Silyl enolates, aldol reactions, scandium

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. 

Quite recently, it has been found that scandium triflate (Sc(OTf)rcatalyzed aldol reactions of silyl enol ethers with aldehydes can be successfully carried out in micellar systems [24], While the reactions proceeded sluggishly in pure water (without organic solvents), remarkable enhancement of the reactivity was observed in the presence of a small amount of a surfactant (cf. Section 4.5). 

Scandium triflate-catalyzed aldol reactions of silyl enol ethers with aldehyde were successfully carried out in micellar systems and encapsulating systems. While the reactions proceeded sluggishly in water alone, strong enhancement of the reactivity was observed in the presence of a small amount of a surfactant. The effect of surfactant was attributed to the stabiMzation of enol silyl ether by it. Versatile carbon-carbon bondforming reactions proceeded in water without using any organic solvents. Cross-linked Sc-containing dendrimers were also found to be effective and the catalyst can be readily recycled without any appreciable loss of catalytic activity.Aldol reaction of 1-phenyl-l-(trimethylsilyloxy) ethylene and benzaldehyde was also conducted in a gel medium of fluoroalkyl end-capped 2-acrylamido-2-methylpropanesulfonic acid polymer. A nanostmctured, polymer-supported Sc(III) catalyst (NP-Sc) functions in water at ambient temperature and can be efficiently recycled. It also affords stereoselectivities different from isotropic solution and solid-state scandium catalysts in Mukaiyama aldol and Mannich-type reactions. 

Catalytic systems comprising of sulfonated calix[n]arenes with alkyl group at the lower rim (n = 4, 6) and scandium inflate were used for the Mukaiyama aldol addition (Scheme 4.14). It was established that cahxarenes stabilized labile silyl enol ester and allowed reaction to be carried out in water [66]. 

Pioneering work by Lubineau and coworkers showed that the aldol type reaction between a silyl enol ether and an aldehyde (the so-called Mukaiyama aldol reaction) occurred in water at room temperature with high syn stereoselectivity, albeit in low yields.However, the development of water-tolerant Lewis acids for this reaction has led to improved rates and chemical yields. Various lanthanides triflates, such as ytterbium triflate [Yb(OTf)3], scandium triflate [Sc(OTf)3], gadolinium triflate [Gd(OTf)3], or lutetium triflate [Lu(OTf)3], have been found to afford the aldol products between various aldehydes and silyl enol ethers in high yields in aqueous media, with good to moderate syn/anti diastereoselectivi-ties (Scheme 8.3, Table 8.1). ... 

As mentioned in the previous section, lanthanide and scandium triflates (Ln(OTf)3 and Sc(OTf)3) are stable Lewis acids in water, and aldol reactions of silyl enol ethers with aldehydes proceed smoothly in aqueous media in the presence of a catalytic amount of the lanthanide salt. While the reactions were successfully carried out in THF-water or toluene-ethanol-water, lower yields were obtained in pure water. In the course of our investigations to develop new synthetic reactions, especially carbon-carbon bond-forming reactions, in aqueous media, we have found that such reactions proceeded smoothly in micellar systems. 

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