Diels clays

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

Porous surfaces of inorganic solids such as clays, silica gel, alumina and zeolites are the commonest systems used as catalysts in Diels-Alder reactions. 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Clay-catalyzed asymmetric Diels-Alder reactions were investigated by using chiral acrylates [10]. Zn(II)- and Ti(IV)-K-10 montmorillonite, calcined at 55 °C, did not efficiently catalyze the cycloadditions of cyclopentadiene (1) with acrylates that incorporate large-size chiral auxiliaries such as cA-3-neopentoxyisobornyl acrylate (2) and (-)-menthyl acrylate (3, R = H) (Figure 4.1). This result was probably due to diffusion problems. 

Acceleration of the Diels-Alder reaction by clays suspended in organic solvents [7]... 

Reaction of pyrones under dassical conditions requires the use of high temperatures to obtain low to moderate yields. The Diels-Alder reaction of pyrones has been performed in a commercial microwave oven under solvent-free conditions on solid supports such as silica gel, montmorillonite, fitrol clay and alumina. The reaction time was dramatically reduced - from 4 h to 4 min (Scheme 9.9) [50],... 

Reaction conditions markedly affect the Diels-Alder reaction.521,522 First observed by Yates and Eaton,526 A1C13 accelerates cycloadditions. Other Lewis acids as well as acidic solids (clays, zeolites) were later shown to exhibit similar effects attributed to the coordination of the catalyst with the dienophile. High pressure521,522 and water521,522,527 were also found to show beneficial effects. Reaction conditions also enhance selectivities of the Diels-Alder reaction (see discussion below). 

Adams, J. M., S. Dyer, K. Martin, W. A. Matear, and R. W. McCabe. 1994. Diels-Alder reactions catalysed by cation-exchanged clay minerals. J. Chem. Soc. Perkin Tram. 1 761-765. 

An interesting phenomenon related to the acceleration of Diels-Alder reactions in aqueous media is found when clays such as montmorillonite are suspended in an organic solvent e.g. ethanol or dichloromethane) [719]. The layered structure of the clay enables it to trap pools of internal water, which are obviously capable of exerting hydrophobic effects. 

Diels-Alder reactions showed a pronounced increase of rate, endo- and regio-selectivity when carried out in organic solvents in the presence of modified clays or zeolites. Similar improvements could be accomplished by co-adsorption of the reactants on dry Si02 in the absence of organic solvents. Hiese trends are summarized below and compared with the results obtained by employing the classical-thermal ,Lewis acid catalyzed, " " high pressure or aqueous reaction conditions. 

Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Modified clays (e.g. montmorilonite K-10) as mild Lewis acid catalysts in Knoevenagel, Michael, Diels-Alder reactions, aromatic chlorination and nitration. 

Until 1960s, proton exchanged or acid treated clays were used as cracking catalysts. They have also proved to be good catalysts for many industrially important reactions like alkylation [1], dimerization and polymerization of unsaturated hydrocarbons [2], Diels Alder condensation [3] etc. Moreover acid treated clays are commonly used commercially for decolourising oils [4] and also in colour formation with leuco dyes in pressure sensitive recording paper [5]. 

The Diels-Alder reaction has been shown to be subject to catalysis by a wide range of solid catalysts (see Chapter 4 for some examples). Acidic mesoporous aluminosilicates can be used to catalyse selective Diels-Alder reactions such as that between cyclopentadiene with methyl acrylate. The zinc-exchanged version of the material is particularly effective and compares well to other more established solid acids such as the ion-exchanged clay Zn2+-K10 as well as homogeneous catalysts such as boron trifluoride (Table 2.7).50... 

Ion exchange is the most important technique for the preparation of clay and zeolite catalysts from the preformed supports. It is used to make several catalysts described here, including montmorillonite-Fe3+, which is a useful Diels-Alder catalyst that can function in water and zeolite-Na+,H+ (i.e. partially proton-exchanged zeolite), which can catalyse some aromatic chlor-inations with shape selectivity.15,21,22... 

Clay-catalyzed Diels-Alder reactions were thoroughly investigated by Laszlo and co-workers. Of particular interest are reactions with furans [14] or acrolein [15] as substrate catalyzed by iron(III)-exchanged KIO montmorillonite. Some represen-... 

Table 2. Examples of clay-catalyzed Diels-Alder reactions.

A variety of solid Lewis and Br0nsted acids has been shown to catalyze Diels-Alder reactions. In several instances the results obtained with heterogeneous catalysts were better than those with homogeneous Lewis acid catalysts. Most of the reported reactions of interest in the synthesis of fine chemicals were catalyzed by (modified) zeolites, clays, alumina, or silica. Catalysts with interesting properties were obtained when support materials such as zeolites, alumina, or silica were treated with Lewis acids. These catalysts were moderately selective in diastereo-selective Diels-Alder reactions with chiral dienophiles and induced enantioselec-tivity (up to 31 % e. e.) in the reaction of cyclopentadiene with methacrolein after treatment with chiral derivatives. Excellent enantioselectivity in this reaction (up to 95 % e. e.) was observed with a polymer-supported chiral oxazaborolidinone. Because of their facile recovery and recycling, we expect that solid-acid catalysts will find increasing use in Diels-Alder reactions in the future. 

K10 montmorillonites exchanged with different cations, dried at 120°C or calcined at 550°C, are used as catalysts in Diels-Alder reactions of methyl and (-)-menthyl acrylates with cyclopentadiene. In general, calcined clays give rise to better conversions and selectivities. Zr(IV) and specially Ti(IV) clays display the best catalytic activities. However, the best asymmetric induction is achieved with Cr(lll) and Ca(ll) calcined clays. Clays containing easily reducible cations behave differently due to the cyclopentadiene polymerization via radical cations. 

Clays [2] and zeolites [3] have been reported as good catalysts for Diels-Alder synthesis. Recently, we have studied the Diels-Alder cycloaddition between methyl acrylate (1) and cyclopentadiene (2) (Figure 1) and have shown that the solvent [4], the calcination of the solid [5] and the exchanged cation [6] play a decisive role. We now report the results obtained from the reactions of cyclopentadiene with methyl and (-)-menthyl acrylates, catalysed by K10 montmorillonites exchanged with different cations and dried at 120°C or calcined at 550°C. 

The heterogeneous catalysis of the benchmark asymmetric Diels-Alder reaction between (-)-menthyl acrylate (4) and cyclopentadiene (2) (Figure 2) with both dried [12] and calcined [5] Zn(ll)-clays has shown that selectivities close to... 

More recently, catalysts such as silica (6), clays (7), zeolites (8-10) were also reported to catalyze the Diels-Alder reactions and two important concepts were invoked from these few reports i- an acidic catalytic effect on the reaction rate and ii- an entropic effect due to the presence of cavities in some zeolites. 

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