Aluminium alkoxide derivatives

The product is the racemic [(R)/(S)] alcohol since the free energies of the two diastereoisomeric transition states, resulting from hydride attack on the si-face of the ketone as shown (order of priorities O > R1 > R2, p. 16) or the re-face, are identical. The use of an aluminium alkoxide, derived from an optically pure secondary alcohol, to effect a stereoselective reaction (albeit in low ee%) was one of the first instances of an asymmetric reduction.48 Here (S)-( + )-butan-2-ol, in the form of the aluminium alkoxide, with 6-methylheptan-2-one was shown to give rise to two diastereoisomeric transition states [(5), (R,S) and (6), (S,S)] which lead to an excess of (S)-6-methylheptan-2-ol [derived from transition state (6)], as expected from a consideration of the relative steric interactions. Transition state (5) has a less favourable Me—Me and Et—Hex interaction and hence a higher free energy of activation it therefore represents the less favourable reaction pathway (see p. 15). 

Recently work has been done on the introduction of a new group of through driers, the aluminium alkoxide derivatives. These compounds may have the general formula... 

The use of lead salts ( through driers) is being abandoned in many countries on toxicity grounds. A new group of through driers are now being introduced which are aluminium alkoxide derivatives. 

Alkaline hydrolysis, 282 Alkanes, 152,324 Alkanediols and their esters, 143 Alk-1-enyldiacylglycerols, 134 Alkoxyhydroperoxide, 463 Alkyd paints, 245 Alkyldiacylglycerols, 134 Alkylglycerols, MS, 436 Alkylsulphates, 36 Allenic acids, 13, 53 Allylic bromination, 466 Almond oil, 3,51,97 Aluminium alkoxide derivatives, 246 Aluminium oxide, chromatography,... 

As a method of avoiding the damage caused by aqueous solutions, an attempt was made to find an aluminium compound which was both soluble in a non-polar solvent and had significant tanning powers. A wide range of compounds was screened and one, an aluminium isopropoxyde derivative chelated with ethyl acetoacetate called, for simplicity, aluminium alkoxide, was found to have the optimum combination of properties. This is used as follows. 

The mechanism of reduction of molecular nitrogen at titanium catalysts continues to be a subject of much research. The inherent complications of these systems, which consist of reagents, catalyst, and co-catalysts, result in a dearth of precise kinetic results, but nonetheless informed speculation on the basis of available kinetic, product, and stoicheiometric knowledge provides useful information about the mechanisms. These have been reviewed in general discussions, while mechanisms have been postulated for specific systems involving catalysis by titanium tetrachloride-aluminium tribromide derivatives, electrolysis in a 1,2-dimethoxyethane solution containing titanium tetraisopropoxide, aluminium trisisopro-poxide, and naphthalene, and catalysis by titanium alkoxide-sodium naphthalenide mixtures. In the last case the role of titanium(n) in the... 

A veiy elegant reaction is the synthesis of 1-hydrosilatrane and its C-substituted derivatives via readily available boratranes. Aprotic conditions exclude side processes involving the Si—H bond, so that the desired silatranes are formed in high yields (60-95%) . For example, triethoxysilane reacts with 4-ethylboratrane in xylene in the presence of magnesium propoxide to give l-hydro-4-ethylsilatrane (92%) (equation 40) . Aluminium alkoxides or aluminum chloride are also used as catalysts. The reaction rate decreases sharply in the absence of catalysts. Attempts to prepare l-(2 -chloroethyl)silatrane by this mild method failed . 

For these and similar reactions recently a variety of Lewis acidic aluminium, rare earth metals, and titanium alkoxides have been applied. Alkoxides have the additional advantage that they can be made as enantiomers using asymmetric alcohols which opens the possibility of asymmetric catalysis. Examples of asymmetric alcohols are bis-naphtols, menthol, tartaric acid derivatives [28], Other reactions comprise activation of aldehydes towards a large number of nucleophiles, addition of nucleophiles to enones, ketones, etc. 

The derivatives of the other new class—the alkoxide halides, MCORj HaU which are more reactive than the alkoxides themselves—were first described by Meerwein and Schmidt for magnesium and aluminium [1103, 1098]. An important input into the development of synthetic and physicochemical methods in the investigation of metal alkoxides was made by an outstanding meta-lorganic chemist H. Gilman [855, 856], who studied the derivatives of uranium in Los Alamos during the Manhattan Project. 

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. 

Thermolysis of tin and lead alkoxozirconates leads to the formation of metals. The mass-spectral data indicate the presence ofbarium and aluminium derivatives in the gas phase, but no preparative data are accessible for them. The major application of zirconium and hafnium alkoxides lies now in the sol-gel technology of zirconate-titanate and solid solutions Zr02-Y203 (see Section 10.3), Except in the synthesis of oxide materials, the alkoxides of zirconium and hafnium are traditionally used in the polymer chemistry, where they are applied as the components in catalysts [1278, 1269] and as additives to polymers, improving their characteristics [825, 1403] and so on. Already in 1930s Meerwein has proposed the use of zirconium alkoxides for the reduction of aldehydes intoprimary alcohols (Meerwein-Schmidt reaction) [1420],... 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

One consequence of LiAlH4 being very reactive, is that it is rather unselective. In order to improve its selectivity, various derivatives have been made that are less reactive. These derivatives usually have three of the hydride ions replaced by alkoxide ions, i.e. A1H(0R)3. We saw earlier that some mixed alkoxide/hydride aluminium anions disproportionate to form the tetra-alkoxide... 

Despite the fact that the metal ions enable to control the polymerisation, they also pose an important downside as the formed polymers will always contain metal residues. These metal ions might have detrimental health effects. Aluminium derivatives have already been linked to Alzheimer s disease, although this hypothesis is controversial [50], while tin octoate, despite being FDA approved, has already been shown to be cytotoxic. As tin levels often exceed 1000 ppm, the FDA has set a limit of 20 ppm for polyesters used for biomedical applications [51]. To circumvent this problem, attention has already been paid to less toxic catalysts including magnesium and calcium alkoxides. Conversely, attempts are being made to remove the catalyst residues from the polymer. 

The fluoride [Me AlF] is tetrameric in the crystal. It is isoelectronic with [Me SiO] (p. 113) and has a very similar structure. The eight-membered ring is pucWed and the bond angles / AlFAl large (146°). Four coordinate aluminium is also present in alkoxides and amino- derivatives which result from partial reaction of R3AI with alcohols or amines (p. 82 ). Cubane type tetramers e.g. (PhAlNPh) are formed when triphenylaluminium is heated with an arylamine. 

Barraclough et studied the infrared specda of a number of metal (aluminium, titanium, zirconium, hafnium, niobium, and tantalum) alkoxides and tentatively assigned the v(C-0)M and v(M-O) bands in these derivatives. It has been observed... 

In addition to the formation of alkali alkoxometallates by the reactions of alkali alkoxides (strong bases) with alkoxides of a variety of metals and metalloids (Lewis acids), formation of heterometal alkoxides has been shown to occur even between alkoxides of such similar metals as aluminium and gaUium as weU as niobium and tantalum. However, the formation constant of the latter derivative has been found to be statistical, which precludes the isolation of this bimetallic alkoxide in view of the equilibrium ... 

The first type of transformations is characteristic of both homo- and heterometallic derivatives and leads to formation of stable fully substituted complexes with chelating ligands from the mixed-ligand derivatives. It has been first observed and described in detail by Wengrovius (1986) for the alkoxide )8-diketonate derivatives of aluminium and then was studied also by Errington (1998) for the same type of titanium derivatives ... 

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