2,2,6,6-Tetramethylpiperidine- 1-oxyl oxidation

The Ru-catalyzed epoxidation of tran -stilbene in the presence of NaI04 was carried out using a bipyridyl ligand with a fluorous ponytail at the 4 and 4 positions. As illustrated by the first equation in Scheme 8, a triphasic system comprising water, dichloromethane and perfluorooctane was employed in the reaction. The reaction was complete in 15 min at 0°C and tran -stilbene oxide 5 was obtained from the dichloromethane layer in a 92% yield. The fluorous layer, containing the catalyst, could be recycled for four further runs without any addition of RuCls. The same perfluoroalkyl-substituted bipyridyl ligand was used successfully in the copper(i)-catalyzed TEMPO (2,2, 6,6 -tetramethylpiperidine (V-oxyl)-oxidation of primary and secondary alcohols under aerobic conditions (Scheme 8, second equation). ... 

The second synthesis of crystalline 43 was reported by Mori as summarized in Scheme 62 [93]. The building block (4.R,5S)-A was prepared by an enzymatic process, while another building block C was synthesized via Sharpless asymmetric epoxidation. Coupling of A with C gave D, which was cyclized under Op-polzer s conditions to give crystalline E. When E was oxidized with Dess-Martin periodinane or tetra(n-propyl)ammonium perruthenate or Jones chromic acid, crystalline 43 was obtained. Swern oxidation or oxidation with 2,2,6,6-tetramethylpiperidin-1 -oxyl of E afforded only oily materials. Accordingly, oxidation of E to 43 must be executed extremely carefully. A synthesis of oily 43 was reported by Gil [94]. 

In other cases, organic modification of the sol gel cages markedly protects the entrapped molecular dopant from degradation by external reactants, as shown for instance by the entrapment of the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). This is a highly active catalyst which in the NaOCl oxidation of alcohols to carbonyls in a CH2CI2-H20 biphasic system becomes highly stabilized upon sol gel entrapment in an ORMOSIL matrix it progressively loses it activity when entrapped at the external surface of commercial silica.25... 

It is known that the nitrosonium cation is a strong oxidant (54). In (55) it was found by multinuclear NMR ( H, 13C, 19F and 14N) that the interaction of nitrosonium tetrafluoroborate with 2,2,6,6-tetramethyl-4-R-piperidine-1 -oxyl radicals 22a-e resulted in formation of 4-R-2,2,6,6-tetramethylpiperidine-l-oxoammonium tetrafluoroborates (Scheme 16). Cations 23a-e could be classified as nitrosonium complexes of biradicals 24a-e. 

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... 

Three nitroxide derivatives of Ru2 species have been reported. The first contains the [Ru2(//-02C Bu)4L2] ion where L = 2,2,6,6-tetramethylpiperidine-l-oxyl. A relatively large antiferromagnetic coupling is observed between the Ru2 " core and the nitroxide radical. The second example is [Ru2(//-02C Bu)4L ] where L = 2-phenyl-4,4,5,5-tetramethyM,5-dihydro-lH-imidazolyl-l-oxy-3-oxide. 

During the induction periods caused by adding antioxidants, a small contraction in volume occurred because of the formation of dimers of chloroprene (14). This reaction occurs during the oxidation but was most easily studied by dilatometry in the absence of oxygen. A few values of the initial rate of dimerization of chloroprene, inhibited against polymerization with 2,2,6,6-tetramethylpiperidine-l-oxyl, are given in Table III. Their dependence on temperature is given by... 

A convenient procedure for the oxidation of primary and secondary alcohols was reported by Anelli and co-workers (8,9). The oxidation was carried out in CH2CI2 with an aqueous buffer at pH 8.5-9.5 utilizing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 1) as the catalyst and KBr as a co-catalyst. The terminal oxidant in this system was NaOCl. The major disadvantage of using sodium hypochlorite or any other hypohalite as a stoichiometric oxidant is that for each mole of alcohol oxidized during the reaction one mole of halogenated salt is formed. Furthermore,... 

We wish to report here on a new and highly efficient catalyst composition for the aerobic oxidation of alcohols to carbonyl derivatives (Scheme 1). The catalyst system is based on 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Mg(N03)2 (MNT) and N-Bromosuccinimide (NBS), utilizes ecologically friendly solvents and does not require any transition metal co-catalyst. It has been shown, that the described process represents a highly effective catalytic oxidation protocol that can easily and safely be scaled up and transferred to technical scale. 

As peracids react very sluggishly with alcohols, it was apparent that the presence of a nitroxide was playing an important role in the oxidation of the alcohol into a ketone. This seminal serendipitous observation led to the development of the first description of the oxidation of alcohols mediated by catalytic 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) (55), published almost simultaneously by Celia et al and Ganem.3 These authors presented two papers with remarkably similar contents, in which alcohols were oxidized by treatment with MCPBA in CH2CI2 at room temperature in the presence of a catalytic amount of TEMPO (55). In both papers, a plausible mechanism is presented, whereby m-chloroperbenzoic acid oxidizes TEMPO (55) to an oxoammonium salt 56. This oxoammonium salt 56, as detailed in Ganem s paper, can react with the alcohol producing an intermediate 57, which can deliver a carbonyl compound by a Cope-like elimination. 

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). 

Our group have developed 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO)-functionalized PEG for biomimetic oxidation of alcohols together with CuCl in compressed C02, through a so-called mono-phase reaction, two-phase separation process to recover the catalyst, thus leading to conducting a homogeneous catalysis in a continuous mode [62]. 

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. 

The primary alcohol of the diol 309 was selectively oxidized to the corresponding hydroxyaldehyde through a 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO)-catalyzed oxidation. The remaining secondary hydroxyl functionality was protected as an acetoxy group and the aldehyde was further oxidized to the acetate carboxylic acid. Treatment of... 

Oxidation of the primary alcoholic group to a carboxyl group in diols with primary and secondary hydroxyls is accomplished by silver carbonate [377]. Unfortunately, an extremely large excess of the reagent is needed. Similar results are obtained with a rather exotic oxidant, 4-methoxy-2,2,6,6-tetramethyl-l-oxopiperidinium chloride, which is prepared by treatment with chlorine of a stable radical, 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl. The compound oxidizes 1,4-butanediol to y-butyrolactone in 100% yield (isolated yield 81%) and 1,5-pentanediol to 8-valerolactone in 61% yield (isolated yield 40%) [995] (equation 292). 

Ragauskas et al. reported the oxidation of benzylic alcohols to their corresponding carbonyl compounds in [bmim][PFg] using 2,2,6,6-tetramethylpiperidine-l-oxyl... 

The use of TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) as an oxidant for the oxidation of primary and secondary alcohols to the corresponding aldehyde or ketone in combination with primary oxidants has also received significant attention in the past few years [18]. It is, therefore, not surprising to see the amount of effort that has been devoted to the synthesis of immobilized versions of this family of reagents (Figure 4.3) [19], on a wide variety of supports (e.g. silica-supported... 

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