Surfactant with Lewis acid

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

Sinou and co-workers [73] studied the influence of different surfactants on the palladium-catalyzed asymmetric alkylation of l,3-diphenyl-2-propenyl acetate with dimethyl malonate in presence of potassium carbonate as base and non-water-soluble chiral ligands. Best results in activity and enatioselectivity (> 90% ee) were observed with 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as ligand and cetyltrimethylammonium hydrogen sulfate as surfactant in aqueous medium. Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [74]. An acceleration of the reaction was indicated in presence of SDS as anionic surfactants. An additional promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant-combined catalysts) as shown in Eq. (3). Surfactant the anion of dodecanesulfonic acid. 

Since the seminal contributions in the 1980s of Breslow [la] for the Diels-Alder reaction and Kuntz and Ruhrchemie for hydroformylation reactions [lb], there has been an upsurge in interest in using water as the solvent, not only to enhance the reaction rates, but also to perform organic reactions that would otherwise be impossible, or to elicit new selectivities. Several reviews have been devoted specially to such a use [2], which nevertheless does not exclude the possibility of further catalyzing the reactions with Lewis acids [3] or the use of Lewis acid-surfactant-combined catalysis (LASC) [4],... 

Likewise, the number of available surfactants (Lewis bases) are also limited, since not all of them have available orbitals to form molecular orbitals with Lewis acid TM centers [74,76,82]. Typical surfactants used in the LAT method are alkyl phosphates and alkyl amines [72,74,76,82]. The second major drawback is the thermal stability of the formed mesostructure. Due to the strong S-I interaction, conventional solvent extraction methods are not enough to remove surfactant from mesostructured TM oxides to form mesoporous oxide materials. In addition, low metal to surfactant ratios ( 1) and essentially high temperature treatments (>500 °C) to remove hydrophobic alkyl chains, which make these materials thermally unstable. 

With the aim of catalysis of the Diels-Alder reaction of 5.1 with 5.2 by metallo micelles, preliminary studies have been performed using the surfactants 5.5a-c and 5.6 (Scheme 5.2). Unfortunately, the limited solubility of these surfactants in the pH region that allows Lewis-acid catalysis of the Diels-... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

In 1950 the Fischer-Tropsch synthesis was banned in Germany by the allied forces. Sinarol, a high paraffinic kerosene fraction sold by Shell, was used as a substitute. This ban coincided with the rapid development of the European petrochemical industry, and in due time Fischer-Tropsch synthesis applied to the production of paraffins became uneconomic anyway. After the war there was a steady worldwide increase in the demand for surfactants. In order to continually meet the demand for synthetic detergents, the industry was compelled to find a substitute for /z-paraffin. This was achieved by the oligomerization of the propene part of raffinate gases with phosphoric acid catalyst at 200°C and about 20 bars pressure to produce tetrapropene. Tetrapropene was inexpensive, comprising a defined C cut and an olefinic double bond. Instead of the Lewis acid, aluminum chloride, hydrofluoric acid could now be used as a considerably milder, more economical, and easier-to-handle alkylation catalyst [4],... 

It has been found that the combination of Lewis acids and surfactants is particularly effective for catalyzing Diels-Alder reactions in water. The effect of micelles of SDS, CTAB, dodecyl heptaoxyethy-lene ether (Q2E7), and copper and zinc didodecyl sulfate [M(DSb] on the Diels-Alder reaction of 3-(p-substituted phenyl)- l-(2-pyridyl)-2-propen-l-ones (Figure 12.1) with cyclopentadiene was studied. 

While the Lewis acid-catalyzed aldol reactions in aqueous solvents described above are catalyzed smoothly by several metal salts, a certain amount of an organic solvent such as THF had still to be combined with water to promote the reactions efficiently. This requirement is probably because most substrates are not soluble in water. To avoid the use of the organic solvents, we have developed a new reaction system in which metal triflates catalyze aldol reactions in water with the aid of a small amount of a surfactant, such as sodium dodecyl sulfate (SDS). 

With these results in hand, we have next introduced new types of Lewis acids, e.g scandium tris(-dodecyl sulfate) (4a) and scandium trisdodecanesul-fonate (5a) (Chart 1).[1S1 These Lewis acid-surfactant-combined catalysts (LASCs) were found to form stable colloidal dispersions with organic substrates in water and to catalyze efficiently aldol reactions of aldehydes with very water-labile silyl enol ethers. 

A variation of this method was used to control the surface properties of aluminum oxide particles, particularly the surface density of Lewis acid sites. Instead of using a long-chain amine surfactant, the solution of aluminum alkoxide precursor was mixed with a small amine to convert the alkoxide dimer (or oligomer in general) into monomeric alkoxide-amine adduct. Controlled hydrolysis of this adduct produces an aluminum oxyhydroxide in which the surface A1 ions are coordinated to amine... 

Prins cyclization of the acetal 327 can be conducted in the presence of a Lewis acid surfactant catalyst in water to afford 2,4,6-trisubstituted tetrahydropyrans. The reaction proceeds via ionization of the crji-iinsatuiated acetal and subsequent reaction with the tethered electron-rich alkene, proving that the interior of micelles are sufficiently anhydrous to protect Prins cyclization intermediates (Equation 138) <20030L4521>. 

Electrostatic interactions occur between the ionic head groups of the surfactant and the oppositely charged solid surface (head down adsorption with monolayer structure) [56]. Acid-base interactions occur due to hydrogen bonding or Lewis acid-Lewis base reactions between solid surface and surfactant molecules (head down with monolayer structure) [57]. Polarisation of jt electrons occurs between the surfactant head group which has electron-rich aromatic nuclei and the positively charged solid surface (head down with monolayer structure) [58]. Dispersion forces occur due to London-van der Waals forces between the surfactant molecules and the solid surface (hydrophobic tail lies flat on the hydrophobic solid surface while hydrophilic head orients towards polar liquid) [59]. 

Surfactant-based synthesis of mesoporous metal oxides and metal sulfides emerged about four years after the initial report of MCM-41 [21-36]. High surface area and thermally robust mesoporous metal oxides and sulfides represent a new class of materials with diverse opportunities for the development of improved fuel and solar cells, batteries, membranes, chemical delivery vehicles, heavy metal sponges, sensors, magnetic devices and new catalysts. All of these applications could benefit from tailorable Bronsted and Lewis acidity and basicity, flexible oxidation states, and tunable electronic, optical and magnetic properties. 

The lanthanide triflate remains in the aqueous phase and can be re-used after concentration. From a green chemistry viewpoint it would be more attractive to perform the reactions in water as the only solvent. This was achieved by adding the surfactant sodium dodecyl sulfate (SDS 20 mol%) to the aqueous solution of e.g. Sc(OTf)3 (10 mol%) [145]. A further extension of this concept resulted in the development of lanthanide salts of dodecyl sulfate, so-called Lewis acid-surfactant combined catalysts (LASC) which combine the Lewis acidity of the cation with the surfactant properties of the anion [148]. These LASCs, e.g. Sc(DS)3, exhibited much higher activities in water than in organic solvents. They were shown to catalyze a variety of reactions, such as Michael additions and a three component a-aminophosphonate synthesis (see Fig. 2.44) in water [145]. 

The main drawback in the use of water (low solubility of most organic substances in water) would be overcome by using surfactants, which solubilize organic materials or form emulsions with them in water. To address this solubility issue, therefore, we planned to use surfactants, hopefully small amounts of them, for the Lewis acid-catalysed reactions in water. 

Homoallenic alcohols 8 yield 3,4-dimethylidenetetrahydropyrans in a Prins reaction with aldehydes <03CC346> and an intramolecular Prins cyclisation occurs in water using a Lewis acidic surfactant <03OL4521>. Application of the Prins reaction to alkynols instead of alkenols yields 5,6-dihydro-2//-pyrans (Scheme 10) <03OL1979>. 

A recent notable finding in this field is Mukaiyama aldol reactions in aqueous medium (THF H20 = 9 1) catalyzed by metal salts. Lewis acids based on Fe(II), Cu(II), and Zn(II), and those of some main group metals and lanthanides are stable in water. Remarkably, the aldol reaction shown in Sch. 29 occurs more rapidly than the hydrolysis of the silyl enol ether [137]. In the presence of surfactants (dodecyl sulfates or dodecane sulfonate salts), reactions of thioketene silyl acetals with benzaldehyde can be performed in water [138]. 

Yb(OTf)3 and Sc(OTf)3 are water-stable Lewis acids [73], and when combined with a surfactant such as SDS, the three-component coupling also proceeded ef-... 

These Lewis acid-surfactant-combined catalysts (LASCs) were found to form stable colloidal dispersion systems with organic substrates in water and efficiently catalyze aldol reactions of aldehydes with very water-labile silyl enol ethers. 

A fluorous surfactant covalently tethered to silica provides a thin film of perfluorinated solvent for reactions and/or extractions. This material was used for the small-scale hydrocyclization of 6-bromo-1-hexene with NaBH and a catalytic amount of a fluorous tin bromide in 1-butanol. The yield of methylcyclopentane was modest, however, and this technology is a long way from being viable on an industrial scale. The same idea has been more successfully employed with fluorous silica-supported tin Lewis acid catalysts for Baeyer-Villiger oxidations. ... 

Several procedures for a one-pot Mannich-type reaction in water to give (3-amino carbonyl compounds catalyzed by either Lewis acid or Bronsted acid with or without addition of surfactants have been developed. The reactions are high yielding however, the diastereoselectivities are moderate. The HBF4-catalyzed reaction between aldimines and ketene silyl acetals in a water/SDS mixture provides high stereoselectivity with very good yields (Scheme 5.19). 

As for surfactants, they have uncertain, sometimes contradictory, consequences on reaction rates [45], but the main advantage of using surfactants as additives lies in their solubilizing effect. Special attention has been paid to the rate-accelerating effect of Lewis acid catalysts. The first study deals with the Diels-Alder reaction between cyclopentadiene and a bidentate dienophile a large acceleration can be achieved by the combined use of copper(II) nitrate as catalyst and water as solvent. The rate enhancement imposed on the catalyzed Diels-Alder reaction is much less pronounced than that for the uncatalyzed reaction... 

Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

Lewis acid catalysis in micellar systems was first found in the model reaction of the silyl enol ether of propiophenone with benzaldehyde. Although the reaction proceeded sluggishly in the presence of 0.2 equiv. Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 0.2 equiv. Yb(OTf)3 in an aqueous solution of sodium dodecylsul-fate (SDS, 0.2 equiv., 35 mM), and the corresponding aldol adduct was obtained in a 50% yield. In the absence of the Lewis acid and in surfactant (water-promoted conditions) [11], only 20% yield of the aldol adduct was isolated after 48 h, while a 33% yield of the aldol adduct was obtained after 48 h in the absence of the Lewis acid in an aqueous solution of SDS. The amounts of the surfactant also influenced the reactivity, and the yield was improved when Sc(OTf)3 was used as a Lewis acid catalyst. Judging from the critical micelle concentration, micelles would be formed in these reactions, and it is noteworthy that the Lewis acid-catalyzed reactions proceeded smoothly in micellar systems [25]. 

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