Clays aldol condensations

Potassium hydroxide-clay Aldol condensation in the vapor phase... 

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

Tichit, D., Bennani, M. N., Figueras, F., Tessier, R. and Kervennal, J. Aldol condensation of acetone over layered double hydroxides of the meixnerite type. Appl. Clay Sci., 1998, 13, 401-415. 

Varma has run many types of reactions on nontoxic supports, such as alumina, silica, and clay, using microwaves.45 (For more on the use of microwaves, see Chap. 15.) The method offers selectivity under safe, fast, mild reaction conditions in open vessels. The supports can be recycled. The reactions include oxidations, reductions, nitrations, aldol condensations, and others. Solvents were used in the workups. Other workers have devised a condensation (8.3) of aromatic aldehydes with malonic acid (without a support) that uses no solvents at all.46 The workup consisted of pouring the glassy reaction mixture from the Erlenmeyer flask into ice water, filtering off the solid, and drying it. The conventional procedure for this reaction requires heating at 120°C for 3 h to give the product in 96% yield. 

There has been a quite limited number of reports of clay-catalysed aldol condensations. One of the more interesting of these is the aluminium-exchanged montmorillonite (Al3+-mont) catalysed cross-aldol reaction of silyl enol ethers with aldehydes (Reaction 6).34 The reaction proceeded smoothly under mild conditions to give the corresponding aldol adduct in good yield. 

The alkoxyalkylation reaction of carbonyl compounds can be considered as an orientated cross-aldol condensation between two masked carbonyl compounds, an acetal and a silyl enol ether. The key step involves the formation of an electrophilic species by reaction of the acetal with catalytic amounts of a Lewis acid and the right catalyst can lead to excellent diastereo- and enantio-selectivities. Clays are satisfactory catalysts in these reactions, with the acid-treated clay K10 performing better than the more powerfully acidic clay KSF,... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

Hydrotalcite clays as solid bases can catalyze aldol-condensations or JCnoevena-gel-condensations... 

Hydrotalcite, an anionic clay mineral with the formula Mg6Al2(0H)i6C034H20, shows a high activity for cross aldol condensation of formaldehyde with acetone to form... 

Hydrotalcite clays, for example, are built up of positively charged brucite layers, for reviews see Cavani et al. (1991). Upon calcination they become active as solid bases in e.g. aldol and Knoevenagel condensations (see Fig. 2.26) (Fgueras et al., 1998 Corma and Martin-Aranda, 1993 Climent e/a/., 1995). 

The use of solid bases as catalysts in organic synthesis is less well-developed than solid-acid catalysis but is becoming increasingly popular [18]. For example, hy-drotalcite anionic clays [19] and mesoporous silicas modified by surface attachment of organic bases [20] are effective and recyclable catalysts for aldol, Knoe-venagel, and related condensations that are widely used in fine chemical synthesis. 

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