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Lewis acids heterogeneous catalysis

Since the revised Biginelli mechanism was reported in 1997, numerous papers have appeared addressing improvements and variations of this reaction. The improvements include Lewis acid catalysis, protic acid catalysis, non-catalytic conditions, and heterogeneous catalysis. In addition, microwave irradiation (MWI) has been exploited to increase the reaction rates and yields. [Pg.511]

Several aluminum- and titanium-based compounds have been supported on silica and alumina [53]. Although silica and alumina themselves catalyze cycloaddition reactions, their catalytic activity is greatly increased when they complex a Lewis acid. Some of these catalysts are among the most active described to date for heterogeneous catalysis of the Diels-Alder reactions of carbonyl-containing dienophiles. The Si02-Et2AlCl catalyst is the most efficient and can be... [Pg.115]

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. [Pg.574]

Another example of the use of Lewis acids in organic reactions in water is the lan-thanide(III) triflate catalysed aza-Diels-Alder reaction, exemplified in Scheme 14. In this reaction the hetero-dienophile is formed in situ from a primary ammonium hydrochloride and a carbonyl compound followed by the actual Diels-Alder reaction288,289. This type of reaction proceeds readily in aqueous media290-296, and a dramatic increase in the yield upon addition of lanthanide triflates was observed288,289. The exact role of the catalyst, however, is not entirely clear. Although it was suggested that the catalyst binds to the dienophile, other mechanisms, such as simple proton catalysis, are also plausible. Moreover, these reactions are further complicated since they are often heterogeneous. [Pg.1075]

A. Corma and H. Garcia, Lewis Acids From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis , Chem. Rev., 2003, 103, 4307-65. [Pg.185]

This type of co-catalyhc influence is well known in heterogeneous catalysis, in which for some reachons an acidic support will achvate a metal catalyst more efficiently than a neutral support In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transihon metal catalysts dissolved in it As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transihon metal complex. The opportunity to opdmize the ionic medium used, by vari-ahon of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for ophmizahon. However, the choice of these parameters may be restricted by some possible incompahbilihes with the feedstock used. Undesired side reachons caused by the Lewis acidity of the ionic Hquid or by strong interaction between the Lewis acidic ionic Hquid and, for example, some oxygen funchonalihes in the substrate have to be considered. [Pg.222]

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. [Pg.30]

Overall there are benefits and limitations to all of the common methods for immobilising Lewis acids. This helps illustrate the apparently inescapable conclusion in heterogeneous acid catalysis that there are likely to be as many optimum catalysts as there are acid-catalysed processes. [Pg.50]

In both homogeneous and heterogeneous catalysis, carbon monoxide activation involves first the coordinaiive interaction of carbon monoxide with a metal acceptor center. Carbon monoxide, being a weak donor base, does not react with a proton and produces only a vety weak interaction with a hard acid center such as BH3, With less hard Lewis centers, such as CuX, AgX, AuXj etc. (X - halogen), more or less stable carbon monoxide adducts can be isolated. A variety of modes of CO coordination in well characterized organometallic complexes is known. Scheme 1 contains some selected examples. [Pg.6]

Corma, A. Garcia, H. Lewis acids from conventional homogeneous to green homogeneous and heterogenous catalysis. Chem. Rev. 2003 103 4307-4365. [Pg.2134]


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See also in sourсe #XX -- [ Pg.476 , Pg.478 ]




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