Tetramethylpiperidine nitroxyl

Fig. 17,16. Mechanism of the TEMPO oxidation of alcohols to aldehydes (TEMPO stands for tetramethylpiperidine nitroxyl).

Multi-redox-mediated electrosynthesis may significantly expand the scope of the electrosynthesis. Various combinations of different kinds of mediators have been smdied intensively so far. Nicotinamide adenine dinucleotide (NAD)/methyl viologen double mediatory systems are utilized successfully for various enzymatic reductions [14]. The combinations of halide ions and 2,2,6,6-tetramethylpiperidine nitroxyl (TMPO)... 

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). 

Nitroxyl radicals can be oxidized to N-oxo ammonium salts that are themselves useful oxidants for primary and secondary alcohols. Recently, the behavior of different nitroxides as catalysts for alcohol oxidation has been studied by quantum chemical calculations [105]. Generally, 2,2,6,6-tetramethylpiperidine Ai-oxyl (TEMPO) (80) is used for the... 

The oxidation of secondary amines with no a-hydrogen atoms leads to hydroxy-lamines, but excessive dioxirane may further oxidize the hydroxylamines to the corresponding nitroxyl radical. For example, when a slight excess of isolated DMD is employed, 2,2,6,6-tetramethylpiperidin-4-ol (15) is quantitatively transformed into the hydroxylamine... 

Stable organic nitroxyl radicals are of relatively recent use as catalysts in the oxidation of alcohols. Nitroxyl radicals are compounds that contain the A ,A -disubstituted NO-group with one unpaired electron, and their uses have been reviewed.124 The most simple radical of this class is 2,2,6,6-tetramethylpiperidin-l-oxyl (43, TEMPO). It is generally assumed that the active oxidizing species, the oxoammonium salt (44), is formed in a catalytic cycle by a one-electron oxidation of the nitroxyl radical by a primary oxidant [two-electron oxidation of the hydroxylamine (45) is also possible, depending on the primary oxidant] (Scheme 21). 

To reveal the mode of dye-DNA binding, we studied quenching of the triplet states of thiaearboeyanine dyes K1 - K4 in solutions and in eomplexes with DNA (aqueous phosphate buffer, pH 7) by the stable nitroxyl radical 4-hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl (4-hydroxy-TEMPO), iodide ion, and oxygen. To study the properties of the triplet states of the dyes, we used flash photolysis technique. 

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). 

Fig. 19. Different ways to introduce oxyl radical reactivity nature employs metal bound tyrosyl radicals (19) or high-valent metal oxo fragments in many active sites (65,153). Nitroxyl radicals such as 2,2,6,6,-tetramethylpiperidin-l-oxyl (TEMPO, 20) are reactive species used in organocatalysis (154). The excited states of carbonyl functional groups (21) and metal oxo-fragments (22) display a radical pair character, which may become very attractive for biomimetic photoredox processes upon spectral sensitization (3,5).

Derivatives of l-hydroxy-2,2,6,6-tetramethylpiperidine have been widely investigated electrochemically, mainly due to the interest in the nitroxyl radicals derived from them [427]. The application of TEMPO and that of A-hydroxyphthalimide [428] as agents for indirect oxidations are discussed in Chapter 29. 

The direct way to increase the number of radicals on a substrate surface is to treat it with stable radical solutions. Nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine series have been found to considerably improve durability of adhesive bonds (62,63). For instance, when sticking together high pressure polyethylene, fluoro-plastic F-4 and polyvinylchloride, pretreatment of their surfaces with nitroxyl radical solutions improved the durability of adhesive bonds by 160-170%. This result is considerably better than that obtained by treatment of the aforesaid polymer surfaces with the most active adhesion modifiers known (64). 

Pretreatment of polymer surfaces with nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine series in the process of chemically forming coatings leads to a considerable improvement in fastening nickel to the substrate and a two to three-fold increase in the... 

Antiwear. It has been shown that the addition of nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine series to lubricating compositions consisting of mineral oil and alkyl adipates (20%) considerably improves their lubricating properties (75). When utilizing the suggested compositions, the maximum load can be increased by 1.2-1.6 times for the same and sometimes even smaller friction factor. At the same time, the weight wearout rate is reduced by 1.2-2.0 times for bronze samples and by 2-8 times for steel samples. 

FD-MS oxidation studies of N-alkyl-3,3-dialkyldecahydroquin-oxalin-2-ones show that only the N-oxide form and no hydrogen atoms are lost. Similarly, the oxidation of 2,2,6,6-tetramethylpiperidines gives only the nitroxyl radical and no hydrogen atoms are lost. The facts prove that both a nitroxyl radical and hydrogen atoms on the bicyclic bridge are necessary for the oxidation of the hydrogen atoms to occur. 

Similar results have been reported by Felder and Schumacher (3, Bellus, Lind, and Wyatt (9). The results indicate that among the tested compounds, the most efficient quencher of singlet oxygen is the N-methyl-substituted piperidine derivative, followed by nickel chelates and nitroxyl compounds of the hindered amines. The 2,2,6,6-tetramethylpiperidines themselves were of relatively low efficiency. 

The polymerization of VAc by NMP was reported by Matyjaszewski group in 1996. To reduce the bimolecular radical termination dnring the polymerization, the authors attached the stable nitroxyl radical (TEMPO 2,2,6,6-tetramethylpiperidine-l-oxyl radical) to the interior of a dendrimer and used this modified TEMPO ([G-3]-TEMPO) as a scavenger combined with 2,2 -azobis(2-methyl-propionitrile) (AIBN) to polymerize VAc. Fignre 5 shows the kinetic plot and the dependence of molecular weight on monomer conversion for the bnlk polymerization of VAc at 80 °C in the presence of [G-3]-TEMPO/AIBN. 

The nitroxyls (a.k.a. nitroxides) are remarkably stable free radicals. Nitroxyls have two major resonance structures, one N-centered and one O-centered the lone electron may also be considered to be in the tt orbital of an N—O tt bond. Nitroxyls are thermodynamically stable because dimerization would give a very weak N—N, N—O, or O—N bond. TEMPO (2,2,6,6-tetramethylpiperidin-l-oxyl), a commercially available nitroxyl, is further stabilized by steric shielding, as shown here. Other thermodynamically stable free radicals include the small molecules O2 (a 1,2-diradical, best represented as -O—O ) and nitric oxide (NO), a messenger molecule in mammals that mediates smooth muscle contraction. 

The stable, commercially available nitroxyl radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) 51 is an excellent catalyst, in conjunction with a co-oxidant, for the oxidation of alcohols. The most popular co-oxidant is buffered sodium hypochlorite (NaOCl). Oxidation of the nitroxyl radical gives the oxoammonium ion 52, which acts as the oxidant for the alcohol to form the carbonyl product. Primary alcohols are oxidized faster than secondary and it is often possible to obtain high chemoselectivity for the former. For example, oxidation of the triol 53 gave the aldehyde 54, with no oxidation of the secondary alcohols (6.44). The use of TEMPO is particularly convenient for the oxidation of primary alcohols in carbohydrates, avoiding the need for protection of the secondary alcohols. 

Scheme 9.15 Oxidation of a 2,2, 6,6 -tetramethylpiperidine (TMP) derivative to the corresponding nitroxyl radical, i.e. the...

The spin probe method was one of the first methods used to evaluate the free volume in polymers [1,7]. It is based on the principle that the rotational frequency of spin probes, usually stable nitroxyl radicals such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is sensitive to the free volume. The relatively complex correlation between spectral data and FV makes this method more suitable for qualitative comparison of different polymers than for quantitative analysis of the FV [1,8]. 

While nitroxyl radicals, e.g., (2,2,6,6-tetramethylpiperidin-l-yl)oxyl (TEMPO) (2014COR459) and 2-aza-adamantane-Al-oxyl (AZADO) (2006JA8412), have been used extensively as catalysts in the oxidation of alcohols, there have been no reports of analogous uses for hydrazyls and... 

The phenomenon of selective inhibition of chain reactions was for the first time explained by N.M. Emanuel, E.A. Bliunberg, L.A. Tavadyan and S.A. Maslov [11]. It was experimentally observed in a mnnber of liquid-phase oxidation reactions. Thus, introducing small amoimts of an additive stable nitroxyl radical (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPOL) and a heterogeneous inhibitor (WSc2) makes it possible to increase the selectivity of formation of imsaturated acids and epoxides in chain oxidation reactions of a-methylacrolein, 2-ethylhexenal, as well as co-oxidation of aldehydes and olefins (Table 5.1). 

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