TEMPO polymer-immobilized

Two interesting recent modifications of Anelli s protocol involve the employment of silica-supported TEMPO40 and a kind of polymer-immobilized TEMPO (PIPO)41 PIPO is easily prepared from a cheap polymer called Chimassorb 944 that is used as an antioxidant and light stabilizer for plastics. 

Selective oxidations of primary alcohols can also be achieved employing less common variants of the Anelli s protocol, such as those involving silica-supported TEMPO11 and polymer-immobilized TEMPO.12... 

This new polymer immobiUzed TEMPO, which we refer to as PIPO (Polymer Immobilized Piperidinyl Oxyl), proved to be a very effective catalyst for the oxidation of alcohols with hypochlorite [17] (Scheme 4.3). Under the standard conditions PIPO is dissolved in the dichloromethane layer. In contrast, in the absence of solvent PIPO was a very effective recyclable heterogeneous catalyst. Furthermore, the enhanced activity of PIPO compared with TEMPO made the use of a bromide co-catalyst redundant. Hence, the use of PIPO, in an amount equivalent to 1 mol% of nitroxyl radical. 

Some successful attempts to immobilize catalysts for the oxidation of alcohols to carbonyl compounds involve the attachment of TEMPO-derivatives to a solid phase. Bolm et al. were the first to immobilize l-hydroxy-2,2,6,6-tetramethylpiperi-dine to modified silica gel (SG-TMP-OH) (11) and applied in the oxidation of multifunctional alcohols [68]. Other groups further investigated the use of polymer-supported TEMPO [69]. This system allowed the oxidation of alcohols to aldehydes and ketones, respectively, using bleach to regenerate the immobilized ni-troxyl radical (Scheme 4.6). 

In another approach, a polymer-bound TEMPO-species, referred to as PIPO (polyamine immobilized piperidinyl oxyl), was derived by oxidation of commercially available Chimasorb 944, an oligomeric sterically hindered amine. Together with bleach as co-oxidant PIPO (12) afforded aldehydes and ketones in short reaction times and with yields from 80 to > 99% (Scheme 4.6) [70]. 

A multipolymer version of the aerobic TEMPO oxidation of alcohols in the presence of Cu(II) was also developed in the Toy group using immobilized TEMPO and polymer-supported 2,2 -bipyridine, which form the oxidative organometalhc complex with copper [31],... 

A similar oxidative protocol has been used for the oxidation of (fluoroalkyl)alkanols, Rf(CH2) CH20H, to the respective aldehydes [146], in the one-pot selective oxidation/olefination of primary alcohols using the PhI(OAc)2-TEMPO system and stabilized phosphorus ylides [147] and in the chemo-enzymatic oxidation-hydrocyanation of 7,8-unsaturated alcohols [148]. Other [bis(acyloxy)iodo]arenes can be used instead of PhI(OAc)2 in the TEMPO-catalyzed oxidations, in particular the recyclable monomeric and the polymer-supported hypervalent iodine reagents (Chapter 5). Further modifications of this method include the use of polymer-supported TEMPO [151], fluorous-tagged TEMPO [152,153], ion-supported TEMPO [154] and TEMPO immobilized on silica [148],... 

If the water used for rewetting contains the weakly polar probe TEMPO, one observes its uptake by the polymer by an increase of the immobilized fraction with... 

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