Lewis silica supported

Chiral Lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein [103]... 

Determination of the acidic sites through IR spectroscopy of adsorbed CO is a valuable tool for the choice of the support when selective or multifunctional processes are to be set up. This technique allowed to identify a particular kind of silica as the support of choice for the selective hydrogenation of citral to citronellal and sepiolite as a Lewis acid support able to promote the one-step transformation of citral into menthol. 

The use of silica-supported Lewis acids as catalysts for the Diels-Alder reactions of 2,5-dimethylfuran leads to fairly good yields of adducts [28]. Solid supports such as... 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

Bound reagents are an excellent alternative in cases where the reagent is used in excess and can be difficult to remove. Purification is now a simple process of filtration and evaporation. For example, silica-supported aluminium chloride is a Lewis acid and an effective catalyst for Friedel-Crafts alkylations (Figure 3.15). 

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

Table 5 Reactions of indole 2 with 196 using silica-supported Lewis acids...

The Michael addition of methyl a-acetamidoacrylate (196) with pyrrole (1) under silica-supported Lewis acid (Si(M) Si(Zn), Si(Al) and Si(Ti)) assisted by microwave irradiation (MW) afforded the alanine derivatives 395 and 396 dependent on the reaction conditions (Scheme 81) [153]. Both MW and thermal activation for pyrrole gave only Michael product 396, whereas alanine derivatives 395, which are the a-Michael addition product, and 396 were observed with A1 and Ti-catalyst. This behavior shows that aluminium and titanium Lewis acids can form a new acceptor in an irreversible way. The Si(M) or p-TsOH catalyzed reactions of N-benzylpyrrolc 397 with the acrylate 196 under MW gave the product 398 as sole product. The reaction yield has been increased by using a catalytic amount of p-TsOH (Scheme 82). 

Scheme 1 Bronsted acidity arising from inductive effect of Lewis acid centre coordinated to a silica support...

The physical properties of silica supported ZnCl2 will not be discussed here, but have been widely studied elsewhere, shown to be a pure Lewis acid21. 

Fig. 8. Diflierential heat of adsorption for pyridine adsorbed on silica-supported oxides that showed only Lewis acidity. (Adapted from Ref. 104.)...

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

An inorganic anion-exchange resin based on a zirconia-silica support was prepared by Chicz, Shi, and Regnier. The Lewis acid sites were employed as adsorption sites that permanently retained a PEI coating which then acted as an anion-exchange ligand. In this study, a continuous film of cross-linked PEI was achieved. These surfaces were then tested for the analysis of ova-bumin and bovine serum albumin. 

Bulk Metal Oxides. Extensive Raman chemisorption studies on high surface area alumina and silica supports have been performed because of the industrial importance of these oxides and their weak background Raman vibrations. The most informative studies resulted from the adsorption of pyridine since this probe molecule is very sensitive to the type of acid sites (Brpnsted and Lewis) present on the silica and alumina surfaces. On the alumia support, Lewis pyridine was predominately observed and on the silica support both Lewis and Brpnsted pyridine were observed. " The surface concentrations of pyridine on the silica surface were very small in comparison to the pyridine coverages on the alumina surface. The signal intensities were dramatically enhanced by the... 

A fluorous surfactant covalently tethered to silica provides a thin film of perfluorinated solvent for reactions and/or extractions. This material was used for the small-scale hydrocyclization of 6-bromo-1-hexene with NaBH and a catalytic amount of a fluorous tin bromide in 1-butanol. The yield of methylcyclopentane was modest, however, and this technology is a long way from being viable on an industrial scale. The same idea has been more successfully employed with fluorous silica-supported tin Lewis acid catalysts for Baeyer-Villiger oxidations. ... 

To assess whether Lewis acid sites are present on zirconium sulfate, we prepared water-free bulk zirconium sulfete. Furthermore, a water-free silica-supported zirconium sulfrite catalyst was prepared by deposition-precipitation of zirconia on silica and subsequent gas-phase reaction with SO3. The activity of these catalysts was compared with that of two conventionally prepared sul ted zirconia catalysts. 

The anhydrous bulk zirconium sulfate preparation did not display any activity in the trans-alkylation of benzene (1) and diethylbenzene (2) to ethylbenzene (3). At 473 K the silica-supported, gas-phase sulfated zirconia showed a very small activity, which rapidly dropped to a negligible level (Fig. 2). The conclusion is that Lewis acid sites are not active with sulfated zirconia catalysts. The low activity of the silica-supported catalyst is due to adsorption of some water leading to Bronsted acid sites. Desorption of water at 473 K leads to the decrease in activity with time. Pre-hydration of the supported catalyst brings about a slightly higher activity as apparent from Fig. 2 the activity drops again due to the loss of water. 

Neither the anhydrous bulk zirconium sulfate nor the silica-supported, sul ted zirconia were active in the addition of acetic acid (4) to camphene (5). The lack of activity is due to the fact that addition of acetic anhydride removes water completely from the reactants. Also the liquid-phase reaction thus demonstrates that Lewis acid sites are not active in our catalysts. Addition of water leads to a well measurable activity with both catafysts. Fig. 5 represents the activity of the silica-supported sulfated catalyst after pre-hydration. The... 

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