LASC Lewis acid-surfactant-combined catalysts

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]. 

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... 

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. 

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]. 

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... 

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). 

This type of catalyst was named Lewis acid-surfactant combined catalyst (LASC), and was expected to possess the characters of both a Lewis acid and a surfactant. Sc(DS)3 showed quite high activity in aldol reactions in water without using any organic solvents (Scheme 12.63). A kinetic study on the initial rate of this reaction revealed that the reaction in water is about 130 times faster than that in dichloromethane. It was assumed that hydrophobic reaction environments were created by combining Sc(DS)3 and substrates under the conditions, and that they were concentrated in water to realize efficient catalysis. Interestingly, under neat conditions, the reaction proceeded much slower to give the desired adduct in a much lower yield. 

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