LASC,

The catalytic activity of micelles bearing catalytically active metal counterions (Lewis acid-surfactant combined catalysts, LASCs) on Diels-Alder reactions was recently investigated [72a, 76]. 

Different results were obtained by Kobayashi and colleagues [76] performing the Diels-Alder reaction of 2,3-dimethyl butadiene with N-butylmaleimide in water in the presence of various dodecyl sulfate (DS) and dodecane sulfonate (DCS) LASCs [M(DS) M = Sc, Cu n = 3, 2 M(DCS) M = Sc, Yb, Mn, Co, Cu, Zn, Na, Ag n = 3,2, 1]. Unexpectedly, no acceleration was observed with respect to the reactions carried out in water only, and no catalytic effect was found also by using a bidentate dienophile which, in principle, should be able to coordinate the metal cation in the LASC system. 

LASC Lewis-acid surfactant combined catalyst... 

Moreover, a new type of catalyst, scandium tris(dodecyl sulfate) [Sc(03S0Ci2H25)3,Sc(DS)3] has been developed.62. The catalyst (a Lewis Acid-Surfactant Combined Catalyst, LASC) acts both as a catalyst and as a surfactant, and aldol reactions proceed smoothly in the presence of a catalytic amount of Sc(DS)3 in water, without using any organic solvents (Scheme 16). 

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. 

During our investigations of the reactions mediated by LASCs, we have found that addition of a small amount of a Bronsted acid dramatically increased the rate of the aldol reaction (Eq. 5).[191 This cooperative effect of a LASC and an added Bronsted acid was also observed in the allylation ofbenzalde-hyde with tetraallyltin in water.1201 Although, from a mechanistic point of view, little is known about the real catalytic function of scandium and proton, this cooperative effect of a Lewis acid and a Bronsted acid provides a new methodology for efficient catalytic systems in synthetic chemistry. 

LASC = 5a Bronsted acid = none 10% yield LASC = none Bronsted acid = HCI 0% yield LASC = 5a Bronsted acid = HCI 67% yield... 

The catalytic asymmetric aldol reaction has been applied to the LASC system, which uses copper bis(-dodecyl sulfate) (4b) instead of CufOTf. 1261 An example is shown in Eq. 6. In this case, a Bronsted add, such as lauric add, is necessary to obtain a good yield and enantioseledivity. This example is the first one involving Lewis acid-catalyzed asymmetric aldol reactions in water without using organic solvents. Although the yield and the selectivity are still not yet optimized, it should be noted that this appredable enantioselectivity has been attained at ambient temperature in water. 

To remove this restriction we introduced another scenario [268] where all of fh j three laser polarizations, 20, eu and s2, are perpendicular to one another. This lasc arrangement now allows for transitions between different Mj levels. The first few a these levels are shown in Figure 8.13. Under these circumstances, control surv v J averaging over Mj levels [268]. J... 

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 results mentioned above prompted us to synthesize a more simplified catalyst, scandium tris(dodecyl sulfate) (Sc(DS)3) [23,24]. This new type of catalyst, Lewis acid-surfactant-combined catalyst (LASC) , was expected to act both as a Lewis acid to activate the substrate molecules and as a surfactant to form emulsions in water. Eng-berts and co-workers also reported a surfactant-type Lewis acid, copper bis(dodecyl sulfate) (Cu(DS)2) [25]. Although they studied detailed mechanistic aspects of Diels-Alder... 

TABLE 13.2. Effect of solvents on LASC-catalysed aldol reaction. 

Various substrates have been successfully used in the present LASC-catalysed aldol reaction. Aromatic as well as aliphatic, a, 3-unsaturated and heterocyclic aldehydes worked well. As for silicon enolates, silyl enol ethers derived from ketones as well as ketene silyl acetals derived from thioesters and esters reacted well to give the corresponding adducts in high yields. It is noted that highly water-sensitive ketene silyl acetals reacted smoothly in water under these conditions. 

In the LASC-catalysed reactions, the formation of stable emulsions seemed to be essential for the efficient catalysis. We thus imdertook the observation of the emulsions by means of several tools. Optical microscopic observations of the emulsions revealed the formation of spherical emulsion droplets in water (Figure 13.1). The average size of the droplets formed from 3 in the presence of benzaldehyde in water was measured by dynamic light scattering, and proved to be ca. 1.1 pm in diameter. The shape and size of the emulsion droplets were also confirmed by transmission electron microscopy and atomic force microscopy. 

As an extension of the studies on the LASC systems, we plaimed to develop a Brpsted acid-surfactant-combined catalyst (BASC) , composed of a Blasted acidic group and a hydrophobic moiety, and found that a BASC was an efficient catalyst for Mannich-type reactions in water. 

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