Aluminium phenoxide

Very recently, Maruoka s team developed two highly sophisticated and efficient aluminium compounds for the Oppenauer oxidation of alcohols. Thus, the complex aluminium phenoxide 67, containing two aluminium atoms, is able to catalyze in a quantity as low as 5 mol%—the oxidation of alcohols with pivalaldehyde at room temperature.37... 

The diastereoselectivity of reduction of 2-substituted cyclohexanones with 4-substituted aluminium phenoxides has been investigated over a wide temperature range (—75 to +80 °C).276 Hydride transfer dominates at high temperature whereas an MPV-type reaction contributes at lower temperatures. 

Phenol at 85° was treated with paraformaldehyde and after /ih, following the addition of powdered anhydrous aluminium phenoxide, reaction was continued for 3h. to afford salicyl alcohol. 

A complication in the preparation of compounds having sec- and tert-alkyl groups in the 2- and in the 4-positions is their vulnerability to thermal dealkylation with acidic catalysts, particularly with 1% oleum in the range 160-200°C, and, as has been mentioned, to aluminium phenoxide when this is used at high temperature (ref.12). Indeed above 250°C all sec- and tert- groups are eliminated from alkylated phenols. The role of thermal dealkylation has been mentioned for the preparation of nonylphenol from dinonylphenol. In the case of 5-methyl-2,4-di-tert-butylphenol and 4-methyl-2,6-di-tert-butylphenol resulting from the tert-butylation of 3-methyl and 4-methylphenol mixtures, the former... 

Compounds such as [Al(R)(OC6H3Bu2-2,6)2] will carry out the polymerization of methyl and ethyl methacrylate when activated with t-BuLi. The nature of the alkyl group was found to affect the tacticity of the polymer. The tetraphenylporphinato aluminium phenoxide has been shown to be active for the ring opening polymerization of epoxides and lactones. The reaction is accelerated by the presence of reagents such as MAD. An adduct [Al(OC6H2Bu2-2,6-Me-4)Me2(methyhnethacrylate)] relevant to this reactivity has been isolated and structurally characterized. ... 

Yamamoto and coworkers followed a promising approach, developing a rich chemistry with aluminium phenoxides prepared from sterically hindered 2,6-substituted phenols. The reaction of hullqr phenols with trimethyl aluminium led to methylaluminium bis(2,6-di-tert-bulyl-4-methylphenoxide) (MAD) and aluminium tris(2,6-diphenylphenoxide) (ATPH), as shown in Scheme 18.8, which are as expected monomeric in organic solvents. 

Related mononuclear aluminium monophenoxides with a morpholine instead of a benzotriazole substituent were investigated by the group of Kerton, as shown in Scheme 18.48. With PPNCl as cocatalyst and styrene oxide as test substrate, the aluminium phenoxide produced styrene carbonate in 37% yield (24 h, 60 °C, 40 bar carbon dioxide, 0.2 mol% catalyst/ cocatalyst). 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

With j -branched aluminium alkyls like Al(i-Bu)3, reduction is often the main reaction [52]. Ligand modification can be used to increase the amount of reduction still further, and also to control the diastereoselectivity. In this respect, the phenoxide-modified complex 7 appears to be particularly effective [53]. A recurring problem in diastereoselective reductions is that the product can epimerize through MPV reduction (see next section) of the starting material [54]. The kinetics of this complicated system have been analyzed in terms of the iso-inversion principle [55]. 

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. 

A series of 2-substituted cyclohexanones was studied over a wide range of temperature in an attempt to optimize the diastereoselectivity of diisobutylaluminium phenoxides in the reduction of ketones.161 Hydride transfer dominates at high temperature, but a Meerwein-Ponndorf-Verley-type interconversion of the aluminium alcoholate intermediates (via the reactant ketone) is an important factor in diastereoselection at low temperature. 

In addition, Sawamoto et al. [137, 140] detail the radical living polymerization of MMA with an initiating system that consists of CC14, RuCl2(PPh3)3 and methyl aluminium bis (2,6-ditertbutyl phenoxide) (MAO-type) with a constant polydispersity index vs conversion rate (Mw/Mn = 1.3). According to the authors, the polymerization is considered to proceed via repeating radical... 

Athar T, Bohra R, Mehrotra RC. Synthesis and characterization of some mixed steri-cally crowded phenoxides-isopropoxides derivatives of aluminium (III), [A1 (OPr )3 (OAr)J. Ind J Chem 1989 28(6) 492-6. 

Aluminium and Thallium.- Two new reagents, methylaluminium bis(2,6-di - t-butyl-4-phenoxide) (MAD) (117), and methylaluminium bis(2,4,6-tri-t-butyl-4-phenoxide) (MAT) (118), have been prepared... 

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