Alcohol TEMPO

Oxoammonium based Oxidation of Alcohols - TEMPO as Catalyst... 

OXIDATION OF PRIMARY ALCOHOLS TO CARBOXYLIC ACIDS WITH SODIUM CHLORITE CATALYZED BY TEMPO AND BLEACH 4-METHOXYPHENYLACETIC ACID... 

A 1-L, three-necked, round-bottomed flask equipped with a mechanical stirrer and two 100-mL addition funnels is charged with 4-methoxyphenethyl alcohol (If) (6.09 g, 40 mmol), TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical) (0.436 g, 2.8 mmol), 200 mL of acetonitrile, and 150 mL of... 

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

Sheldon et al. have combined a KR catalyzed by CALB with a racemization catalyzed by a Ru(II) complex in combination with TEMPO (2,2,6,6-tetramethylpi-peridine 1-oxyl free radical) [28]. They proposed that racemization involved initial ruthenium-catalyzed oxidation of the alcohol to the corresponding ketone, with TEMPO acting as a stoichiometric oxidant. The ketone was then reduced to racemic alcohol by ruthenium hydrides, which were proposed to be formed under the reaction conditions. Under these conditions, they obtained 76% yield of enantiopure 1-phenylethanol acetate at 70° after 48 hours. 

Another iron porphyrin complex with 5,10,15,20-tetrakis(2, 6 -dichloro-3 -sulfonatophenyl)porphyrin was applied in ionic liquids and oxidized veratryl alcohol (3,4-dimethoxybenzyl alcohol) with hydrogen peroxide in yields up to 83% to the aldehyde as the major product [145]. In addition, TEMPO was incorporated via... 

Oxidations Using Oxoammonium Ions. Another oxidation procedure uses an oxoammonium ion, usually derived from the stable nitroxide tetramethylpiperidine nitroxide, TEMPO, as the active reagent.31 It is regenerated in a catalytic cycle using hypochlorite ion32 or NCS33 as the stoichiometric oxidant. These reactions involve an intermediate adduct of the alcohol and the oxoammonium ion. 

A new interesting branch of the modern antioxidant chemistry deals with the cyclic mechanisms involving acid catalysis. The first inhibiting system of this type was discovered in 1988 [44]. It consisted of an alcohol (primary or secondary), a stable nitroxyl radical TEMPO, and... 

As the initiator, a common radical initiator and arenesulfonyl chloride are also used [286,287]. As shown in Table 6, this polymerization has a significantly large polymerization rate, and it is hardly disturbed by impurities such as alcohol and water [288]. ATRP with Cu complex was also applied to the polymerization of acrylates [289,290], methacrylates [290-297], and AN [298] as well as St [288, 297, 299]. Because of the suppressed bimolecular termination, hyperbranched polymers are readily prepared [292], being similar to the polymerization with TEMPO previously described. 

Scheme 9 a TBDMSiCl2, imidazole, CH2CI2 AD mix a (Sharpless asymmetric dihydrox-ylation reagent), methanesulfonamide, H20/t-butyl alcohol b NaOCl, KBr, TEMPO in CH2C12/H20 c NH2OH HCl, pyridine/EtOH, 90 °C d NaOH, 1,2-propanediol, 160 °C e PPh3, DIAD, toluene, 0 °C, reflux [54]... 

Figure 1.9 Alcohols are oxidized at the inner surface of TEMPO DE (1 > 2). But not so hydrated aldehydes (3 -> 4) which cannot enter the pores due to the hydrophobicity of the material s surface. (Reproduced from ref. 20, with permission.)... 

The method uses a simple electrode made of a thin film of sol-gel organosilica doped with nitroxyl radicals deposited on the surface of an indium tin oxide (ITO) electrode. Thus, whereas in water benzyl alcohol is rapidly oxidized to benzoic acid, the use of the hydrophobic sol-gel molecular electrode TEMPO DE affords benzaldehyde only (Figure 1.9), with an unprecedented purity, which is highly desirable for the fragrance and pharmaceutical industries where this aromatic aldehyde is employed in large amounts. 

Figure 5.9 TEMPO DE is obtained by electrodeposition of a thin layer of orga-nosilica doped with TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) upon application of — 1.1 V (v.v. Ag/AgCl) for 15 min to a solution of suitable organosilanes (left). The electrocatalytic film thereby obtained selectively converts benzyl alcohol dissolved in 0.2 M NaHC03 (right). 

In other cases, organic modification of the sol gel cages markedly protects the entrapped molecular dopant from degradation by external reactants, as shown for instance by the entrapment of the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). This is a highly active catalyst which in the NaOCl oxidation of alcohols to carbonyls in a CH2CI2-H20 biphasic system becomes highly stabilized upon sol gel entrapment in an ORMOSIL matrix it progressively loses it activity when entrapped at the external surface of commercial silica.25... 

Figure 5.14 Sol-gel immobilized TEMPO is an off-the-shelf alcohol oxidation catalyst. In a biphasic reaction system and in organic solvent it yields carbonyls in water it yields carboxylates.

More recently, a series of sol-gel hydrophobized nanostructured silica matrices doped with the organocatalyst TEMPO (SiliaCat TEMPO) entered the market as suitable oxidation catalysts for the rapid and selective production of carbonyls and carboxylic acids. In the former case, SiliaCat TEMPO selectively mediates the oxidation of delicate primary and secondary alcohol substrates into valued carbonyl derivatives (Scheme 5.2), retaining its potent activity throughout several reaction cycles (Table 5.2).33 Using this catalyst, for example, enables the synthesis of extremely valuable a-hydroxy acids with relevant selectivity enhancement by coupling of SiliaCat TEMPO with rapid Ru04-mediated olefin dihydroxylation (Scheme 5.3).34... 

R. Ciriminna, C. Bolm, T. Fey and M. Pagliaro, Sol-Gel Ormosils Doped with TEMPO as Recyclable Catalysts for the Selective Oxidation of Alcohols, Adv. Synth. Catal., 2002, 344, 159. 

Human life, furthermore, certainly benefits from a less polluted world, and here, again, sol-gel entrapped catalysts are, literally, able to have transferred to within their large inner porosity the whole chemistry of fine chemicals production. Think for instance of an innocuous easily handled orange powder called SiliaCat TEMPO (Chapter 5) that added to a mixture of alcohols at 0 °C with a modest excess of aqueous, cold bleach rapidly converts them into all those fragrances, vitamins, hormones and drugs made of carbonyl compounds. 

Scheme 11.12 TEMPO mediated oxidation of primary alcohols. (After [95]).

Oxidation of primary alcohols by hypochlorite moderated by TEMPO... 

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... 

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