Tetramethylpiperidine nitroxyl TEMPO

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

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... 

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 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. 

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 amorphous regions were studied using EPR spectroscopy, which allows the estimation of the molecular mobility in disordered regions [17], A stable nitroxyl radical, 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) was used as a paramagnetic probe. The radical was introduced into the films from vapor at a temperature of 30 C. The rotational mobility of the radical probe was determined by the correlation time The following formula is used to calculate Eq. (2) r ... 

A combination of RuQaCPhjP) and the stable nitroxyl radical, 2, 6,6-tetramethylpiperidine-N-oxyl (TEMPO) is a remarkably effective catalyst for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, respectively, in >99% selectivity. The best results were obtained using lm% of RuCl2(Ph3P)3 and 3m% of TEMPO (Reaction 4). 

Figure 5 Structures of the nitroxyl radicals (from left to right) 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO), 2-azaadamantane W-oxyl (AZADO), l-methyl-2-azaadamantane W-oxyl (1-Me-AZADO), 2-azanoradamantane W-oxyl (Nor-AZADO), 9-azabiciclo[3.3.1]nonane W-oxyl (ABNO), and 9-azabiciclo[3.3.1]non-3-one W-oxyl (ketoABNO).

Electron-transfer/organocatalytic reactions were postulated in oxamination reactions [9] later it was demonstrated [10] that the reaction follows preferentially a polar enamine activation manifold when FeCls was used in DME as oxidizing agent the metal coordinates rapidly to the nitroxyl radical of TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) (Scheme 39.2) [llj. SOMO conditions were find by replacing the metal salt by a non-coordinating oxidizer such as tris(p-bromophenyl) aminium hexachloroantimonate [12]. Under these conditions the ET reaction arguably afforded the enamine radical cation, which was trapped by TEMPO, or by styrene as a purported SOMOphile partner. 

The oxidation of alcohols to carbonyl products with stable nitroxyl radicals as catalyst has been recently reviewed [323-326]. The best-known oxidation catalyst is the tetraalkylnitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl). 

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