Oxoammonium based oxidation

Table 4.9 Aerobic oxoammonium-based oxidation of alcohols.

Oxoammonium based Oxidation of Alcohols - TEMPO as Catalyst... 

The major drawbacks of this method are the use of NaOCl as the oxidant, the need for the addition of bromine ions and the necessity to use chlorinated solvents. Recently a great deal of effort has been devoted towards a greener oxoammonium-based method, by, for example, replacing TEMPO by heterogeneous variations or the replacement of NaOCl by a combination of metal as the co-catalyst and molecular oxygen as the oxidant. Examples of heterogeneous variants of TEMPO are anchoring... 

Other organic mediators act as hydride atom-abstracting agents. This is true, for example, with 2,2-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and the oxoammonium ion which is anodically accessible from 2,2,6,6-tetramethylpiperidyl oxide (TEMPO). DDQ has been electrochemically regenerated either externally or internally The in situ electrochemical oxidation, of TEMPO to the active oxoammonium ion is performed in lutidine-containing acetonitrile. Thus, primary alcohols can be oxidized to the aldehydes, while secondary ones are stable Primary amines are transformed to nitriles. If water is present, the amines are cleaved via the Schiff bases to the corresponding carbonyl compounds... 

Nitroxide-mediated oxidation based on oxoammonium salts is a very common application of nitroxides in organic synthesis. In addition to a variety of alcohol oxidations, applications using oxoammonium species as one-electron oxidants have been utilized with a number of different substrates [30]. 

TEMPO and other organic nitroxyls have been used as catalysts in combination with numerous stoichiometric oxidants, such as sodium hypochlorite [24], PhI(OAc)2 [25], and sodium chlorite [26]. A number of recent studies have shown that NO -based redox cocatalysts enable these reactions to be conducted with O2 as the terminal oxidant [27]. The general catalytic cycle for these aerobic nitroxyl/NO -catalyzed alcohol oxidation reactions is depicted in Scheme 15.6a. A variation of this approach features halides as additives, in which the X2/HX redox couple is believed to mediate the NO2/NO and oxoammonium/hydroxylamine redox couples (Scheme 15.6b). 

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. 

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