2.2,6,6-tetramethylpiperidinyl-1 -oxide

Cu2+ ions, e.g. Cu(N03)2, catalyze the epoxication of alkenes by iodosylbenzene.664 Oxidation of alcohols to aldehydes can be effected by 02 in the presence of Cu2 ions and Tempo (2,2,6,6-tetramethylpiperidinyl 1-oxide).665 Arene hydroxylation of the binucleating ligand (206)... 

Organic Mediators Organic mediators are very useful in electroorganic chemistry since their structural modification may increase the selectivity of the oxidation. The first organic mediator exploited for oxidation in synthesis was thio-anisole. Since then, there appeared a variety of other organic mediators such as tris-arylamines, tetramethylpiperidinyl-1 -oxy (TEMPO), and Ai-hydroxyphthalimides (NHPIs). 

TEMPO 2,2,6,6-Tetramethylpiperidinyl-1-oxy (20 TEMPO) works as a mediator for the oxidation of primary alcohols to aldehydes. The oxidation of secondary alcohols is much slower than that of primary alcohols as exemplified by the oxidation of (19) to (21) (Scheme 7) [48]. Active species is the oxo-ammonium generated from TEMPO. 

A variety of phenol couplings have been described. Those reported before 1991 have been reviewed [66]. 2-Naphthol (27) was oxidized to l,l -binaphthol (28) in high current efficiency on a graphite felt electrode coated with a thin poly(acrylic acid) layer immobilizing 4-amino-2,2, 6,6-tetramethylpiperidinyl-l-oxy (4-amino-TEMPO) (Scheme 10) [67]. 

Foohiotes (a) Data from [9], adjusted to see using data for oxidation of 2,2,6,6-tetramethylpiperidinyl-1 -oxyl. 

Table 3 Physical constants of the radicals TEMPO (tetramethylpiperidinyl-iST-oxyl), 1,3,5-triphenylverdazyl (10), and their complexes with the oxidized cation salts...

Early publications on [VO(/3-diketonato)2] have been reviewed.355 More recently, complexes with benzoyl m-nitroacetanilide, benzoyl acetanilide545 and l,l -(l,3-phenylene)-bis(butane-1,3-dione546 have been synthesized. Other [VO(/S-dik)2] adducts have been isolated, for example [VO(acac)2] adducts with a series of pyridine N-oxides547 and several pyridine carboxamides, 48 and [VO(bzac)2] adducts with pyridine, methylamine, isoquinoline and 4-picoline.549 Equilibrium constants of 1 1 and 2 1 adducts of pyrazine with [VO(tfacac)2] have been determined (equation 38).550 In the 2 1 complex, the pyrazine bridge between two equatorial sites of adjacent vanadium atoms promotes a weak exchange interaction. The nitroxide radical 2,2,6,6-tetramethylpiperidinyl N-oxide also forms an adduct with [VO(hfacac)2] in which there is a strong interaction between the electrons on the metal and nitroxide.551... 

Table 2 Ruthenium/2,2, 6,6 -tetramethylpiperidinyl-W-oxyl (TEMPO) catalysed oxidation of primary and secondary alcohols to the corresponding aldehyde using molecular oxygen. 15 mmol substrate, 30 ml chlorobenzene, RuCl2(PPh3)3/TEMPO ratio of 1/3, 10 ml mur1 02/N2 (8/92 v/v), P=10 bar, T=100 °C...

Fig. 15 Ruthenium/2,2, 6,6 -tetramethylpiperidinyl-Ar-oxyl catalysed aerobic oxidation of alcohols...

TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) is a stable free radical that catalyzes many types of oxidations. For example, a catalytic amount of TEMPO added to a hypochlorite oxidation of an alcohol increases the rate by enabling lower-energy reaction mechanisms involving reversible oxidation of the N—0 bond. 

The aerobic oxidation of benzylic and allylic alcohols to their corresponding carbonyl compounds in [C4Ciim][PF6] with TEMPO-CuCl (TEMPO = 2,2,6,6-tetramethylpiperidinyl-l-oxy) as catalyst was found to proceed at higher rates than that of aliphatic alcohols, which is in agreement with results in classical solvents.[75] After product extraction with diethyl ether, the ionic liquid was washed with water and dried at 70°C, prior to the next run. In that manner catalyst activity remained relatively stable for 8 cycles. 

This type of chemoselectivity has been observed in the oxidations of alcohols using copper(II) chloride and a catalytic quantity of 2,2,6,6-tetramethylpiperidinyl-l-oxyl (11 equation 11). The oxidizing species which is generated, in fw example the oxidation of (12) to (13), is the cation (14), which may also be produced using electrochemicai oxidation (in the wesence of the weak base 2,6-lutidine). Allylic and ben lic alccAols are easily oxidized by this reagent, whereas secondary alcdiols react slowly. 

Great attention has been paid to HAS and their safety application in plastics and coatings. The 4-unsubstituted 2,2,6,6-tetramethylpiperidine is considered as relatively toxic, the acute oral toxicity being about 1 g/kg. The substitution in position 4 (i.e. the general mode in the synthesis of HAS for polymer purposes) dramatically improves the situation. Therefore, commercial HAS like 28 (R = H), 34,35a or 35b were approved for stabilization of packaging materials in contact with food [307]. Some data are available on properties of TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxyl) and its 4-amino or 4-hydroxy derivatives. They were found to act as weak intrinsic direct mutagens in Salmonella typhimurium. TEMPO increases intracellular hydroperoxide concentration. This may indicate its pro-oxidative effect which does not result, however, in cellular toxicity [314]. 

Kim and Jung (2003) oxidized benzylic and allylic alcohols by molecular oxygen in presence of catalytic amounts of 2,2,6,6-tetramethylpiperidinyl-l-oxyl (TEMPO) and CAN (scheme 64). Molecular oxygen is reduced to water by oxidation of Ce + to Ce . The nitroxyl radical of TEMPO is oxidized to the A-oxoammonium cation by Ce +. The N-oxoammonium cation then oxidizes the alcohol to a carbonyl compound. The reactions were carried out in refluxing acetonitrile and molecular oxygen was bubbled through the reaction mixture. The amount of CAN varied between 10 and 20 mol%, and the amount of TEMPO... 

Evans, W. I, Perotti, J. M., Doedens, R. I, ZUler, J. W. The tetramethylpiperidinyl -1-oxide anion (TMPO-) as a ligand in lanthanide chemistry Synthesis of the per(TMPO-) complex [(ONC5HeMe4)2Sm(m-ONC5He,Me4)]2. Chem. Common., 2326-2327 (2001). 

It is noteworthy that the addition of a radical scavenger such as 2,2,6,6-tetramethylpiperidinyl-l-oxyl (TEMPO) completely suppressed the reactivity, showing that a radical mechanism is most likely involved in the transformation. Furthermore, a significant amount of 1,2-diphenylethane was obtained under such standard conditions, which also supports the formation of a benzyl radical 6-D during the reaction (Scheme 4.6). Firstly, ferf-butyl peroxide decomposes to give a tert-butyl peroxide radical and oxidizes Fe(ii) 6-A into Fe(iii) 6-B, which reacts with the 1,3-dicarbonyl derivative leading to an Fe(iii) enolate 6-C. Meanwhile, a benzyl radical 6-D obtained by abstraction of H can then react with 6-C. 

Acidic solvents, and particularly fluorinated alcohols, activate hydrogen peroxide, and even in the absence of a catalyst the rearrangement proceeds faster [308]. Other more uncommon non-metaUic oxidants such as perhydrates [309] and TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxyl) in combination with sodium hypochlorite [309] have also been applied. 

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