Scandium triflate-catalyzed aldol reactions

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. 

Quite recently, we have found that scandium-triflate-catalyzed aldol reactions of silyl enol ethers with aldehydes were successfully carried out in micellar systems. While the reactions proceeded sluggishly in water, remarkable enhancement of the reactivity was observed in the presence of a small amount of a surfactant. In these systems, versatile carbon-carbon bond-forming reactions proceeded in water without using any organic solvents. 

Huorous compounds are also potentially useful as additives to promote organic reactions in carbon dioxide. For example, a fluorous alcohol RfCH20H assists asymmetric hydrogenations with non-fluorous ruthenium BINAP catalysts, and a fluorous aryl alkyl ether (C8F17C6H4-P-OC12H25) does so in scandium-triflate-catalyzed aldol and Friedel-Crafts reactions. These additives are presumed to act as solubilizers or emulsifiers to promote contact among the various reaction components. Since they are fluorous, they can be readily recovered from the otherwise organic reaction mixtures for reuse. 

Several groups have described aldol reactions on the solid phase in which either the aldehyde or the enolate are resin bound (Figure 5). Kobayashi et al. reported the scandium triflate catalyzed condensation of thiosilylenol ethers with aryl aldehydes... 

Scandium triflate was found to be an effective catalyst in the aldol reactions [33]. The activities of various triflate catalysts were evaluated in the aldol reaction of 1-trimethylsiloxycyclohexene (2) with benzaldehyde in dichloromethane (Table 8.7). While the reaction scarcely proceeded at -78°C in the presence of Yb(OTf)3 or Y(OTf)3, the aldol adduct was obtained in an 81% yield in the presence of Sc(OTf)3. Obviously, Sc(OTf)3 is more active than Y(OTf)3 or Yb(OTf)3 in this case. (However, Y(OTf)3 and Yb(OTf)3 did catalyze the aldol reaction of 2 with benzaldehyde at room temperature in dichloromethane to afford the aldol adduct in 58% yield (synlanti = 32/68) and 81% yield synlanti = 29/71), respectively.)... 

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