TEMPO catalyzed

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

A copper-centered mechanism for the Cu-TEMPO-catalyzed aerobic oxidation of alcohols was proposed by Sheldon and co-workers, wherein the active catalytic Cu" species is generated by oxidation of a Cu species with TEMPO, in the presence of alcohol, with formation of TEMPOH (Scheme 3) [146]. The resulting Cu" species is then capable of oxidizing the alcoholate to the aldehyde or ketone species. Regeneration of the TEMPO radical species was achieved by rapid oxidation of TEMPOH with O2. 

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. 

The reaction appears to be well suited for selective conversion of biomass carbohydrates into their corresponding oxidized derivatives. This system serves to oxidize several polymeric carbohydrates including starches and pullulan.445 49 More recent work has described the TEMPO-catalyzed introduction of carboxyl groups in native cellulose and its different morphological forms,45(M52 cellulose derivatives,453 and the surface of cellulose nanocrystals.454 The related biopolymer, chitin, also is oxidized under these conditions.455... 

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... 

Ansari lA, Gree R (2002) TEMPO-catalyzed aerobic oxidation of alcohols to aldehydes and ketones in ionic hquid [bmim][PPJ. Org Lett 4 1507-1509... 

Another option is the use of both solid-supported versions of the catalyst and the co-oxidant. The Toy group in Hong-Kong has developed a multipolymer system for the TEMPO-catalyzed alcohol oxidation in which both the pre-catalyst and the co-oxidant are attached to a polymer. As it was clearly impossible to use two insoluble supported reagents, the idea was to use an insoluble polymer in conjunction with a soluble one. An insoluble polymer-supported diacetoxyiodosobenzene (PSDIB), an analog of 10, and a soluble polymer-supported TEMPO were used,... 

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). 

Fig. 27. [Cu -TEMPO]-catalyzed aerobic oxidation of alcohols using fluorous biphasic conditions 97,98).

TEMPO-Catalyzed Aerobic Oxidation of Alcohols in Ionic Liquid [bmim][PF6] (99)... 

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).

Recently, Giacomelli and co-workers have reported an efficient oxidation of primary alcohols to carboxylic acids through a TEMPO-catalyzed procedure. The procedure is based on the addition of 2 molar equiv of TCCA to an acetone solution of the alcohol followed by catalytic amounts (0.1 equiv) of TEMPO, NaBr, and then 1 equiv of aq. NaHCOs (Equation 65). This system operates at room temperature, the oxidation of the primary alcoholic group being practically quantitative. Secondary alcohols are oxidized to ketones. The mild conditions of this procedure and the total absence of any transition metal make this reaction suitable for safe laboratory use <2003JOC4999>. 

Scheme 6.4 BASF adaptation of Cu/TEMPO-catalyzed aerobic alcohol oxidation.

Scheme 15.7 Minisd s MntNOjIj/CotNOjjj/TEMPO-catalyzed aerobic alcohol oxidation and Hu s NaNOj/Brj/TEMPO-catalyzed aerobic alcohol oxidation.

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],... 

A similar TEMPO-catalyzed system for the oxidation of alcohols using l-chloro-l,2-benziodoxol-3(l//)-one (130) (Section 2.1.8.1.1) as the terminal oxidant in ethyl acetate in the presence of pyridine at room temperature has been reported [158], Various alcohols 129 can be oxidized to the corresponding carbonyl compounds in high to excellent yields under these conditions (Scheme 3.55). The oxidation of primary alcohols (129, = H) in this reaction proceeds generally faster compared to the secondary alcohols. 

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]. 

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