Alkoxides, aluminum

First investigations of hydrolysis of aluminum alkoxides were performed in the 1950s and 1960s [1252, 285]. Stolarek [1536] studied hydrolysis reactions with the aim of the preparation of aluminum oxide as a carrier for catalysts. 

The investigations on the preparation and properties ofalkoxy-derived sols and gels of A1203 performed by Yoldas [1793], along with the numerous works on hydrolysis of Si(OR)4, laid the foundation of the sol-gel method as the technique employing the process of continuous transformation from colloid (sol) to gel. The structures of sols and gels prepared in accordance with the procedures described by Yoldas were in detail characterized by means of NMR, 

Aluminum oxide sols are successfully used for application of films [1794, 1793], such as high area films for hybrid circuits [1236]. The same technique was used for the preparation of J-A1203 (NaAlnO,7) in this case alcoholic solutions of NaOR were added to aluminum alkoxide solutions [1795, 1038]. To increase the stability of sols, aluminum alkoxides are frequently modified with acetic acid [1570] or (3-diketones [1659]. Alumina sheets are commercially produced by the sintering of high-purity fine A1203 powders prepared by hydrolysis of modified Al(OPr )3 [1236]. To prepare powders with spherical species from dilute solutions, hydroxopropylcellulose was added to prevent 

Akamanchi and co-workers reported that aldehydes and ketones can be reduced very rapidly in good yield at room temperature by addition of trifluoroacetic acid (TFA) to Al(OPr )3 as shown in Sch. 6 [27], 

This modified reagent can be used in catalytic amounts and very efficiently catalyzes hydride transfer from isopropanol. In the MPV reduction of m-nitrobenzalde-hyde (7, R = m-N02-CgH4, = H), it was revealed that Al(OPr )3 and TFA in as 

Akamanchi also reported that reaction of Al(OPr )3 with TFA in CH2CI2 produced a white solid that is stable when stored under dry conditions. Using this new off-the-shelf reducing agent, various aldehydes and ketones were reduced to the corresponding alcohols in moderate to good yields at room temperature in a short time [28]. 

Maruoka and co-workers reported a conceptually new MPV reduction system based on bidentate Lewis-acid chemistry [29]. The initial formation of bidentate aluminum catalyst 9 derived from (2,7-dimethyl-l,8-biphenylenedioxy)bis(dimethylalu-minum) (8 prepared from 2,7-dimethyl-l,8-biphenylenediol and 2 equiv. MesAl) and i-PrOH (4 equiv.), followed by treatment of benzaldehyde with the in situ generated (2,7-dimethyl-l,8-biphenylenedioxy)bis(diisopropoxyaluminum) (9) at room temperature instantaneously produced the reduced benzyl alcohol almost quantitatively (Table 2, entry 2). Even with 5 mol% catalyst 9 the reduction proceeds quite smoothly at room temperature to furnish benzyl alcohol in 81 % yield after 1 h (Table 2, entry 3). This remarkable efficiency can be ascribed to the double electrophilic activation of carbonyls by the bidentate aluminum catalyst (Sch. 7). 

Entry Substrate A1 reagent Hydride source Conditions Yield (%)  

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The widely used Moifatt-Pfltzner oxidation works with in situ formed adducts of dimethyl sulfoxide with dehydrating agents, e.g. DCC, AcjO, SO], P4O10, CCXTl] (K.E, Pfitzner, 1965 A.H. Fenselau, 1966 K.T. Joseph, 1967 J.G. Moffatt, 1971 D. Martin, 1971) or oxalyl dichloride (Swem oxidation M. Nakatsuka, 1990). A classical procedure is the Oppenauer oxidation with ketones and aluminum alkoxide catalysts (C. Djerassi, 1951 H. Lehmann, 1975). All of these reagents also oxidize secondary alcohols to ketones but do not attack C = C double bonds or activated C —H bonds. 

Hydrolysis. Aluminum alkoxides are hydrolysed using either water or sulfuric acid, usually at around 100°C. In addition to the alcohol product, neutral hydrolysis gives high quaUty alumina (see Aluminum compounds) the sulfuric acid hydrolysis yields alum. The cmde alcohols are washed and then fractionated. 

Manufacture. Hydroxypivalyl hydroxypivalate may be produced by the esterification of hydroxypivaUc acid with neopentyl glycol or by the intermolecular oxidation—reduction (Tishchenko reaction) of hydroxypivaldehyde using an aluminum alkoxide catalyst (100,101). 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

Alkaline earth metal alkoxides decompose to carbonates, olefins, hydrogen, and methane calcium alkoxides give ketones (65). For aluminum alkoxides, thermal stability decreases as follows primary > secondary > tertiary the respective decomposition temperatures are ca 320°C, 250°C, and 140°C. Decomposition products are ethers, alcohols, and olefins. 

This reaction is important in the manufacture of long-chain alcohols by means of hydrolysis of the aluminum alkoxide. Examples of oxidation of metal alkoxides (40,42) include ... 

Aluminum alkoxides are easily soluble in hydrocarbons and in chlorinated hydrocarbons, but sparingly soluble in alcohols. They are sensitive to moisture and dry storage is essential. Aluminum alkoxides are used extensively as intermediates, for example, in the Meerwein-Poimdorf reaction (94). 

The class of compounds identified as basic aluminum chlorides [1327-41 -9] is used primarily ia deoderant, antiperspirant, and fungicidal preparations. They have the formula Al2(OH)g where x = 1 5, and are prepared by the reaction of an excess of aluminum with 5—15% hydrochloric acid at a temperature of 67—97°C (18). The same compounds are obtained by hydro1y2ing aluminum alkoxides with hydrochloric acid (19,20) (see Alkoxides, METAl). Basic aluminum chloride has also been prepared by the reaction of an equivalent or less of hydrochloric acid with aluminum hydroxide at 117—980 kPa (17—143 psi) (20). 

Hydrolysis of aluminum alkoxides is also used commercially to produce precursor gels. This approach avoids the introduction of undesirable anions or cations so that the need for extensive washing is reduced. Although gels having surface area over 800 m /g can be produced by this approach, the commercial products are mosdy pseudoboehmite powders in the 200 —300 m /g range (28). The forming processes already described are used to convert these powders into activated alumina shapes. 

Tlie amphoteric behavior of aluminum hydroxide, wliich dissolves readily in strong acids and bases, is shown in Figure 4. In the pH range of 4 to 9, a small change in pH towards the neutral value causes rapid and voluminous precipitation of colloidal hydroxide wliich readily fomis a gel. Gels are also fomied by the hydrolysis of organoaluminum compounds such as aluminum alkoxides (see Alkoxides, metal). 

The effects of both pH and temperature of aluminum alkoxide hydrolysis on gelation is shown in Eigure 8. Addition of acid into the mixture hydrolyzed at 90°C, and by consequence reduction of pH, reduces the gelation time of the samples, whereas in mixtures hydrolyzed at room temperature, acidic addition increases gelation time. 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Sulfuric acid is also a very satisfactory catalyst aluminum alkoxides also are useful, especially when the alcohols would be adversely affected by strong acids. Sodium alkoxides produce undesirable side reactions and give lower yields. When alkaline catalysts are employed, an alkaline polymerization inhibitor, such as j j-phenylenediamine or phenyl-d-naphthylamine, should be used instead of hydroquinone. 

The oxidation of a hydroxyl group by an aluminum alkoxide-catalyzed hydrogen exchange with a receptor carbonyl compound is known as the Oppenauer oxidation. For oxidation of steroidal alcohols the reaction is generally... 

The reduction of ketones to secondary alcohols and of aldehydes to primary alcohols using aluminum alkoxides is called the Meerw>ein-Ponndorf-Verley reduction. The reverse reaction also is of synthetic value, and is called the Oppenauer oxidation. ... 

The aldehyde or ketone, when treated with aluminum triisopropoxide in isopropanol as solvent, reacts via a six-membered cyclic transition state 4. The aluminum center of the Lewis-acidic reagent coordinates to the carbonyl oxygen, enhancing the polar character of the carbonyl group, and thus facilitating the hydride transfer from the isopropyl group to the carbonyl carbon center. The intermediate mixed aluminum alkoxide 5 presumably reacts with the solvent isopropanol to yield the product alcohol 3 and regenerated aluminum triisopropoxide 2 the latter thus acts as a catalyst in the overall process ... 

In preparing the membrane, a clear sol was obtained by the addition of acid into the aluminum sec-butoxide sol to peptise the sol and obtain a stable colloid solution. Aluminum monohydroxides formed by the hydrolysis of aluminum alkoxides, which are peptisable to a clear sol. Peptisation was performed by the addition of acid and heat treatment for a sufficient time. It was found that stable sols cannot be obtained when the concentration of the peptisation acid is too low. The critical range for inorganic acids such as nitric, hydrochloric and perchloric acids is 0.03-0.1 mole/mole of hydroxide. In this study, nitric acid was used as the peptising agent. The resulting sols are poured into Petri dishes and dried in an oven at a controlled drying rate to obtain a gel layer. The molar ratio of zirconia salt... 

The products are liberated by hydrolysis of the aluminum alkoxide at the end of the reaction. Lithium aluminum hydride reduction of esters to alcohols involves an elimination step in addition to hydride transfers. 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Anionically prepared hydroxy-terminated PBd was reacted with AlEt3 to form the corresponding aluminum alkoxide macroinitiator, capable of initiating the polymerization of L-lactide [117]. Using ratios [PBd-OH]/[AlEt3] between 1 and 6, reaction temperatures between 70 and 120 °C and maintaining the conversion of the lactide polymerization below 90%, products with narrow molecular weight distribution were obtained. 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

Aluminum(III) complexes are amongst the most common Lewis acids. In particular, aluminum halide species (e.g., A1C13, AlBr3) are commercially available and are widely used for various reactions. Other types of Lewis acid such as aluminum alkoxides, alkylaluminum halides, and trialkylaluminum species are also used for many kinds of Lewis-acid-mediated reactions. 

We recently reported a modified Meerwein-Ponndorf-Verley reduction in which low-boiling alcohols such as EtOH and w-PrOH, but preferably i-PrOH, were used at temperatures near 225 °C in the absence of aluminum alkoxides [42]. The carbonyl moiety of an olefinic aldehyde such as cinnamaldehyde was reduced selectively to the alcohol without the carbon-carbon double bond being affected (Scheme 2.7). Since base was not present, aldol and Claisen-Schmidt condensations were avoided. 

Reduction of Carbonyl Compounds with Aluminum Alkoxides... 

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