Nitroxyl radicals TEMPO

The theory of diffusion in polymers as heterogeneous media was discussed in Refs. [68,74,81-85], The correlation between the frequency of rotation vT of the nitroxyl radical (TEMPO) and diffusion coefficient of oxygen D (298 K) was found [86]. 

The addition of an acceptor decreases the rate of POOH decomposition. The increase of added [InH] creates a tendency for k-% to decrease to the kA value, i.e., Ax —> A d at [InH] —> DC. Acceptors, which do not react with hydroperoxide groups, were used sterically hindered phenols and stable nitroxyl radicals (TEMPO) were found to be efficient acceptors. The ratio kinA(2kt)m can be calculated from the values Ax and A d according to the formula ... 

A new interesting branch of the modern antioxidant chemistry deals with the cyclic mechanisms involving acid catalysis. The first inhibiting system of this type was discovered in 1988 [44]. It consisted of an alcohol (primary or secondary), a stable nitroxyl radical TEMPO, and... 

The oxidation of alcohols to carbonyl compounds with the stable nitroxyl radical TEMPO (86) as catalyst is a well-known preparative method [134, 135]. Hypochlorite or peracetic acid is usually used as the final oxidizing agent and ca. 1 mol% of catalyst 86 is used. In 1996 Rychnovsky et al. reported the synthesis of the chiral, binaphthyl-derived TEMPO analog 87 [136], Results obtained by use of 0.5-1 mol% of catalyst 87 [136] are listed in Table 10.12. In these oxidation reactions 0.6-0.7 equiv. sodium hypochlorite were used as the final oxidizing agent (plus... 

The nitroxyl radical TEMPO (18a) is an active catalyst for the selective oxidation of alcohols, with hypochlorite as the oxidant. The actual oxidizing species is the oxoaminium ion (18b), which in the alcohol oxidation (I in the structure) is reduced to the hydroxylamine (18c). A catalytic amount of bromide is used to generate BrO , which is capable of reoxidizing the hydroxylamine or the aminoxyl radical (18a) to the oxoaminium stage (408). 

A considerable amount of recent work has focused on the oxidation of polymeric and monomeric carbohydrates in aqueous media. In the context of the biorefinery, these processes could be used for the preparation of oxidized carbohydrates as primary outputs of biomass deconstruction. Of particular interest are processes catalyzed with stable oxygen-centered radicals such as the nitroxyl radical TEMPO (2) (2,2,6,6-tetramethylpiperidi-noxyl) and using bleach as the stoichiometric oxidant. 

TEMPO and Related Reagents. The nitroxyl radical TEMPO has been used in... 

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. 

Huang, W.-W, Henry-Riyad, H., and Tidwell, T.T., Reactions of the Stable Nitroxyl Radical TEMPO with Ketenes Formation of a Unique Peroxidic Source of Aminyl Radicals, /. Am. Chem. Soc., 121, 3939, 1999. 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

The polymerization kinetics have been intensively discussed for the living radical polymerization of St with the nitroxides,but some confusion on the interpretation and understanding of the reaction mechanism and the rate analysis were present [223,225-229]. Recently, Fukuda et al. [230-232] provided a clear answer to the questions of kinetic analysis during the polymerization of St with the poly(St)-TEMPO adduct (Mn=2.5X 103,MW/Mn=1.13) at 125 °C. They determined the TEMPO concentration during the polymerization and estimated the equilibrium constant of the dissociation of the dormant chain end to the radicals. The adduct P-N is in equilibrium to the propagating radical P and the nitroxyl radical N (Eqs. 60 and 61), and their concentrations are represented by Eqs. (62) and (63) in the derivative form. With the steady-state equations with regard to P and N , Eqs. (64) and (65) are introduced, respectively ... 

The method uses a simple electrode made of a thin film of sol-gel organosilica doped with nitroxyl radicals deposited on the surface of an indium tin oxide (ITO) electrode. Thus, whereas in water benzyl alcohol is rapidly oxidized to benzoic acid, the use of the hydrophobic sol-gel molecular electrode TEMPO DE affords benzaldehyde only (Figure 1.9), with an unprecedented purity, which is highly desirable for the fragrance and pharmaceutical industries where this aromatic aldehyde is employed in large amounts. 

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... 

A report was concerned with the ability of nitroxyl radicals, such as TEMPO and other related structures, to act as catalysts in the asymmetric oxidation of alcohols. Cyclic voltammetry was used to measure the oxidation potentials of the nitroxyl... 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

The inclusion of stable free radicals such as TEMPO (18, TO ) in free radical polymerizations leads to precise control of chain length by restricting the number of polymerizing chains (equation 96). This process is known as nitroxyl radical mediated polymerization (NRMP). 

Obviously there is a definite need in the fine chemical and pharmaceutical industry for catalytic systems that are green and scalable and have broad utihty [10]. More recently, oxidations with the inexpensive household bleach (NaOCl) catalyzed by stable nitroxyl radicals, such as TEMPO [17] and PIPO [18], have emerged as more environmentally friendly methods. It is worth noting at this juncture that greenness is a relative description and there are many shades of green. Although the use of NaOCl as the terminal oxidant affords NaCl as the by-product and may lead to the formation of chlorinated impurities, it constitutes a dramatic improvement compared to the use of chromium(VI) and other... 

CAUTION With the DMSO methods, dimethyl sulfide and toxic gaseous side products are produced. With the TEMPO method, care should be taken when handling highly toxic nitroxyl radicals. 

Less commonly used TEMPO-related nitroxyl radicals include 4-PhC02-TEMP019 (61), 4-NC-TEMPO20 (62), 63,20 4-(4-tBuC6H4C02)-TEMP019c (64), 65,19c 6619c and 66a.21... 

In -OH-induced reactions of Thy, Tg and 5-hydroxymethyluracil yields are markedly enhanced in the presence of nitroxyl radicals such as TEMPO (Kagiya et al. 1983). Adducts maybe the intermediates. In the case of the nitroxyl radical TAN and for the Thd system, such adducts have been characterized (Berger et al. 1985). 

Several systems have been proposed to fulfill these conditions. Among them is the one mediated by stable nitroxyl radicals such as 2, 2, 6, 6-tetra-methylpiperidinyl-l-oxy (TEMPO) ... 

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). 

Heterogeneous TEMPO catalysts have much lower catalytic efficiency than their homogeneous counterparts, however. A convenient compromise [4, 5a, 6] is the use of the nitroxyl radical 6, prepared from the commercially available antioxidant Chimassorb 966. This catalyst, which is soluble in acidic medium, is quite effective in promoting the aerobic oxidation under mild conditions and, after easy recovery, can be conveniently reused [4, 5a, 6],... 

Fig. 17.38. A sec-amine —> nitroxyl radical oxidation exemplified by the preparation of the oxidizing agent TEMPO (see Figure 17.16 for a synthetics application).

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