Asymmetric a-alumina

For a composite membrane that is relatively free of pinholes, there is apparently a limiting palladium thickness that depends primarily on support quaUty and somewhat on deposition method. For example, unmodified PSS requires at least 10 pm of palladium to bridge a majority of the pores [149, 227]. Invariably, a few macrodefects remain that preclude the attainment of perfect hydrogen permselectivity. On porous asymmetric a-alumina supports with 100-200 nm pore size on the surface, approximately 7 pm appears to be the limiting thickness that is proba-... 

The research group coordinated by Professor Gabriele Centi prepared a series of tubular catalytic membranes (TCM) and tested in the direct synthesis of H202. " Such TCMs are asymmetric a-alumina mesoporous... 

Silica gel [11] or alumina [11a, 12] alone, or silica and alumina together modified by Lewis-acid treatment [13] and zeolites [14], have been widely used as catalysts in Diels-Alder reactions, and these solids have also been tested as catalysts in asymmetric Diels-Alder reactions [12,13b,14]. Activated silica gel and alumina at 140 °C were used [15] to catalyze the asymmetric cycloaddition of (-)-menthyl-N-acetyl-a, S-dehydroalaninate (3) (R = NHCOMe) with cyclopentadiene in the key step for synthesizing optically active cycloaliphatic a-amino acids. When the reactions were carried out in the absence of solvent, a higher conversion was obtained. Some results are reported in Table 4.5 and compared with those obtained by using silica and alumina modified by treatment with Lewis acids. Silica gel gives a reasonable percentage of conversion after 24 h with complete diastereofacial selectivity in exo addition. 

Fig. 1. P MAS NMR spectrum of (a)Ru-BrNAP/PTA/y-Al203, and (b)Ru-BINAP crt rlex In order to find the characteristics of the immobilized catalyst, asymmetric hydrogenation of the prochiral C=C bond was performed as a model reaction. Firstly, three different homogeneous Ru-BINAP complexes including [RuCl2((R)-BINAP)], [RuCl((R)-BINAP)(p-cymene)]Cl and [RuCl((R)-BINAP)(Benzene)]Cl were immobilized on the PTA-modified alumina. Reaction test of immobilized catalysts showed that [RuCl2((R)-BINAP)] was the most active and selective so all the experiment were done using this catalyst afterwards.

A completely different type of inorganic membrane also has its origin in the nuclear industry the asymmetric alumina membranes obtained by the anodic oxidation of an aluminum sheet were first developed for uranium enrichment... 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

Membranes are classified as organic or inorganic, taking into account the material used for their syntheses porous or dense, based on the porosity of the material applied and symmetric and asymmetric for a membrane made of a single porous or dense material or for a membrane made of a porous support and a dense end, respectively [16,64], We are fundamentally interested here in asymmetric inorganic membranes made of a porous end to bring mechanical stability to the membrane and made of alumina, silica, carbon, zeolites, and other materials, and a dense end to give selectivity to the membrane (see Chapter 10). However, we also analyze the performance of porous polymers. 

Further reports on asymmetric synthesis in the presence of Cinchona alkaloids have been made.142 " For example, hydrogenation of methyl pyruvate with a platinum-alumina catalyst containing quinine gives (+)-(/ )-methyl lactate in 87% optical yield.1426 Asymmetric induction with optical yields up to 36 and 26% has been observed in the Michael addition of thiols and nitro-alkanes to ct/ -unsaturated ketones in the presence of quaternary salts derived from the Cinchona alkaloids.142"... 

In principle, racemic molecules may be separated into their enantiomers through any interaction with an asymmetric influence, and related to the method of less soluble diastereoisomers is the resolution by Piper (81) of the trisacetylacetonates of chromium(III) and cobalt(III) on a column either of alumina saturated with (+)tartaric acid or of (+) -quartz. The less strongly adsorbed enantiomers seem likely to have related configurations, if the sites of adsorption are the same. 

T. Nagata, K. Yorozu, T. Yamada, T. Mukaiyama, Angew. Chem. 1995, 107, 2309-2311 Angew. Chem. Int. Ed. Engl. 1995, 34, 2145-2147. For a recent review on the asymmetric reduction of carbonyl groups with hydrides,. see J. Seyden-Penne, Reductions by the Alumina- and Boro-hydrides in Organic Synthesis, 2nd edn., Wiley, New York, 1997, 55-84. 

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