Aldol hydrotalcites

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). 

As part of a search for environmentally friendly solid acid-base catalysts, a modified Mg-Al hydrotalcite has been used as a base catalyst for aldol and Knoevenagel condensations. Yields are often quantitative, reaction times are about Ih, the catalyst can be recovered by filtration, and only moderate temperatures are required (60 °C for the aldol, ambient for the Knoevenagel). 

Figure 2 shows illustrated mechanism for acetone self-condensation over calcined hydrotalcites, where the enolate ion is formed in a first step followed by two possible kinetic pathways 1) In the first case the subtracted proton is attracted by the basic sites and transferred to the oxygen of the enolate ion to form an enol in equilibrium. 2) In the second case the enolate ion reacts with an acetone molecule in the carbonyl group, to produce the aldol (diacetone alcohol). Finally, the p carbon is deprotonated to form a ternary carbon and then loses an OH group to obtain the final products. 

While much effort has been put into obtaining materials with basic sites of different strength, the exact nature of the basic sites acting on those catalysts is not always known. For instance, reactions like aldolization, hydrogen transfer or double bond isomerization require either Brpnsted or Lewis type sites or even the association of strong or medium basic sites with weak acid sites/13-161 Many of these requirements (acid-base associations, versatility of the basic strength) can be accomplished by hydrotalcites and this would explain their significant development as catalysts precursors/13-15,171... 

Lakshmi Kantam, M., Choudary, B. M., Reddy, C. V., Koteswara Rao, K. and Figueras, F. Aldol and Knoevenagel condensations catalyzed by modified Mg-Al hydrotalcite a solid base as catalyst useful in synthetic organic chemistry, Chem. Commun., 1998, 1033-1034. 

Lopez, J., Jacquot, R. and Figueras, F. Heterogeneous catalysis of aldolizations on activated hydrotalcites, Stud. Surf. Sci. Catal., 2000, 130A, 491 496. 

Abello, S., Medina, F., Tichit, D., Perez-Ramirez, J., Groen, J. C., Sueiras, J. E., Salagre, P. and Cesteros, Y. Aldol condensations over reconstructed Mg-Al hydrotalcites structure-activity relationships related to the rehydration method, Chem. Eur. J.,... 

Aldol and related condensation reactions such as Knoevenagel and Claisen-Schmidt condensations are also widely used in the fine chemicals and specialty chemicals, e.g. flavors and fragrances, industries. Activated hydrotalcites have been employed as solid bases in many of these syntheses. Pertinent examples include the aldol condensation of acetone and citral [107, 108], the first step in the synthesis of ionones, and the Claisen-Schmidt condensation of substituted 2-hydroxyacetophenones with substituted benzaldehydes [109], the synthetic... 

Fig. 2.33 Hydrotalcite-catalyzed aldol and Claisen-Schmidt condensations...

Interesting recent developments are the use of hydrotalcite supported on carbon nanofibers [119], to facilitate recovery of the catalyst by filtration, and the use of synthetic hydroxyapatite, Ca10(PO4)6(OH)2 as a solid base catalyst in a variety of reactions including Michael additions [120]. The supported hydrotalcite exhibited higher activities and selectivities than the conventional unsupported material in the aldol condensation of citral with acetone [119]. 

It has previously been reported that hydrotalcite catalyzes the aldol condensation of acetone (25). Polyoxometalates are known to dehydrate alcohols due to their acidic nature (IS ). In order to compare the relative basicity of polyoxometalate-pillared hydrotalcites to that of hydrotalcite itself, a variety of hydrotalcites were screened for 2-propanol conversion (Table II). This reaction is known to give propylene when the catalyst contains acidic sites (such as alumina) and acetone when the catalyst contains basic sites (such as magnesium oxide). 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

Figure 2.26 Cascade reactions requiring both strong acid and basic Bronsted sites reaction scheme and pictorial representation of tandem deprotection-aldol reaction with different acids and basic steps using a combination of Ti -montmorillonite (Ti -mont) and hydrotalcite (HT). Elaborated from the results of Kaneda et al. [219], Source adapted from Centi and Perathoner [211].

Note, however, that rehydration of Mg-Al hydrotalcites in the liquid phase using ultrasound or a high stirring speed [267] leads to nanoplatelets with surface areas of400 m g , displaying catalytic activities in aldol condensations up to eight-times higher than the best catalytic system reported in the literature. There are thus alternative methods to increase the performances of HT materials. 

Figure 2.42 Examples of reactions catalyzed by hydrotalcite-based catalysts for the synthesis of flavors and fragrances (a) aldol condensation (b) Knoevenagel condensation (c) double-bond isomerization.

Nucleophilic reactions of unmodified aldehydes are usually diiScult to control, affording complex mixture of products, often due to the high reactivity of the formyl group under either basic or acidic reaction conditions. The activity order of the supported amines was secondary > primary > tertiary, which may suggest the intervention of an enamine pathway the enals were exclusively obtained as ( ) isomers. Notably, FSM-16-(CH2)3-NHMe exhibited higher activity than conventional solid bases such as MgO and Mg-Al-hydrotalcite [hexanal self-aldol condensation FSM-16-(CH2)3-NHMe 97% conversion and 85% yield in 2h, MgO 56% conversion and 26% yield in 20 h, Mg-Al-hydrotalcite 22% conversion and 11% yield in 24 h]. 

This carbanion further adds to the carbonyl compound to form an intermediate, which removes a proton from water to give the final nitro aldol product (Figure 3.9). Solid bases without Bronsted hydroxy groups such as decarbonated hydrotalcites are not active in this reaction. The Mg-Al hydrotalcite catalyst is non-toxic and can be recycled with little loss in activity.42... 

The aldol condensation of formaldehyde and acetone to methylvinylketone has been investigated by Suzuki and Ono [20] on a series of solids MgAl hydrotalcite was found to be the best catalyst, leading to 21 % conversion of acetone at 673 K, with 96 % selectivity for acetone and 64 % for formaldehyde. The carbonate form was the most suitable. Exchange by sulfates or chromates resulted in poor activity. 

The aldolization of acetaldehyde on calcined hydrotalcites of different composition has been studied by Kagunya and Jones [21] in a flow reactor at 10 bar and LHSV = 4 at temperatures between 363 and 403 K. MgAl carbonate was the best catalyst conversion reached ca 40 % at 110 °C with a broad range of products with a larger selectivity to Cg isomers. 

Aldolizations on Hydrotalcites First Calcined then Rehydrated... 

The specific properties of hydrated hydrotalcites appear not only in the aldoli-zation of acetone, but in many other aldolization reactions. For example, in the aldol condensation of benzaldehyde with acetone the hydrated form catalyzes the reaction at 273 K, yielding aldol as the main product instead of benzalacetone, obtained on the calcined sample. Competitive adsorption kinetics are still observed, with a much greater adsorption coefficient for benzaldehyde. As suggested earlier from Hammett relationships, this reaction can be generalized with success to many substituted benzaldehydes [32], although the reaction could be performed selectively at 273 K with benzaldehyde only, and substituted benzaldehydes required a reaction temperature of 333 K. Because of this high temperature the reaction usually gives a, unsaturated ketones isolated yields are > 95 %. 

MgAl hydrotalcite impregnated with potassium t-butylate has recently been used for aldolization reactions [35]. The results are summarized in Table 4 for the condensation of acetone on benzaldehydes of general formula R2CHO. 

Aldolization with MCM 41, a mesoporous material modified by anchoring of 3-trimethoxysilylpropylethylenediamine, has also been reported [36]. This solid is expected to have the medium basicity of a diamine. Results for some aldolization reactions are reported in Table 5. The temperature was 323 K and relatively long times are required to reach high conversions, so these solids seem to be weaker bases than activated hydrotalcites. The high selectivity for dehydrated products is, moreover, evidence of the residual acidity of the solid. 

The aldol condensation of benzaldehyde with acetophenone has been used to compare hydrated hydrotalcites with many solids claimed to be strong solid bases-KF and KNO3 supported on alumina, X zeolites containing excess Cs or Mg, and lanthanum oxide. Only hydrated HDT and KF/AI2O3 could be used to perform this reaction at 273 K-HDT with 100% selectivity for chalcone and KF with lower selectivity, because of secondary Michael addition of the chalcone to acetophenone. The other solids were inactive, but could induce comparable conversions at 423 K. 

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