Heterogeneous Catalysis of Diels-Alder Reactions

This contribution provides an overview of known heterogeneous catalysts for Diels-Alder reactions with emphasis on methods which enable efficient synthesis of fine chemicals. It covers the literature up to the end of 1998. 

Other zeolite-catalyzed Diels-Alder reactions have since been reported these concentrate mainly on studies of standard model compounds [9]. Pindur and Haber [10] described the reaction of in situ generated A-benzoyl-2,3-dihydro- 

A variety of Diels-Alder reactions have been catalyzed by Na-ZSM-5, Na-Y, or Ce-Y [12]. Some results obtained with furan are included in Table 1. Reactions with other dienes were catalyzed more effectively when the zeolites were treated with ZnBr2. Zeolite H-Y modified with anhydrous ZnCl2 has subsequently been 

6-octadiene) and acrolein [13], Diels-Alder adducts of myrcene are fragrance intermediates. 

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- 

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Fraile J. M., Garcia J. I., Mayoral J. A. Heterogeneous Catalysis of Diels-Alder Reactions Recent Res. Dev. Synth. Org. Chem. 1998 1 77-92... 

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... 

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. 

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... 

Note at the outset that asymmetric catalysis in the synthesis of fine chemicals is rarely a single-step process that converts a reactant directly to the final product. It is usually one of the steps in a total synthesis but is often the key step. Hence the analysis of the overall yield will be based on the methods described in Chapter 5. There are many types of reactions where asymmetric catalysis can be applied. The most important of these are C-C bond-forming reactions such as alkylation or nucleophilic addition, oxidation, reduction, isomerization, Diels-Alder reaction, Michael addition, deracemization, and Sharpless expoxidation (of allyl alcohols). A few representative examples (homogeneous and heterogeneous) are given in Table 9.6. 

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