidazolyl-l-oxy-3-oxide. 

Shorthand notations such as ET (electron transfer), HAT (hydrogen atom transfer), BDE (bond dissociation energy), NHE (normal hydrogen electrode), CV (cyclic voltammetry), LFP (laser flash photolysis), EPR (electron paramagnetic resonance) and KIE (kinetic isotope effect) will be used throughout the chapter. In addition, recurring chemical compounds such as TEMPO (2,2,6,6-tetramethylpiperidine-Ai-oxyl), HBT (1-hydroxyben-zotriazole), BTNO (benzotriazole-A-oxyl), HPI (iV-hydroxyphthalimide), PINO (phthal-imide-iV-oxyl), NHA (A-hydroxyacetanilide) and a few others will be referred to by means of the capital-letter acronym. 

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... 

During the induction periods caused by adding antioxidants, a small contraction in volume occurred because of the formation of dimers of chloroprene (14). This reaction occurs during the oxidation but was most easily studied by dilatometry in the absence of oxygen. A few values of the initial rate of dimerization of chloroprene, inhibited against polymerization with 2,2,6,6-tetramethylpiperidine-l-oxyl, are given in Table III. Their dependence on temperature is given by... 

A convenient procedure for the oxidation of primary and secondary alcohols was reported by Anelli and co-workers (8,9). The oxidation was carried out in CH2CI2 with an aqueous buffer at pH 8.5-9.5 utilizing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 1) as the catalyst and KBr as a co-catalyst. The terminal oxidant in this system was NaOCl. The major disadvantage of using sodium hypochlorite or any other hypohalite as a stoichiometric oxidant is that for each mole of alcohol oxidized during the reaction one mole of halogenated salt is formed. Furthermore,... 

We wish to report here on a new and highly efficient catalyst composition for the aerobic oxidation of alcohols to carbonyl derivatives (Scheme 1). The catalyst system is based on 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Mg(N03)2 (MNT) and N-Bromosuccinimide (NBS), utilizes ecologically friendly solvents and does not require any transition metal co-catalyst. It has been shown, that the described process represents a highly effective catalytic oxidation protocol that can easily and safely be scaled up and transferred to technical scale. 

As peracids react very sluggishly with alcohols, it was apparent that the presence of a nitroxide was playing an important role in the oxidation of the alcohol into a ketone. This seminal serendipitous observation led to the development of the first description of the oxidation of alcohols mediated by catalytic 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) (55), published almost simultaneously by Celia et al and Ganem.3 These authors presented two papers with remarkably similar contents, in which alcohols were oxidized by treatment with MCPBA in CH2CI2 at room temperature in the presence of a catalytic amount of TEMPO (55). In both papers, a plausible mechanism is presented, whereby m-chloroperbenzoic acid oxidizes TEMPO (55) to an oxoammonium salt 56. This oxoammonium salt 56, as detailed in Ganem s paper, can react with the alcohol producing an intermediate 57, which can deliver a carbonyl compound by a Cope-like elimination. 

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