Oxoammonium salts

A double mediatory system consisting of A-oxoammonium salts and active bromine species, generated from 2,2,6,6-tetramethylpiperidine-1 -oxyl derivatives... 

Chloride dioxide (CIO2) forms red charge-transfer complexes with piperidine and imidazoline nitroxyl radicals that slowly transform into oxoammonium salts. ... 

In a similar way, 1,4-oxathian-2-one and l,4-dioxan-2-one were obtained from the condensation of thioglycolic acid and glycolic acid, respectively <1979J(P1)1893, 1998T11445>, and from palladium-catalyzed <1999JOC6750> or oxoammonium salt <2004JOC5116> oxidations of 2,2 -thiodiethanol or diethylene glycol. Asymmetric syntheses... 

There are several methods reported in the literature for transforming vicinal diols into ct-diketones while avoiding the risk of C-C bond cleavage.26 Examples include the standard Swem conditions (dimethyl sulfoxide and oxalyl chloride followed by triethylamine), or the use of DMSO activated by acetic anhydride, pyridine-sulfur trioxide complex, or dicyclohexylcarbodiimide (Mq/J-att oxidation). Diones are also obtained by treatment with benzalacetone as a hydride acceptor in the presence of catalytic amounts of tris(triphenylphosphine)ruthenium dichlonde [(PPh RuCFl.27 Recent developments include the use of w-iodoxyben/.oic acid28 or the oxoammonium salt of 4-acctamidoletramethylpipcridine-1-oxyl and y -toluencNulfonic acid.29... 

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. 

As soon as, it was learnt that oxoammonium salts, which are unstable compounds, are very efficient in the oxidation of alcohols, and that they can be generated in situ by treating catalytic TEMPO, or related compounds, with MCPBA acting as a secondary oxidant, it became apparent that other secondary oxidants would be more practical than MCPBA in Synthetic Organic Chemistry. MCPBA is a very energetic oxidant that reacts with many functionalities including alkenes and ketones. 

Thus, Semmelhack et al.8 in 1983 published the oxidation of alcohols by an oxoammonium salt, generated by electrooxidation of catalytic TEMPO and, in 1984, Semmelhack et al.9 published a similar oxidation of alcohols, in which catalytic TEMPO is oxidized by Cu (II), which itself can be used in catalytic quantities, being generated by the oxidation of catalytic Cu (I) by excess of gaseous oxygen. 

Potassium bromide produces an accelerating effect that has been attributed to the generation of HOBr, which is a stronger oxidant than HOC1. Interestingly, the oxidation proceeds at a higher speed at 0°C than at room temperature, a fact that can be explained by the instability of the primary oxidant—that is an oxoammonium salt—above 0°C. 

Oxoammonium salts react with water resulting in the generation of hydrogen peroxide.14 This side reaction is minimized at 0°C. A substantial amount of heat is evolved in oxidations following Anelli s protocol therefore, on multigram scale reactions it may be very difficult to keep a temperature as low as 0°C. In such cases, an efficient oxidation can be achieved at 10-15°C, a temperature in which the decomposition of oxoammonium compounds does not compete substantially with the desired oxidation of alcohols.15... 

One important limitation of TEMPO-mediated oxidations, under Anelli s conditions, originates from competing reactions produced by HOC1, generated in situ from NaOCl. This problem can be solved by the use of [bis(acetoxy)iodo]benzene (BAIB) as a secondary oxidant following the protocol of Piancatelli and Margarita27 which has proved to be particularly efficient in difficult substrates,34 and it is a highly recommended alternative to Anelli s procedure when oxidations with oxoammonium salts are desired. 

Stoichiometric oxoammonium salts have proved to be able to selectively oxidize less hindered secondary alcohols in 1,2-diols containing two secondary alcohols.39... 

It is convenient to keep the internal temperature as low as practical because the primary oxidant—consisting of an oxoammonium salt—is decomposed by reaction with water at a higher temperature. 

A selective oxidation of a primary alcohol, in the presence of a secondary one in a complex substrate, is achieved by using an oxoammonium salt as primary oxidant under the protocol ... 

During the oxidation of primary alcohols with oxoammonium salts, sometimes dimeric esters are formed.20a This can be minimized by increasing the quantity of TEMPO. 

An oxoammonium salt operating as a primary oxidant is generated by oxidation of catalytic TEMPO with Br2, which, in turn, is formed by electrooxidation of bromide anion. The formation of a dimeric ester side-compound is minimized increasing the quantity of TEMPO. 

Among common alcohol oxidants, TEMPO-mediated oxidations have been the subject of a close scrutiny, aimed at finding optimum conditions for the selective oxidation of primary alcohols. In fact, TEMPO-mediated oxidations, that is oxidations in which an oxoammonium salt acts as a primary oxidant, have a great tendency to operate quicker with primary alcohols, regardless of the secondary oxidant employed and the exact experimental conditions. 

A scant look at the facts might suggest that the selective oxidation of primary alcohols in TEMPO-mediated oxidations can be explained solely on steric grounds. Things are not so simple, as it was found8 that the primary oxidants, that is oxoammonium salts, when used stoichiometrically, react quicker with primary alcohols when present as oxoammonium chlorides, while the reverse selectivity, that is selective oxidation of secondary alcohols, is observed when oxoammonium bromides are employed. 

Systems involving oxoammonium salts, electrolitically generated from TEMPO19 or employed in stoichiometric amounts,8 can also show useful selectivities for the oxidation of primary alcohols. The use of stoichiometric oxoammonium salts is sometimes more satisfactory in the selective oxidation of primary alcohols than the employment of catalytic TEMPO... 

Thus, isolation of oxoammonium salts on insoluble, cross-linked polymer supports was investigated along with their implementation in polymer-assisted solution-phase synthesis.23 These isolated oxoammonium salts could be employed in a water-free system to generate highly reactive oxidation agents without the overoxidation problems normally seen in the presence of water. 

Fig. 4. Polymer-supported oxoammonium salts resins 2 are highly reactive oxidants generated in situ by oxidation of TEMPO radical resin 1 with. /V-chlorosuccinimide.

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). 

In the course of their exploration of structure-activity relationships for ketone catalysts, Denmark et al. noted that oxoammonium salts such as 29-33 are very efficient catalysts of the epoxidation of olefins [34a]. Unfortunately, enantiomeric excesses achieved with this class of ketone catalyst have not yet exceeded 40% (30, epoxidation of tram-fl-rn eth yI styrene . With the fhiorinated oxoammonium catalyst 23 already mentioned, however, 58% ee was achieved in the asymmetric epoxidation of trans-stilbene [34b]. 

The 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor. TEMPO is a highly persistent radical, resistant to air and moisture, which is stabilized primarily by the steric hindrance of the NO-bond. Paramagnetic TEMPO radicals can be used as powerful spin probes for investigating the structure and dynamics of biopolymers such as proteins, DNA, and synthetic polymers by ESR spectroscopy [7]. A versatile redox chemistry has been reported for TEMPO radicals. The radical species can be transformed by two-electron reduction into the respective hydroxyl-amine or by two-electron oxidation into the oxoammonium salt [8]. One-electron oxidations involving oxoammonium salts have also been postulated [9]. The TEMPO radical is usually employed under phase-transfer conditions with, e.g., sodium hypochlorite as activating oxidant in the aqueous phase. In oxidations of primary alcohols carboxylic acids are often formed by over-oxidation, in addition to the de-... 

For the sake of completeness we also note that oxygen transfer processes can be mediated by organic catalysts which can be categorized on the same basis as metal catalysts. For example, ketones catalyze a variety of oxidations with mono-peroxysulfate (KHS05) [14]. The active oxidant is the corresponding dkmrane and, hence, the reaction can be construed as involving a peroxometal pathway. Similarly, TEMPO-catalyzed oxidations of alcohols with hypochlorite [15, 16] involve an oxoammonium salt as the active oxidant, i.e. an oxometal pathway. 

In addition a very useful and frequently applied method in the fine chemical industry to convert alcohols into the corresponding carbonyl compounds is the use of oxoammonium salts as oxidants [15]. These are very selective oxidants for alcohols, which operate under mild conditions and tolerate a large variety of functional groups. 

Preparation of tetramethylpiperidine-l-oxoammonium salts and their use as oxidants in organic chemistry 04OPP1. 

In acid medium, nitroxyl radicals disproportionate to produce hydroxylamines and oxoammonium salts. 

Strong oxidants such as chlorine and bromine oxidize nitroxyl radicals to corresponding oxoammonium salts (30) ... 

A reduced formation of NO was confirmed in HAS doped preoxidized PP films exposed to hydrogen chloride [228], although the salt 148 was reported to decompose ROOH. Salts of hydroxylamine 149 and oxoammonium salt 158 are formed from NO and a protonic acid (Eq. 44) and block the free-radical scavenging activity of ] NO [64,229]. Protonation of NO takes place in the first step of Eq. (44). 

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