Lanthanide Triflate-Promoted Reactions in Aqueous Media

1 Lanthanide Triflate-Promoted Reactions in Aqueous Media [2] 

Feringa et al. developed aqueous Michael reactions catalyzed by Yb(OTf)3 (Eq. 1) [4]. fi-keto esters and a-nitro esters could be used as Michael donors for the reaction. a,y3-Unsaturated ketones and a,/5-unsaturated aldehydes without /S-substituents were good Michael acceptors for the reaction but the reaction did not proceed with ethyl acrylate or acrylonitrile. The reported yields were excellent, though the reaction required prolonged reaction time (3-5 days). 

Kobayashi et al. studied the catalytic activity of many metal salts in Mukaiyama-aldol reactions in aqueous THE They came to the conclusion that the catalytic activity of a metal in aqueous media should be related both to the hydrolysis constant, /Ch, and water exchange rate constant (WERC) of the metal [8]. All metals with good catalytic activity had p/Ch values ranging between 4.3 and 10.08 and WERC 3.2 X 10 s This was because when for a metal is 4.3, the metal cation is readily hydrolyzed to generate oxonium ion, which then helps the decomposition of the silyl enol ethers. When pMh 10.08 the Lewis acidity of the metal is too low to promote the reaction. When the WERC is 3.2 x 10 m s, exchange of water molecules seldom occurred and aldehydes had a very little chance to coordinate to the metal to be activated. The metals which fulfill these criteria are Sc(III), Fe(II), Cu(II), Zn(ll), Y(IIl), Cd(Il), Ln(Ill) and Pb(ll). 

The high sensitivity of lanthanide reagents to steric factors is also observed in the cyanosilylation reaction of ketones catalyzed by ytterbium cyanide, Yb(CN)3 (Eq. 7) [10], Other reactions, for example epoxide and the aziridine opening by tri-methylsilyl cyanide, TMSCN, are also efficiently catalyzed by Yb(CN)3 [11]. This Yb reagent is not regarded as a Lewis acid but as the active species in these reactions. 

Kobayashi et al. also reported interesting chemoselectivity of aldehydes and imines in the Yb(OTf)3-catalyzed addition reactions of silyl enol ether, allylstannane or trimethylsilyl cyanide [12]. In the competitive reactions between aldehydes and imines, the imines reacted faster than the aldehydes (Tables 4-6). This tendency is not unique to Yb as catalyst selectivity is similar for other Ln(OTf)3. Nuclear magnetic resonance (NMR) studies revealed selective formation of an imine-Yb(OTf)3 complex in the presence of an aldehyde. This preference was reversed when conventional Lewis acids (SnCE, TiCU, TMSOTf, and BF3 OEt2) were used. 

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