Tetramethyl piperidine oxide TEMPO

As expected, 2-halo-l,3-dithianes react with nucleophiles under Sn conditions. Suitable nucleophiles are enamines <2002TL9517, 2004T6931> and phenols <1997MOL7>. The reaction with EtOC(S)S K, followed by oxidation, provided a xanthate which generated a 1,3-dithiane 1-oxide radical upon treatment with Bu3SnH (Scheme 69) <2004T7781>. An efficient one-carbon radical precursor has also been obtained by addition of 2,2,6,6-tetramethyl-piperidine-l-oxyl (TEMPO) to 2-lithio-l,3-dithiane. The reactivity of this compound has been demonstrated <2005S1389>. 

Optically active (—)-sparteine, a piperidine-derived alkaloid complexed with palladium chloride 33, has been used in the enantioselective oxidation of benzyl alcohol derivatives <2005JA14817>. 4-Hydroxy-TEMPO 34 serves as an efficient catalyst for the continuous production of aldehydes from alcohols and bleach in a tube reactor (TEMPO = 2,2,6,6-tetramethyl-piperidine-l-oxyl) <20050PD577>. 

However, PEG supported metal-free catalysts have also been shown to perform well in water. For example the synthesis of a PEG-supported TEMPO (2,2,6,6-tetramethyl-piperidine-l-oxyl), and its use as a highly efficient, recoverable and recyclable catalyst in oxidation reactions was described (Pozzi et al. 2004). 

Benzeneselenol is an extremely fast reducing agent for alkyl radicals. The rate constant for benzeneselenol trapping of alkyl radicals is 1.2 x 10 s at 20 °C [99]. This is faster than the coupling reaction of alkyl radicals with 2,2,6,6-tetramethyl-piperidine-N-oxide (TEMPO) [100]. This exceptionally large rate constant makes benzeneselenol a very useful radical clock for the measurement of very fast radical processes [99]. 

Selective oxidation of primary OH groups in carbohydrate derivatives has been achieved using A -oxoammonium salts generated from (2,2,6,6-tetramethyl-piperidin-l-yl)oxy (TEMPO) and its derivatives as catalysts. The stoichiometric oxidants employed include sodium hypochlorite [48-50], sodium hypobromite [51, 52], and ammonium peroxodisulfate (using silver on alumina as a co-catalyst) [53, 54]. A representative protocol is shown in Scheme 12. 

Conversion of substituted benzoins to corresponding benzils occurs in quantitative yield when ZnO—DABCO eomplex is used to catalyse aerobic oxidation by O2 in toluene in the presence of K2CO3. The use of chiral ligand (118), in the presence of ZnS04 7H20 and 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perchlorate (TEMPO), resulted in isolation of benzil along with enantiomerically enriched benzoin (43% ee). ... 

The oxoammonium is generated in situ from its precursor, 2,2, 6,6 -tetramethyl-piperidine-N-oxyl (TEMPO), or derivatives thereof, which is used in catalytic quantities (see Figure 5.2). Various oxidants can be applied as the final oxidant [7-12]. In particular, the TEMPO-bleach protocol using bromide as co-catalyst introduced by Anelli et al. is finding wide application in organic synthesis [7]. TEMPO is used in amounts as low as 1 mol% relative to the substrate, and full conversion of substrates can commonly be achieved within 30 min. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Primary alcohol groups can be exclusively oxidized to aldehyde groups with pyridinium dichromate [149,150] and to carboxyl groups with the 2,2,6,6-tetramethyl-1-piperidine oxoammonium ion (TEMPO) [151]. The aldehydes can then be reduced to primary alcohols by reaction with NaB H4 [150,152], giving radiolabeled H-starch and the carboxyl group can be inverted by the action of Azotobacter vinlandii poly- 8-D-marmuronic acid C-5-epimerase to give L-iduronic acid [153]. 

Primary alcohol groups can also be mildly and selectively oxidized to carboxyl groups by reaction with 2,2,6,6-tetramethyl-l-piperidine oxoanunonium ion (TEMPO) in the presence of hypochlorite and bromide [129,130]. The specificity for the oxidation of primary alcohols in the presence of secondary alcohols in carbohydrates occurs because of the bulky nature of the TEMPO reagent, similar to the specificity obtained with the bulky trityl chloride. The mechanism for the oxidation of primary alcohols with TEMPO is given in reaction 4.140. 

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