TEMPO oxidation, mechanism

Fig. 17,16. Mechanism of the TEMPO oxidation of alcohols to aldehydes (TEMPO stands for tetramethylpiperidine nitroxyl).

Figure 7.18 Mechanisms of TEMPO oxidations. The top line shows the disproportionation of TEMPO in acid and the rest of the figure the acidic and basic mechanisms for oxidation of alcohols, with the reaction of hydrated aldehydes also indicated.

It was shown that the NFC may be obtained using an efficient process, where 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) oxidation was utilized as a pretreatment step prior to mechanical treatment (Lavoine et al., 2012 Saito et al.,... 

NFC depend on the cellulose source. Besides, it was found that the degree of crystallinity of NFC slightly decreases with an increase in the number of passes through the high-pressure homogenizer (Iwamoto et al., 2007). Such pretreatments as enzymatic hydrolysis and TEMPO-oxidation do not change the crystalline stracture of the final NFC. However, the nanofibrils produced after preliminary carboxymethylation have lower crystallinity and size of crystallites than NFC obtained by conventional mechanical disintegration. 

There are basically two families of nanosized cellulosic particles (1) Nanofibrillar cellulose, which includes mechanically isolated microfibrils, chemically isolated microfibrils (TEMPO-oxidation), bacterial cellulose and can be considered spaghetti-like, and (2) Cellulose nanocrystals - rods of highly crystalline cellulose which are isolated by acid hydrolysis. Cellulose nanocrystals are represented in literature by synonyms like cellulose whiskers, cellulose nanowhiskers, cellulose microfibrils, micro-crystalline cellulose and nanocrystalline cellulose because they are not yet commercially available. These are needle-shaped (100 run to 200 run X 10 nm), highly crystalline, strong (E - 150 GPa) and form liquid crystal suspensions. 

Nanofibrillar cellulose which includes mechanically isolated microfibrils, chemically isolated microfibrils (TEMPO-oxidation), bacterial cellulose and can be considered as spaghetti-like ... 

FIGURE 113 Schematic illustration of TEMPO-mediated oxidation mechanism from hydroxyl methyl groups of cellulose to carboxylic form. Reprinted with permission from Ref. [4]. 

Fujisawa S, Ikeuchi T, Takeuchi M, Saito T, Isogai A (2012) Superior reinforcement effect of TEMPO-oxidized cellulose nanofibrils in polystyrene matrix optical, thermal, and mechanical studies. Biomacromolecules 13 2188-2194... 

Apart from the production process of NFC by homogenization, their final application is a critical issue. Because of the hydrophilic nature of cellulose nanofibers, their incorporation and dispersion with common polymers, which are hydrophobic, are very critical issues [38]. Low interfacial adhesion between these two parts in composite leads to reduction in the mechanical and other properties of the final product. Thus, a wide variety of modifications like carboxymethylation [65], 2,2,6,6-tetramethylpiperdine-1-oxyl (TEMPO) oxidation [66, 67], acetylation [68, 69], and silylation [70, 71] have been designed to overcome this problem. The modification strategies of cellulose nanosize materials are discussed in Section 11.6. 

The efficiency of defibrillation in the ultrasonic process is dependent on power, concentration, temperature, size of fibers, time and distance from probe tip to collector [77]. In some cases, researchers have been used a combination of ultrasonication with other methods to increase fibrillation of nanoscale cellulose. For example, Li et al. [78] prepared nanocrystalline cellulose by ultrasonication and acid hydrolysis with H SO from bleached softwood pulp. They found that ultrasonication led to folding and erosion of the cellulose surface, and thus provided more reactive site to penetrate acid and prepare high-crystalline and small-size nanocellulose. Furthermore, Wang and Chen [77] reported that a combination of ultrasonication and homogenization boosts uniformity and fibrillation of cellulose nanofiber in comparison to ultrasonication solely. In addition, when compared to mechanical blender, ultrasonic bath and ultrasonic probe, Mishra et al. [79] concluded that TEMPO-oxidized fiber treatment with ultrasonic probe was more efficient for nanocellulose production than the other three methods. 

Bulota, M., Vesterinen, A.H., Hughes, M., Seppala, J., 2013. Mechanical behavior, structure, and reinforcement processes of tempo-oxidized cellulose reinforced poly(lactic) acid. Polymer Composite 34, 173—179. 

Figure 21.21 Reaction scheme of TEMPO-mediated oxidation mechanism of the hydroxymethyl groups of cellulose. Reprinted with permission from Habibi, Y, Lucia, L. A., Rojas, 0. J., Cellulose nanociystals Chemistiy, self-assembly, and applications, Chem. Rev., 110, 3479-3500. Copyright (2010) American Chemical Society [37].

Although these reactions are formulated as ionic reactions via 947 and 949, because of the apparent partial formation of polymers and inhibition of the fluoride-catalyzed reaction of pyridine N-oxide 860 with aUyl 82 or benzyltrimethylsilane 83 by sulfur or galvinoxyl yet not by Tempo, a radical mechanism caimot be excluded [61, 62]. The closely related additions of allyltrimethylsilane 82 (cf. Section 7.3) to nitrones 976 are catalyzed by TMSOTf 20 to give, via 977, either o-unsatu-rated hydroxylamines 978 or isoxazoHdines 979 (cf also the additions of 965 to 962a and 969 in schemes 7.20 and 7.21). 

Synthesis of aldehydes from alcohols is an important transformation in several applications. In small scale oxidations still chromic acid is being used as a stoichiometric oxidant of alcohols, which leads to a large amount of toxic waste and it is also expensive. Catalytic routes have been reported using palladium catalyst [18], or TEMPO (see also Figure 15.13) as a radical catalyst for the oxidation of alcohols [19], or combinations of TEMPO and copper [20] related work is mentioned in the references of these articles. The mechanism of... 

Although this point has been known for many years " , and despite the important synthetic applications of TEMPO-oxoammonium that will be summarized later, the nature of the involved reactive species and the ensuing mechanism of oxidation are still a matter of debate, and this is why the term oxoammonium ion is put within quotation marks in the title of the present section. The reactivity subtleties will be summarized here. 

SCHEME 15. Ionic mechanism of oxidation of alcohols by TEMPO-oxoammonium ion. Redrawn from Reference 63 by permission of The Royal Society of Chemistry on behalf of the Centre National de la Recherche Scientifique... 

SCHEME 17. Sheldon s mechanism of an aerobic copper-centred oxidation of alcohols by TEMPO... 

Operation of the latter mechanism has also been invoked for the oxidation of X-substituted benzyl alcohols with TEMPO and the enzyme laccase becanse the redox potential of the enzyme (0.78 is adeqnate for the oxidation of TEMPO to oxoammonium ion (0.8 Strangely enough, no linear correlation of the log A x/ h ratios... 

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. 

Scheme 3 Proposed catalytic mechanism for the copper catalyzed oxidation of alcohols with TEMPO as oxidant...

Oxidation of Cu with O2 is markedly slower than the analogous oxidation by TEMPO. The existence of a copper-centered dehydrogenation step was supported by investigating kinetic isotope effects and Hammett correlation studies using different substituted benzyhc alcohols. The /1-hydrogen abstraction was postulated to occur in a concerted mechanism with an -coordinated TEMPO radical bgand (Scheme 4). As such, this TEMPO-mediated copper-catalyzed oxidation of alcohols bears resemblance... 

Scheme 4 Postulated concerted mechanism for the 8-hydrogen abstraction dm-ing oxidation of alcohols with TEMPO...

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