Clays Diels-Alder reaction

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. 

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... 

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. 

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. 

Many Diels-Alder reactions have been shown to be accelerated in the presence of Lewis acid species (Inukai and Kojima, 1971). Several workers have capitalized on the highly organized character of the clay surface, in conjunction with its ability to hold water and bind potentially catalytic Lewis acids, to successfully carry out Diels-Alder syntheses that have been difficult to accomplish in solution. For example, 1,3-cyclohexadiene dimerized at 0°C in dichloromethane containing Fe(III)-doped... 

Collet, C. and P. Laszlo. 1991. Clay catalysis of the non-aqueous Diels-Alder reaction and the importance of humidity control. Tetrahedron Lett. 32 2905-2908. 

Laszlo, P. and J. Lucchetti. 1984. Catalysis of the Diels-Alder reaction in the presence of clays. Tetrahedron Lett. 25 1567-1570. 

An interesting aspect of thiophene chemistry is the differences in reactivity between thiophene and its more aromatic isostere, benzene, and its less aromatic isosteres, furan and pyrrole. One interesting facet of this contrast is that metal cation-exchanged clay catalyzed Diels-Alder reactions work for fiiran and pyrrole to produce reaction with a,p-unsaturated carbonyl compounds the thiophene examples do not react <94JCS(P1)761>. 

Clay treated with transition metals, Fe and Cr were especially effective, has been found to catalyze Diels-Alder reactions and pyrrole was among the dienes which was examined. Using the Cr catalyst, pyrrole gave a 4 1 mixture of exo and endo adducts with methyl vinyl ketone. f94JCS(Pl)761>The relative effectiveness of the various metals examined does not appear to correlate with either Lewis acidity or redox potential. A tentative suggestion is that the metal cations may facilitate reaction through coordination complexes. 

In the clay-catalysed reaction even oleate will furnish a cyclic dimer along with monomer which is a mixture of saturated and unsaturated (mainly tram), straight-chain and branched-chain Cis compounds formed by hydrogen transfer and rearrangement. Dimers are formed by diene synthesis (Diels Alder reaction) between a monoene and a conjugated diene produced from monoene by hydrogen transfer. The cyclohexene derivatives are converted by hydrogen transfer to cyclohexane and benzene derivatives. These monocyclic dimers are accompanied by acyclic and bicyclic dimers such as those formulated below. Linoleate reacts in a similar manner. 

Melchione s team reported the asymmetric catalysis of Diels-Alder reactions of IQDs (Scheme 11, equation 1) [69, 70], In addition to other nitro-substituted arylethenes, methyleneindolinones were employed as dienophiles. A limited selection of the compounds synthesized is shown in Scheme 11 (30-32). The third compound (32) is the result of a final cross-benzoin condensation. Chen and colleagues effected an asymmetric Diels-Alder reaction of IQDs (33) generated under mild acidic conditions from 2-methyl-3-indolemethanols and a,p-unsaturated aldehydes (equation 2) [71], Three representative indoles that were prepared in this fashion are 34 to 36. The IQD 33 is presumed to be in equilibrium with the 3-vinylindolenium species. A wide range of substituted trani-cinnamalde-hydes was successfully employed. Although other acids (HOAc, TFA, PhCO H, silica gel) effected the reaction, Montmorillonite KIO clay was superior in terms of yield, enantioselectivity, and diastereoselectivity. 

The acidic clay KIO can also be used to replace conventional Lewis acid catalysts [155]. It gives higher yields and selectivities than aluminium trichloride in reactions between acryloylferrocene and substituted butadienes, for example [156]. Iron(III)-exchanged KIO can exhibit excellent selectivities [157] and/or high yields in Diels-Alder reactions [158]. Indeed, the selectivity is greatly affected by the type of cation present on KIO and calcination of the solid prior to use is vital for optimum activity. 

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