Oxoammonium cation

Israeli, A., Patt, M., Oron, M., Samuni, A., Kohen, R., and Goldstein, S. 2005. Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxy-lamine derived from cyclic nitrones. Free Radical Biology and Medicine 38 317-324. 

In this mechanism, RNO and RNO+ are nitroxide radical and its oxoammonium cation. The rate constants for Reactions (5) and (6) determined by pulse radiolysis are equal to ... 

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. 

E chiral imine, F chiral oxaziridine, G chiral oxoammonium cation Scheme 10.1... 

Fe(II)-tris[2,2 -bi(tetrahydropyrimidine)]m that exhibits an unusual negative AH" and oxidation of nitroxides to oxoammonium cations by superoxometal complexes and by inorganic radicals. Some examples are shown in Table 8.7. 

This result can be explained by assuming that the initially formed TEMPOH (see earlier) undergoes disproportionation to TEMPH and the oxoammonium cation (Eq. 23). Reduction of the latter by the alcohol affords another molecule of TEMPOH and this leads, ultimately, to the formation of the ketone and TEMPH in the observed stoichiometry of 3 2. The observation that attempts to prepare TEMPOH [91] under an inert atmosphere always resulted in the formation of TEMPH is consistent with this hypothesis. 

Goldstein S, Samuni A, Russo A. (2003) Reaction of cyclic nitroxides with nitrogen dioxide The intermediacy of oxoammonium cations. J Am Chem Soc 125 8364-8370. 

The mechanistic details of these laccase/mediator catalyzed aerobic oxidations are still a matter of conjecture (51-54). However, experiments with a probe alcohol point towards one-electron oxidation of the mediator by the oxidized (cupric) form of the laccase followed by reaction of the oxidized mediator with the substrate, either via electron transfer (ET), e.g., with ABTS, or via hydrogen atom transfer (HAT), e.g., with N-hydroxy compounds which form N-oxy radicals (55). TEMPO and its derivatives form a unique case one-electron oxidation of TEMPO affords the oxoammonium cation which oxidizes the alcohol via a heterolytic pathway (Fig. 6), giving the carbonyl product and the hydroxylamine. The Tl copper center in fungal laccases has a redox potential of ca. 0.8 V vs. NHE. Consequently, fungal laccases can easily oxidize TEMPO to the corresponding oxoammonium cation, since the oxidation potential of the latter, which was first measured by Golubev and co-workers (55,57), is 0.75 V. This was confirmed by EPR measurements, which showed that laccase is reduced in the presence of TEMPO One equivalent of laccase could oxidize at least three equivalents of TEMPO within a few minutes under anaerobic conditions (58). 

There are various alternatives for reoxidizing the hydroxylamine back to TEMPO to complete the catalytic cycle. It can be oxidized by dioxygen, laccase or the oxoammonium cation. The active oxidant is the same as that in the TEMPO catalyzed oxidations of alcohols with hypochlorite (or other single oxygen donors), a method which is widely used in the oxidation of a broad range of alcohols using low catalyst loadings (1 mol % or less) (59). 

Li and coworkers developed an effective system for the oxidation of alcohols under an atmosphere of oxygen, without the need for any additional solvent or transition metal catalyst, by using catalytic amounts of (diacetoxyiodo)benzene, TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) and potassium nitrate (Scheme 4.56) [88]. A tentative mechanism for this catalytic oxidation involves the oxoammonium cation 109, which... 

Loss of the EPR signal during Fe(CN)g (hcf) oxidation of imidazolidine nitroxides having a hydrogen at position 4 of the heterocycle (HIMD) supports the assumption that the HIMD fragmentation is facilitated by proton abstraction from the position 4 of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. ... 

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... 

Most of the nitroxides containing functional groups undergo disproportionation to oxoammonium and hydroxyammonium cations in... 

SemmeUiack et al. [104] reported that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzyhc and allylic alcohols and activities were low (10 mol% of catalyst was needed for smooth reaction). They proposed that the copper catalyzes the reoxidation of TEMPO to the oxoammonium cation. Based on our results with the Ru/TEMPO system we doubted the validity of this mechanism. Hence, we subjected the Cu/ TEMPO to the same mechanistic studies described above for the Ru/TEMPO system [105]. The results of stoichiometric experiments under anaerobic conditions, Hammett correlations and kinetic isotope effect studies showed a similar pattern to those with the Ru/TEMPO system, i.e., they are inconsistent with a mechanism involving an oxoammonium species as the active oxidant. Hence, we propose the mechanism shown in Scheme 4.18 for Cu /TEM PO-catalyzed aerobic oxidation of alcohols. 

The nitroxyl radical serves as a catalyst precursor which is oxidized by using different primary oxidants (e.g. sodium chlorite [327], electrochemical oxidation [328-330] or periodic acid [331]). The resulting oxoammonium cation is considered to be the true catalytic oxidant which is reduced to the corresponding hydroxyl-amine during the oxidation of the alcohol. Subsequently, the hydroxylamine is re-oxidized with a suitable stoichiometric oxidant to the nitroxyl radical. 

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