Aluminium alkoxides

Al(OR)3 were first prepared by Gladston and Tribe in 1876 on the action of metal on anhydrous alcohols in presence of I2, HI or A1I3. Wislizenus and Kaufman, studying the reductive properties of the aluminium amalgam, proposed useing it for obtaining water-free alcohols. They found, however, that after all water has reacted, the alcohols themselves start to react with aluminium. In 1897 Hillyer and Crooker reported the preparation of Al(OR)3 on the action of alcohols on the metal in the presence of Al, Sn, Pt, and Fe chlorides [1301]. 

The classic study of Al(OR)3 was carried out by Tishchenko [1585] at the end of nineteenth century. His dissertation, entitled On the Action of Amalgamated Aluminum on Alcohols (Aluminum Alkoxides, Their Properties and Reactions), became a great resonance for the chemistry of alkoxides (see also Chapter 1). The synthetic approaches, that he developed are still in use the purity of the samples obtained by Tishchenko, taking into account the data he reported, was definitely not worse than that of those described at present. In 1929 Meerwein and Bersin [1101], performing the acidimetric titration of Al(OR)3 solutions by solutions of alkali alkoxides, discovered the existence of bimetallic alkoxides (Meerwein salts), which play an important role in the modern chemistry of metal alkoxides. 

The most widespread synthetic approach to Al(OR)3 remains even at present the reaction of the aluminium amalgam with anhydrous alcohols proposed by Tishchenko and then described in detail by Adkins [10]. The most attractive 

The derivatives of lower alcohols and also Al(OR )3, Al(OC5HnI,e°)3 are solids derivatives of other amyloxides and hexyloxides are liquids substituted phenoxides are crystalline matters. The oxocomplexes display a much clearer trend to crystallization than the homoleptic alkoxides. Nearly all the derivatives of aluminium can be sublimed or distilled without decomposition. 

Numerous investigations have noted the instability of the physicochemical characteristics of Al(OR)3—the first members of the homologous series, which can display varied physical states, m.p., density, vapor pressure (hysteresis on measurement), viscosity, refraction coefficient, solubility in alcohols, and so 

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It is interesting to note that under the influence of aluminium alkoxides (in alcohol or, better, in benzene solution) aldehydes produce the ester (Tischenko reaction) ... 

Aldehydes and ketones can be reduced smoothly to the corresponding alcohols by aluminium alkoxides. The most satisfactory alkoxlde for general use Is aluminium tsopropoxide ... 

Acetone in conjunction with benzene as a solvent is widely employed. With cyclohexanone as the hydrogen acceptor, coupled with toluene or xylene as solvent, the use of higher reaction temperatures is possible and consequently the reaction time is considerably reduced furthermore, the excess of cyclohexanone can be easily separated from the reaction product by steam distillation. At least 0 25 mol of alkoxide per mol of alcohol is used however, since an excess of alkoxide has no detrimental effect 1 to 3 mols of aluminium alkoxide is recommended, particularly as water, either present in the reagents or formed during secondary reactions, will remove an equivalent quantity of the reagent. In the oxidation of steroids 50-200 mols of acetone or 10-20 mols of cyclohexanone are generally employed. 

With weak catalysts such as aluminium alkoxides with C0C13, cobalt acetyl acetonate, etc. polymerization was exceedingly slow. 

Butanol, used as a solvent in an autoclave preparation at around 100°C, severely attacked the aluminium gasket, liberating hydrogen which caused a sharp rise in pressure. Other alcohols would behave similarly, forming the aluminium alkoxide. 

The asymmetric reduction of prochiral ketones to their corresponding enantiomerically enriched alcohols is one of the most important molecular transformations in synthetic chemistry (20,21). The products are versatile intermediates for the synthesis of pharmaceuticals, biologically active compounds and fine chemicals (22,23). The racemic reversible reduction of carbonyls to carbinols with superstoichiometric amounts of aluminium alkoxides in alcohols was independently discovered by Meerwein, Ponndorf and Verley (MPV) in 1925 (21—26). Only in the early 1990s, first successful versions of catalytic... 

Finally, /i-hydrogen transfer is the key step in the Meerwein-Pondorf-Verley (MPV) reduction of ketones by alcohols, catalyzed by aluminium alkoxides and many other catalysts. In that case, competition is not an issue, since polymerization is usually not thermodynamically favourable. The accepted mechanism for this reaction is direct transfer of the hydride from alkoxide to ketone. 

Aluminium Alkoxides and Ketones Meerwein-Pondorf-Verley Reduction... 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

The procedure described in this experiment exemplifies a general method [225] for the reduction of propargylic alcohols to -allylic alcohols. The first step in the reaction is the formation of the aluminium alkoxide -C=C-C-OAlH3. Subsequently one of the three hydrogen atoms attached to aluminum is transferred to the triple bond with formation of a 5-membered cyclic aluminum compound. Hydrolysis affords the -allylic alcohol. In the present case an -enyne alcohol is formed. 

More recently Teyssie determined the rate constants in the polymerization of e-caprolactone (eCL) initiated with aluminium alkoxides, believing that the covalent species are the only ones responsible for propagation [4]. For the same monomer Yamashita estimated tentatively rate coefficients of propagation using an anionic initiator [ ]. Lenz in his studies of substituted g-propiolactones (gPL) observed peculiar influence of structure on reactivity that can have its origin in the multiplicity of ionic structures involved [fi]. 

A new type of approach to thiocrown ether synthesis involving a [2 + 2] coupling of a>-dithiol and w-dihalide units in the presence of a boron aluminium alkoxide couple has been described [27]. 

In this way, an aldehyde or ketone could be reduced to the corresponding alcohol after hydrolysis of the resulting aluminium alkoxide. This reaction is known as the Meerwein-Ponndorf-Verley reduction. 

The so-called Oppenauer oxidation proved to be extremely successful in the oxidation of sterols. On the other hand, its application—in the original formulation—to the obtention of ketones outside the field of steroids and to the preparation of aldehydes met a more limited success because of less favourable thermodynamics and side reactions, induced by the basic character of the aluminium alkoxides. 

Normally, Oppenauer oxidations are performed employing Al3+ cations as catalyst because aluminium alkoxides possess a good balance of a desired high hydride transfer capability versus a low propensity to promote undesired base-induced reactions, like aldol condensations and Tischtschenko reactions. In the reaction, as originally described by Oppenauer, aluminium t-butoxide is used as catalyst,4 because its high basicity allows a very favourable equilibrium towards the formation of the aluminium alkoxide of the alcohol whose oxidation is desired. However,... 

The available experimental data supports a mechanism for the Oppenauer oxidation, involving an initial complexation of a carbonyl group with the aluminium from an aluminium alkoxide, followed by a rate-determining hydride transfer via a six-membered transition state.22... 

The Oppenauer oxidation presents two important limitations on one side it is unable to oxidize certain alcohols because of unfavourable thermodynamics, and on the other side, base-induced reactions between the oxidant and the product may become dominant. That is why, it is seldom employed for the obtention of aldehydes because these compounds react readily under basic conditions. On the other hand, although aluminium alkoxides promote aldol condensations, many base-sensitive functional groups such as most esters but not formates—25 resist its action. 

It must be mentioned that about 1 equivalent of aluminium isoprop-oxide is needed in Oppeanuer oxidations using the classical protocol. Supposedly, compound 67 reacts with the alcohol, resulting in an aluminium alkoxide able to form a complex in which both free electron pairs of the oxygen atom in pivalaldehyde are coordinated with aluminium atoms, resulting in a very efficient activation of pivalaldehyde as hydride acceptor via a mechanism represented in Figure 6.1 ... 

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