Aluminium reductive stabilization

The mechanism of formation of Pt particles by the or-ganometallic reduction route, however, was found to proceed differently, for example in the reductive stabilization of Pt nanoparticles produced by reacting Pt-acetylacetonate with excess trimethylaluminium. Here, derivates of aluminium alkyls act as both reducing agents and colloidal stabilizers. As was shown by a combination... 

Reductive Stabilization of Metal Colloids by Aluminium Alkyls... 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

The tlrree impurities, iron, silicon and aluminium are present in the metal produced by the Kroll reduction of zirconium tetrachloride by magnesium to the extent of about 1100 ppm. After dre iodide refining process tire levels of these impurities are 350, 130 aird 700ppm respectively. The relative stabilities of the iodides of these metals compared to that of zirconium can be calculated from the exchange reactions... 

Solid lithium aluminium hydride can be solublized in non-polar organic solvents with benzyltriethylammonium chloride. Initially, the catalytic effect of the lithium cation in the reduction of carbonyl compounds was emphasized [l-3], but this has since been refuted. A more recent evaluation of the use of quaternary ammonium aluminium hydrides shows that the purity of the lithium aluminium hydride and the dryness of the solvent are critical, but it has also been noted that trace amounts of water in the solid liquid system are beneficial to the reaction [4]. The quaternary ammonium aluminium hydrides have greater hydrolytic stability than the lithium salt the tetramethylammonium aluminium hydride is hydrolysed slowly in dilute aqueous acid and more lipophilic ammonium salts are more stable [4, 5]. 

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. 

Middleton established that reaction of 1,2-dialkylhydrazines with sulfur and reduction of sulfinylhydrazines (R2N—N=S=0) with lithium aluminium hydride yielded highly colored N-thionitrosoamines (95), which are stable at < ca. -30°C (66JA3842). However, it was deduced from spectroscopic data that these were not true RN=S species a high contribution from the dipolar resonance form accounts for their stability. Nonetheless, compound 95 will act as a 277 component in an inverse electron demand Diels-Alder reaction with a tetrazine derivative to yield triazole 96 (79CZ230). 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

Recently Creyghton et al. [6,7] reported the use of zeolite beta in the MPVO reduction of 4-t-butylcyclohexanone. ITie high selectivity towards the thermodynamically less favoured ds-alcohol is explained by a restricted transition-state around a Lewis-acidic aluminium in the zeolite pores. When using an aluminium-free zeolite, titanium beta, in the epoxidation of olefins, we have shown that Ti-beta has acidic properties when alcoholic solvents were employed [8], This was ascribed to the Lewis-acidic character of titanium in the zeolite framework. As we reported very recently [9], Ti-beta is found to be an excellent catalyst in MPVO reactions with a tolerance for water. Here, results are presented on the high selectivity, stability and low by-product formation of the catalyst, Ti-beta, in both the liquid-phase and gas-phase MPVO reactions. 

Unfortunately, investigations with ionic liquids containing high amounts of AlEtCl2 showed several limitations, including the reductive effect of the alkylaluminium affecting the temperature stability of the nickel catalyst. At very high alkyl-aluminium concentrations, precipitation of black metallic nickel was observed even at room temperature. 

The stability of water with respect to oxidation to O2, and reduction to H2, limits the range of electrode potentials and the oxidation states possible in any system containing water. These limits are the dashed lines in Table 4.2. Dissolved F2 and CI2 gases are unstable because of their high electrode potentials. The reduced states F and Cl- are the stable states in water because they have accepted an electron and thereby discharged their oxidizing power. Metals are likewise unstable in water and soils because of their tendency to oxidize, to donate electrons. Then oxidized states (Al3+, Ca2+, K h, etc.) are stable in water. Table 4.2 implies correctly that the common metals are unstable and will corrode. Exceptions such as aluminium and zinc metals are metastable an oxide layer that forms initially on their surfaces inhibits further oxidation. Soils and seawater catalyze the breakdown of these protective layers and speed up their corrosion (oxidation). Iron and steel do not form this protective layer. 

In the niobium containing alloy which shows a better oxidation resistance the doping of titania with niobium may reduce the dissolution of AlON. By this means a thin layer AlON is formed at the interface leading to a reduced oxidation rate. Thus it is assumed that the oxidation behaviour of titanium aluminides could be improved by stabilizing the aluminium oxide at the metal/oxide interface either by prevention of aluminium depletion of the metal subsurface zone or by reduction of A1203 dissolution in Ti02. 

Diisobutylaluminium hydride (DIBAL-H or DIBAL, BU2AIH) is a very useful derivative of aluminium hydride and is available commercially as a solution in a variety of solvents. At ordinary temperatures, esters and ketones are reduced to alcohols, nitriles give amines and epoxides are cleaved to alcohols. However, it is particularly useful for the preparation of aldehydes. At low temperatures, esters and lactones are reduced directly to aldehydes (or lactols) nitriles and carboxylic amides give imines which are readily converted into the aldehydes by hydrolysis (7.79-7.81). The lack of further reduction of the aldehyde lies in the relative stability of the intermediate hemiacetal (or imine salt), which hydrolyses to the aldehyde only upon work-up. 

Photoisomerization of tranj-13-demethylretinal (115) gave a mixture of C-7, C-9, C-11, and C-13 stereoisomers which were separated by h.p.I.c. and identified by 400 MHz n.m.r. The photochemistry of (115) and of 14-methylretinal (116) in polar and non-polar solvents has been examined. Conditions for the photoisomerization of methyl retinoate (117) to various ci5-isomers have been studied.Photoisomerization studies of the all-tran -isomers of 3,4-dehydroretinal (118), 10-fluororetinal (119), and 14-fluororetinal (120) suggest Zwitterion intermediates, e.g. (121), the stability of which is influenced by the fluorine substituent or the extra double bond. Conditions have been optimized for reduction of the retinal-hydroquinone complex to retinol by aluminium isopropylate. The electrodimerization of retinal in the presence of electron donors has been studied. ... 

The reductive dissolution of the outer y-FejOj layer exposes the inner magnetite layer of the oxide him. In acid solutions (pH less than 4) the magnetite layer rapidly dissolves but in near neutral solution it may be stable and protective, depending on the nature of the anion present and its concentration The magnetite layer is stable in inhibitive solutions of anions, e.g. benzoate , carbonate ", hydroxide ", borate (though not bicarbonate ). The stability of the magnetite layer controls the inhibition of corrosion of iron when coupled to electronegative metals such as aluminium, zinc or cadmium . 

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