TEMPO-catalyzed oxidations

Table 1. TEMPO Catalyzed Oxidation of Primary Alcohols...

Table 5.1 molecular Copper(II) complex 1 oxygen and TEMPO catalyzed oxidation of alcohols with... 

Oxidation of the crude product mixtures containing both regioisomers 44 and 45 gave Cbz-protected amino acid 46 and amino ketone 47, which could then be separated by simple acid-base-extraction. As oxidizing reagents, periodic acid and a catalytic amount of ruthenium trichloride were found to be suitable. For some cases in which chemical yields were low, a TEMPO-catalyzed oxidation with bleach as oxidant was more effective. 

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. 

Fig. 4.70 CuBr2(bipy)-TEMPO catalyzed oxidation of alcohols...

The primary alcohol of the diol 309 was selectively oxidized to the corresponding hydroxyaldehyde through a 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO)-catalyzed oxidation. The remaining secondary hydroxyl functionality was protected as an acetoxy group and the aldehyde was further oxidized to the acetate carboxylic acid. Treatment of... 

TEMPO-catalyzed oxidations are chemoselective for primary alcohols.Secondary alcohols are oxidized slowly, as illustrated below. ... 

Rychnovsky, S. D. Vaidyanathan, R., TEMPO-Catalyzed Oxidations of Alcohols Using m-CPBA The Role of Halide Ions. /. Org. Chem. 1999, 64,310. 

The TEMPO-catalyzed oxidation of alcohols to carbonyl compounds with buffered aqueous NaOCl has found broad apphcation even in large-scale operations. Indeed, this selective methodology involves the use of safe and inexpensive inorganic reagents under mild reachon condihons. A supported TEMPO 7, which is soluble in CH2CI2 and acetic acid but insoluble in ethers and hexane, was prepared and proved to be an effective catalyst for the selective oxidahon of 1-octanol with various stoichiometric oxidants. When 7 was employed at 1 mol% as a catalyst with a stoichiometric amount of NaOCl, the aldehyde was obtained in 95% yield after only 30 min of reaction. The recycling of catalyst 7 was shown to be possible for seven reaction cycles in the oxidahon of 1-octanol, that occurred in undiminished conversion and selectivity under similar reachon conditions. 

Jiang N, Ragauskas AJ (2005) TEMPO-catalyzed oxidation of benzylic alcohols to aldehydes with the HjO /HBr/ionic liquid [bmim][PFJ system. Tetrahedron Lett 46 3323-3326... 

Notably, the co-oxidant problem also occurs for TEMPO-catalyzed oxidations. For the aerobic oxidation of alcohols with supported TEMPO see the end of Section 4.3.1.2. 

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). 

Dijksman et al. studied the mechanism of the CuCl-TEMPO catalyzed oxidation of alcohol first reported by Semmelhack et al. (96). Thus, various alcohol substrates were reacted with dioxygen in DMF at room temperature. 

Fig. 30. Proposed mechanism for the [CuCl-TEMPO]-catalyzed oxidation of benzylic and allylic alcohols 100).

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],... 

The reactivity pattern of recyclable hypervalent iodine reagents 65-67,69-71,73,74 and 76 is similar to the common iodobenzene-based reagents [34], For example, the recyclable (diacetoxyiodo)arenes 65,69,73 and 76 can be used instead of (diacetoxyiodo)benzene in the KBr or TEMPO-catalyzed oxidations of alcohols [74-78], while [hydroxy(tosyloxy)iodo]arenes 67, 71 and 74 are excellent reagents for tosyloxylation of ketones (Scheme 5.28) [76]. 

Saito, T., Nishiyama, T., Putaux, J.L., Vignon, M., Isogai, A., 2006. Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 7, 1687—1691. 

Roohani M, Habibi Y, Belgacem NM et al (2008) Cellulose whiskers reinforced polyvinyl alcohol copol Tners nanocomposites. Eur Polym J 44 2489-2498 Ruiz MM, Cavaille JY, Dufresne A et al (2000) Processing and characterization of new thermoset nanocomposites based on cellulose whiskers. Compos Interface 7 117-131 Ruiz MM, Cavaille JY, Dufresne A et al (2001) New waterborne epoxy coatings based on cellulose nanofillers. Macromol Symp 16 211-222 Saito T, Nishiyama Y, Putaux JL et al (2006) Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 7 1687-1691... 

Figure 5.2 TEMPO-catalyzed oxidation of alcohols using hypochlorite as the oxidant...

Table 5.3 Ruthenium-TEMPO-catalyzed oxidation of primary and secondary alcohols to the corresponding aldehyde using molecular oxygen. ...

TEMPO-catalyzed oxidations are replacing more familiar oxidation methods, especially in the pharmaceutical industry. An early step in the synthesis of an HIV protease inhibitor is the oxidation shown where the desired ketone is formed in 98% yield in the presence of 1 mol % of TEMPO. 

Figure 7.17 TEMPO-catalyzed oxidation of aicohois with NaOCi.

Individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose [66]... 

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