Lewis acid surfactant combined catalyst

Sc(() l f) ( is an effective catalyst of the Mukaiyama aldol reaction in both aqueous and non-aqueous media (vide supra). Kobayashi et al. have reported that aqueous aldehydes as well as conventional aliphatic and aromatic aldehydes are directly and efficiently converted into aldols by the scandium catalyst [69]. In the presence of a surfactant, for example sodium dodecylsulfate (SDS) or Triton X-100, the Sc(OTf)3-catalyzed aldol reactions of SEE, KSA, and ketene silyl thioacetals can be performed successfully in water wifhout using any organic solvent (Sclieme 10.23) [72]. They also designed and prepared a new type of Lewis acid catalyst, scandium trisdodecylsulfate (STDS), for use instead of bofh Sc(OTf) and SDS [73]. The Lewis acid-surfactant combined catalyst (LASC) forms stable dispersion systems wifh organic substrates in water and accelerates fhe aldol reactions much more effectively in water fhan in organic solvents. Addition of a Bronsted acid such as HCl to fhe STDS-catalyzed system dramatically increases the reaction rate [74]. 

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... 

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. 

Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [22]. A high promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant combined catalysts as shown in Eq. (2)). The surfactant is here the anion of dodecanesulfonic add. 

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]. 

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. 

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... 

The concept of surfactant-type catalysts described above was also found to be applicable to catalytic systems other than Lewis acid-catalysed reactions. For example, we have developed palladium-catalysed allylic substitution reactions using a combination of Pd(PPh3)4 and a non-ionic surfactant, Triton X-100 [32]. 

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. 

Lewis acid-surfactant combined catalysts (LASCs) are possible solutions to address this issue. LASCs designed from water-compatible Lewis acids are expected to act as surfactants as well as Lewis acids in water. One example of LASCs that can be readily prepared from scandium chloride and sodium trisdodecylsulfate in water is scandium trisdodecylsulfate ([ScCDSIj]), shown in Figure... 

Manabe K, Mori Y, Wakabayashi T et al. (2000) Organic synthesis inside particles in water Lewis acid-surfactant-combined catalysts for organic reactions in water using colloidal dispersions as reaction media. J Am Chem Soc 122 7202-7207... 

Recently, a new type of catalyst, namely Lewis acid-surfactant-combined catalyst (LASC), has shown high efficiencies. These reactions are promoted in water without organic cosolvents (09T587). LASCs as metal dodecyl sulfates (07M174) are efficiently employed in the Friedlander quinoline synthesis (06M1253, 07ASC1047). 

Related Reagents. Related polymer-supported scandium tri-flates, i.e. Nafion-Sc, MC Sc(OTf)3, PA-Sc-TAD, and a polymer-supported scandium that works efficiently in water. A Lewis acid-surfactant combined scandium catalyst, scandium tris(dodecylsulfate). ... 

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