Tetramethylpiperidine 1-oxyl free radical

Where more selective oxidations are required, oxidants such as (2,2,6,6-tetramethylpiperidin-l-yl)oxyl (free radical) (TEMPO) 12 have been successfully employed, as illustrated in Scheme 6.7, for the bleach 13 oxidation of alcohols to aldehydes.Other homogeneous oxidants employed under flow conditions include oxone and iron nitrate, " with Koch et al. using the periodic acid/H2S04 mediated oxidative deprotection of / -methoxyphenyl protected amines. 

The Novartis process starts with the protection of the Roche ester 304 as a 4-methoxybenzyl ether. Reduction and subsequent oxidation furnished aldehyde (S)-15 that is ready for the Evans et al. aldol reaction (Scheme 10.58). It is interesting to note that (2,2,6,6-tetramethylpiperidin-l-yl)oxyl (free radical) (TEMPO)/bleach oxidation in biphasic conditions (water/CH2Cl2) produced the unstable aldehyde in quantitative yield. The Evans group syn-aldol... 

Figure 7.2.12 shows the principal set-up for this experiment. The column, containing the immobilized free radicals consists of an adapted PEEK tube, which fits into the flow probe below the detection cell. Figure 7.2.13 depicts a spectrum of d-n-butylphthalate recorded under the influence of a ( free-radical ) column filled with 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO), immobilized with an aminopropyl spacer on silica. The spectrum is taken from an on-line separation of a two-compound mixture. The line width is of the same order as... 

The mechanism of the aerobic oxidation of alcohols depends on the particular catalyst used. Two general mechanisms can be considered (1) the direct oxygenation of alcohols by 02 through a free-radical chain process initiated by the catalyst, and (2) the direct oxidation of the alcohol by the catalyst, which is then regenerated by 02. Both mechanisms are well illustrated [6] by the aerobic oxidations catalyzed by the persistent tetramethylpiperidine-N-oxyl (TEMPO) radical 1 and the nonpersis-tent phthalimide-N-oxyl (PINO) radical 2. 

Direct 1,2-bis azidation of triisopropylsilyl enol ethers was possible when catalytic amounts of 2,2,6,6-tetramethylpiperidine-/V-oxyl (TEMPO) were used, at - 45°C. The reaction under these conditions was often highly stereoselective, since only one (trans) adduct was obtained. In contrast to the other bis azidations of alkenes, which proceed ionically or through cycloaddition, this addition is a free radical process [98], A radical pathway occurred also when cyclohexene was treated with PhIO-Me3SiN3-TEMPO the yield of 1,2-bis azide was doubled (80%) in relation to the system PhIO-AcOH-NaN3 (in both cases the trans adduct prevailed). 

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). 

A method for the conversion of iodides to alcohols via free radical intermediates consists of trapping by 2,2,6,6-tetramethylpiperidin-l-oxyl and subsequent reduction. ... 

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 hyperfine coupling constant of the EPR spectrum of 4-amino-2,2,6,6-tetramethylpiperidine-l-oxyl, a commercially available stable free radical, has been... 

One of the limitations of anionic polymerization with respect to preparation of block copolymers is the rather limited range of monomers that can be polymerized anionically to form polymers with well-defined stmctures. One solution to this problem is to utilize anionic polymerization to form a well-defined polymer that is functionalized with an end group that can be used to initiate polymerization via another polymerization method, for example, controlled free-radical polymerization. One such functional group is the aminoxy group which can be used to initiate nitroxide-mediated radical polymerization (NMP). °° PSLi has been reacted with 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (MTEMPO), a stable nitroxide free radical, in THF at -78 °C as shown in eqn [30]. The mechanism of this functionalization was presumed to occur... 

Since its discovery in 1993 [27], nitroxide-mediated polymerization (NMP) has been the most extensively studied technique from the dissociation-combination dass of LRP mechanisms (Scheme 13.7). This method is also commonly termed stable free radical polymerization (SFRP). NMP reactions are distinguished by the use of stable free radical nitroxide molecules (N ) as the controlling agent [e.g. 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO), (l-diethylphosphono-2,2-dimethyl)propyl nitroxide (DEPN)]. 

Nitroxides suppressed tumorigenesis (Metodiewa etal. 1997). The medication of Tempicol-2 [4-hydroxy-4-(2-picolyl-2,2,6,6-tetramethylpiperidine-1-oxyl], a stable free radical, to rats bearing 3 day-old Yoshida sarcoma (promotion phase) induced both growth inhibition and apoptotic cell death, comparable to the effects of Tempace and Rutoxyl [ rutin/4 - acetamide-1 -hydroxyl-2,2,6,6-tetramethyl-piperidinium] under the same experimental conditions (Metodiewa et al. 1998). 

Stable free radicals are frequently employed as inhibitors (436,437). The most commonly used species are nitroxides, eg, 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) 18. They are far too stable to be able to initiate polymerization, but they are reactive enough to imdergo reaction with other free radicals (438). Nitroxides are very efficient inhibitors, being capable of producing induction periods when present in concentrations of less than 10 mol L" Nitroxides, such as TEMPO, react with carbon-centered radicals at close to diffusion controlled reaction rates (439-441). The stoichiometry between the number of the chains... 

Not many initiation events are needed, but the propagation cycle must be fast and efficient to afford a successful process. Radical probes have been used to demonstrate the formation of radicals along the propagation cycle and also as a synthetic tool. Thus, products from ring closure or ring opening or from rearrangement of the radicals were taken as evidence of the presence of these intermediates. Inhibition by radical traps with stable free radicals 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO), di-t-butyl nitroxide (DTBN), etc. has been extensively used to provide mechanistic... 

Recently, it has been reported that 4-vinylpyridine undergoes controlled radical polymerization in the presence of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). In calorimetric experiments a polymerization behavior similar to that observed for the controlled living polymerization of styrene was found. Furthermore, the resulting polymers showed a small polydispersity compared to polymers obtained by free-radical polymerization [572]. 

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],... 

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]. 

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