Oxidation with aluminium isopropoxide

The isomerization reaction is also encountered in chemical manipulations of steroids. Thus, many natural steroids contain a 5-en-3-ol combination of functionalities, e.g. cholesterol. Treatment of cholesterol with an oxidizing agent (aluminium isopropoxide is particularly suitable) leads to cholest -en-3-one, the tautomerism occurring spontaneously under the reaction conditions. 

This alcohol is oxidized using an Oppenauer reaction under typical conditions with aluminium isopropoxide and cyclohexanone in boiling toluene. 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

The crystalline fraction was found to be formed via a cleavage of the C1g-0(CH2—O) bond in the monomer molecule and proved many years ago to be an isotactic polymer (with regular head-to-tail linkages) (Figure 9.1) [42]. The structure of the amorphous fraction, on the other hand, varies depending on the kind of catalyst. Some amorphous polypropylene oxide)s prepared with catalysts such as diethylzinc-methanol [43] or aluminium isopropoxide-zinc chloride [44] consist of regular head-to-tail linked units, but they are atactic (the mole fraction of isotactic diads is less than 0.6) [43]. Some other amorphous polypropylene oxide)s obtained with catalysts derived from reactions in the diethylzinc-water [44,45], and triethylaluminium-water [46] systems, and with aluminium isopropoxide [44], have been found to be irregular, i.e. to contain head-to-head and tail-to-tail enchained monomer units. 

The cyclic vinyl ether (182),170 postulated last year as an intermediate in the 21-acetoxylation of the hemiacetal (183), can be prepared in good yield by heating the hemiacetal with aluminium isopropoxide in toluene under argon, followed by direct chromatography on basic alumina.171 The vinyl ether was converted into the 20,21-diol (184) either by reaction with osmium tetroxide or by acetoxylation with lead tetra-acetate, followed by hydrolysis. The hypoiodite reaction, followed by oxidation, allows direct conversion of a 21-acetoxypregnan-20/3-ol (185) into the 18-iodo-20-ketone (186), which is solvolysed in the presence of silver acetate to provide a new route to the 21-acetate of 18-hydroxydeoxycorticosterone hemiacetal (187).171... 

The secondary alcohols are oxidized to ketones by refluxing with aluminium isopropoxide, A1[0CH(CH3)2]3 [or Al(0-iPr)3], or potassium t-butoxide, KOC(CH3)3 [or KO-t-Bu]. A ketone such as acetone used in the reaction as refluxing agent is reduced to alcohol, 2-propanol. The reaction is known as the Oppenauer oxidation. The reverse reaction known as the Meerwein-Ponndorf-Verly reduction is the reduction of ketones to alcohols in the presence of alcohol such as 2-propanol. Potassium fert-butoxide can be used for the oxidation of primary alcohols. Aluminium isopropoxide in acetone is particularly used for... 

Fig. 8.36. As the asymmetric center of citronellal is unaffected by the reactions, all of the isopulegol and menthol isomers formed have the correct stereochemistry at Cl of the /i-menthane skeleton. There are therefore two strategies for recycling unwanted isomers. The first is to purify the ( )-isopulegol (172) by crystallization and recycle (178-180) back to citronellal by pyrolysis [221, 223, 224]. The second is to hydrogenate the mixture, separate the (—)-menthol by crystallization and treat the remainder with aluminium isopropoxide, which converts all of them, by Oppenauer oxidation, enoliza-tion, reketonization and Meerwein-Ponndorf-Verley reduction, to (—)-menthol, which is the thermodynamically most stable isomer (225).

The alcohol in this sensitive indole could be oxidized to the desired ketone by heating with cyclohexanone and aluminium isopropoxide. A very similar substrate could no be oxidized efficiently after trying a wide variety of reagents.47... 

One can note, in this connection, that styrene oxide undergoes polymerisation in the presence of aluminium isopropoxide which involves a selective cleavage of the Ca-0[CH(Ph)—O] bond [47]. On the other hand, polymerisation of styrene oxide with the diethylzinc-water catalyst was found to proceed via 0,8-0(0142—0) bond scission [48],... 

Sol-gel chemistry offers flexible methods for the preparation of porous metal oxides such as the transition aluminas used as catalyst supports. The physical properties of sol-gel materials depend on the nature of the reactants, the rate of mixing and the conditions of drying. High surface area aluminas have been prepared from various alkoxide and salt solutions and their textures have been examined extensively. The interest in the use of these chemically prepared materials is largely for catalyst and absorbent applications [1]. The first user of alkoxide precursor was Teichner who prepared pure aluminas by the water vapour action on aluminimn methoxide [2] and reported materials with surface area of 200 m /g. Harris and Sing [3] reported gels prepared from aluminium isopropoxide with water and... 

Oppenauer oxidation, using alkoxides other than aluminium, operates via a hydride transfer mechanism similar to the one depicted in the above Equation, although a complexation of the metal with the carbonyl group may not be present.22d Evidence for a radical mechanism was put forward in the case of the interaction between lithium isopropoxide and benzophenone.24... 

Rehydrated Mg/Al-OH catalysts have been found to be inactive in the MPV reduction of 4-ferf-butylcyclohexanone by isopropanol, whereas the bifunctional character of the Mg(Al)0 mixed oxides made them highly active in this reaction. The aluminium alkoxidc intermediate of the MPV reaction indeed involves a cooperation between basic and acidic sites. On mixed oxides, the abstraction of a proton from isopropanol on O2 sites gives isopropoxide anions, which are then stabilized on Al3+ and form intermediates with the aldehyde. The high activity of mixed oxide comes from the synergetic effect of strong Lewis basicity and mild acidity. 

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