Clayfen oxidants

Scheme 4.3 Oxidation of alcohols by iron(lll) nitrate supported on clay (clayfen).

Selective Solvent-free Oxidation with Clayfen... 

A facile method for the oxidation of alcohols to carbonyl compounds has been reported by Varma et al. using montmorillonite K 10 clay-supported iron(III) nitrate (clayfen) under solvent-free conditions [100], This MW-expedited reaction presumably proceeds via the intermediacy of nitrosonium ions. Interestingly, no carboxylic acids are formed in the oxidation of primary alcohols. The simple solvent-free experimental procedure involves mixing of neat substrates with clayfen and a brief exposure of the reaction mixture to irradiation in a MW oven for 15-60 s. This rapid, ma-nipulatively simple, inexpensive and selective procedure avoids the use of excess solvents and toxic oxidants (Scheme 6.30) [100]. Solid state use of clayfen has afforded higher yields and the amounts used are half of that used by Laszlo et al. [17,19]. 

Scheme 6.30 Solvent-free selective oxidation of alcohols with clayfen.

Laszlo introduced an oxidative cleavage of dithioacetals by their clayfen (4) and claycop reagents under mild conditions with excellent yields [46, 47]. These reagents are convenient sources of the nitrosonium ion NO+, a soft reactive Lewis acid species, well adapted for attack of the soft sulfur atom. 

The dithioacetal (0.01 mol) was stirred for a few hours at room temperature with clayfen (4) (10.4 g, 11 mmol of ferric nitrate) or with claycop (12.1 g, 20 mmol of copper nitrate) in toluene, n-pentane or, preferably, dichloromethane (120ml). Evolution of nitrogen oxides occurred rapidly. Stirring was maintained until gas evolution ceased. The clay was then filtered off and washed twice with portions (50 ml) of the solvent. The resulting pale-yellow or slightly green solution was filtered through a small quantity of neutral aluminium oxide and the solvent was evaporated under vacuum. In the case of dithiane and dithiolane derivatives, this afforded the pure carbonyl compound in excellent yield. 

Oxidation of alcohols. 1 Secondary alcohols are oxidized to carbonyl compounds by Clayfen in pentane or hexane under vigorous stirring. Primary benzyl alcohols are oxidized satisfactorily, but primary aliphatic alcohols are oxidized to complex mixtures. Isolated yields are generally>80%. Nitrite esters (RONO) have been identified as intermediates. 

Scheme 3.45 Example of oxime group oxidation with Clayfen.

Oxidation of Oximes A different type of N-oxidation reaction involves the direct oxidation of oximes to nitro compounds. Although a variety of oxidizing agents have been described for this reaction, the use of non-heme iron-based systems is rather limited. In this context, the oxidation of the oxime group in tetrahydro-4H-pyrido[ 1,2-a]pyrimidines was carried out at room temperature in the presence of 50 mol% of Clayfen [K10 montmorillonite-supported iron(III) nitrate] (Scheme 3.45) [147]. Under these conditions the corresponding nitro derivatives were obtained in 32-35% yield. 1SN mass studies revealed that the reaction involved the direct oxidation of the hydroxyimino group. 

Table 3.3 Synthesis of disulfides using Clayfen-type oxidants.

Nitration and oxidation. Clay-supported Cu(N03)2, unlike clayfen (12,231), > shelf-stable for months. Like clayfen, it is a convenient source of N02+ and can leave thioacetals or selenoacetals to the carbonyl compound at 25° in high yield. It effects aromatization of 1,4-dihydropyridines in 80-92% yield. In the presence i acetic anhydride, it can effect nitration even of halobenzenes at 25° with marked, - jra-preference, which can be enhanced by use of lower temperatures. 

Oxidation of 1,4-dihydropyridines. Clay-supported Fe(NO,).i or Cu(NO,) (Clay-cop) oxidizes 1,4-dihydropyridines to pyridines at 25". Reactions are generally slower with Claycop than with Clayfen, but yields are usually higher. 

A rapid MW oxidation protocol for the oxidation of alcohols to carbonyl compounds has been reported by Vanna et al. using montmorillonite KIO clay-supported iron(III) nitrate (clayfen) (Scheme 7). The simple solvent-free experimental procedure involves mixing of neat substrates... 

Ri = Alkyl, aryl R2 = H, alkyl, aryl Scheme 2.2-38 Solventless oxidation of alcohols with clayfen. 

Oxidation of the cyclic sulfide 4-oxothiophene to the respective sulfoxide 4-ox-othiophenesulfoxide was achieved by heating with iron(III)nitrate impregnated on montmorillonite Kio clay in methylene chloride for 12h to give a 79% yield. Much faster reaction took place using clayfen under MWI to give 83% yield within... 

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