Oxidation with Clayfen

Selective Solvent-free Oxidation with Clayfen... 

Scheme 3.45 Example of oxime group 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.

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 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. 

Ri = Alkyl, aryl R2 = H, alkyl, aryl Scheme 2.2-38 Solventless 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. 

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 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... 

The oxidation of benzoin is representative of die general procedure used for the oxidation of alcohols. Clayfen (0.125 g) was thoroughly mixed with benzoin (0.106 g, 0.5 mmol) in the solid state using a vortex mixer and the material was placed in an alumina bath inside a household microwave oven equipped with a turntable and irradiated at full power (900 Watts). The reaction was monitored by TLC using, hexane AcOEt (10 1, v ). Upon completion, the product was extracted into methylene chlori. The bulk temperature of the alumina bath, whidi serves as a beat sink and convenient site to bold the reaction vessel inside the microwave oven, reached 65 °C after 30 seconds of irradiation as measured by inserting a thermomet into the alumina bath. 

A rapid microwave oxidation protocol for the oxidation of alcohol to carbonyl compound has been reported by Varma (2001). Different types of oxidizing reagents have been used with microwave irradiations. Alcohols were converted into corresponding carbonyl compounds in the presence of montmorillonite KIO clay-sup-ported iron (111) nitrate (Clayfen), Oxone-alumina, IBD-alumina, 35% MnO doped silica, CrO,-alumina or CuSO-alumina under microwave irradiation and solvent-free conditions in few minutes (0.25-3.5 min). 

The first part of this article specifically deals with representative laboratoiy applications to fine chemistry of clearly identified, unaltered KIO, excluding its modified forms (cation-exchanged, doped by salt deposition, pillared, etc.) and industrial uses in bulk. This illustrative medley shows the prowess of KIO as a strong Brpnsted acidic catalyst. The second part deals with cation-exchanged (mainly Fe ") montmorillonite. Clayfen and claycop, versatile stoichiometric reagents obtained by metal nitrate deposition on KIO, are used in oxidation and nitration reactions. They are treated under Iron(III) Nitrate-KlO Montrrufrillonite Clay and Copperfll) Nitrate-KlO Bentonite Clay. 

---