Heterogeneous aldol reaction

Clays are abundant and available substances that promote various chemical reactions. The characteristic layered structure of clays attracted the attention of synthetic chemists who have used it as a support material [117]. For example, chiral organic-inorganic hybrid materials (229) based on (S)-proline-exchanged clay such as montmorillonite are used for heterogeneous aldol reactions (Scheme 28.31). Co-inclusion of ammonium cations within the layers enhances yields and... 

Until then, only heterogeneous catalyst had been successful. However, in the mid-1980s, the work of Ito et al. led to an outstanding discovery in a catalytic asymmetric aldol reaction. In this case, enantioselectivity was given by a chiral ferrocene diphosphine ligand, with a carbon nucleophile addition to a carbonyl... 

In the work concerning the mechanism of solid-catalysed aldol reactions, the analogy between the homogeneous and heterogeneous mechanisms is usually assumed [370,372—375]. The mechanism of base-catalysed condensations, which has received much attention (cf. ref. 371), may be pictured in general as... 

Simple L-alanine, L-valine, L-norvaline, L-isolecucine, L-serine and other linear amino acids [ 121 ] or chiral amino acids with a binaphthyl backbone [ 122] and peptides have also been used as asymmetric catalysts [123,124,125,126]. Solid-supported proline-terminated peptides have been used for heterogeneous catalysis of the asymmetric aldol reaction [ 127]. Apart from proline and derivatives, other cyclic compounds such as 5,5-dimethyl thiazolidinium-4-car-boxylate (DMTC) [128], 2-fert-butyl-4-benzyl imidazolidinones [129], (l/ ,25)-2-aminocy-clopentanecarboxylic acid [130], (5 -5-(pyrrolidin-2-yl)tetrazole, (5)-l,3-thiazolidine-4-car-boxylic acid, (5)-5,5-dimethyl-l,3-thiazolidine-4-carboxylic acid, and (5)-hydroxyproline are effective catalysts in asymmetric aldol reactions [126,131,132,133,134,135]. 

Zhong L, Xiao JL, Li C (2007) Direct asymmetric aldol reactions on heterogeneous bifunctional catalyst. Chin J Catal 28 673... 

Mayoralas et al. [70] reported the aldol reaction of hydroxyacetone with different aldehydes catalyzed by immobilized L-proline on a mesoporous support. Heterogenized L-proline on MCM-41 showed higher enantioselectivity (80% ee) than its homogeneous counterpart (75% ee) in the aldol reaction of benzaldehyde with hydroxyacetone in dimethylsulfoxide (DMSO) solvent with the assistance of microwave heating. 

The use of water in Mannich reactions is counterintuitive since it is supposed to shift to the left the imine formation reaction. However, as in the case of aldol reactions catalysed by species 3-12 (Table 1.1), the success is due to the biphasic heterogeneous conditions adopted - all three components of the Mannich reaction and the catalyst forming an organic phase where the process takes place, the water/organic interphase providing a local environment rich in free OH bonds that can co-activate the substrate. ... 

The direct asymmetric aldol reaction between unmodified aldehydes and ketones plays an important role in nature as a source of carbohydrates and it is used for the synthesis of chiral p-hydroxycarbonyl compounds. This reaction was performed by using (5)-proline/poly-(diallyldimethylammonium) hexafluorophosphate heterogeneous catalytic system 36. The catalyst was simply prepared by mixing a suspension of the commercially available polyelectrolyte 34 in methanol with a solution of (,S )-prolinc (35) in the same solvent (Scheme 3.11). 

Heterogeneous catalysts such as cation-exchanged montmwillonite clay or Nation also catalyze the aldol reaction of enol silyl ethers with aldehydes and acetals. 

Fluorous solid catalyst 8 is highly effective for the Mukaiyama aldol reaction [Eq. (9)] and Sakurai-Hosomi allylation reaction [Eq. (lO)j. These reactions have been performed at -78 °C and room temperature, respectively, under heterogeneous conditions. Post-reaction, 8 has been recovered in high yield by decanting the liquids at room temperature. 

A key role of the water-organie emulsion and/or heterogeneous reaetion conditions in reactivity and stereoselectivity of direct asymmetric aldol reactions of cyclic ketones 8 (R -R =-(CH2) -, -(CH2)2XCH2-) with aryl aldehydes 9 catalysed by amphiphilie, in particular surfactant-like, eatalysts 19a-e in the aqueous environment was later confirmed by Chinese and Italian researchers (Scheme 10.1). These catalytic reactions could be performed on a 0.2 mole seale with the enantiomeric purity of products 10 maintained at the same level, whieh shows great promise for industty. ... 

Inspired by the reported investigations of the group of Barbas on direct asymmetric aldol reactions using hydrophobic catalysts in aqueous emulsions, Miravet, Escuder and coworkers developed supramolecular hydrogel catalyst 33 as a reusable heterogeneous catalyst for the direct aldol reaction between 4-nitrobenzaldehyde and cyclohexanone (Scheme 13.21b). ... 

A cationic rathenium monomer spedes, prepared by ligand exchange of Cl with SbFu or OTf, on hydroxyapatite (HAP) (Scheme 6.15), acted as a heterogeneous Lewis acid toward carbonyl and cyano compounds for promoting Diels-Alder and aldol reactions [106]. The fine structure of an intermediate, cationic Ru species... 

A combination of ion-exchange and molecular sieve properties - as well as their chemical stability has led to their use as heterogeneous catalysts. If the counter ion in a zeolite is a proton this means the materials can essentially behave as solid phase Bronsted adds will promote all the various acid-catalysed reactions in organic chemistry (e.g. cracking, isomerisation, esterifications and aldol reactions) and they have a long history of application as acidic catalysts in the oil refining industries (where the molecular sieving properties are also important) [37],... 

---