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Group 3-15 metal chlorides (Lewis acids) were classified on the basis of activity and aldehyde- and aldimine-selectivity in an addition reaction of a silyl enol ether. Based on the experimental results, metal chlorides (Lewis acids) were classified as follows: A, active; B, weak; C, inactive for the activation of the aldehyde and/or aldimines. Groups A and B were further divided into A-1 or B-1 (aldehyde-selective), A-2 or B-2 (aldimine-selective), and A-3 or B-3 (neutral). The final classification is as follows: A-1, BCl3, AlCl3, TiCl4, GaCl3, ZrCl4, SnCl4, SbCl5, SbCl3, HfCl4, ReCl5; A-2, ScCl3, FeCl3, InCl3, BiCl3; A-3, NbCl5, MoCl3. MoCl5, SnCl2, TaCl5, WCl5. WCl6, ReCl3, TlCl3; B-1, none; B-2, SiCl4, FeCl2, CoCl2, CuCl, CuCl2, GeCl4, YCl3, OsCl3, PtCl2; B-3, ZnCl2, RuCl3; C, VCl3, CrCl3, MnCl2, NiCl2, RhCl3, PdCl2, AgCl, CdCl2, IrCl3, AuCl, HgCl2, HgCl, PbCl2. This classification has revealed several new fundamental aspects of elements (metal chlorides) as Lewis acids.

In the presence of a catalytic amount of Ln(OTf) 3 or Cu(OTf) 2 , three-component Mannich-type reactions of aldehydes, amines, and silyl enolates proceeded smoothly in micellar systems to afford the corresponding b-amino ketones or esters in high yields. These reactions were successfully carried out in water without using any organic solvents.Much attention has been focused on organic processes in water without use of harmful organic solvents, which is an environmentally-friendly methodology. 1 Recently, we found that lanthanide triflates are stable Lewis acids in aqueous media that efficiently catalyze reactions of carbonyl and related compounds. 2 In addition, Sc(OTf) 3 -catalyzed aldol reactions of silyl enolates with aldehydes in pure water were found to proceed smoothly with the aid of a small amount of a surfactant such as sodium dodecyl sulfate (SDS). 3 In this micellar system, ketene silyl acetals, which are known to be hydrolyzed easily in the presence of even a small amount of water, reacted with aldehydes to afford the corresponding aldol adducts in high yields. These results indicated that an excellent hydrophobic reaction field would be formed under these conditions, and gave us an idea that Mannich-type reactions of aldehydes, amines, and silyl enolates would also proceed smoothly in such a micellar system.The Mannich and related reactions provide one of the most fundamental and useful methods for the synthesis of b-amino ketones and esters. 4 Although the classical protocols include some severe side reactions, new modifications using preformed iminium salts and imines have improved the process. Some of these materials are, however, unstable and difficult to isolate, and deaminations of the products that occur under the reaction conditions still remain as problems. The direct synthesis of b-amino ketones from aldehydes, amines, and silyl enolates under mild conditions is desirable from a synthetic point of view. 5 Our working hypothesis is that aldehydes could react with amines in the hydrophobic micellar system in the presence of a catalytic amount of lanthanide triflate to produce imines, which could react with hydrophobic silyl enolates. 6,7 Milder reaction conditions using lanthanide triflate as a Lewis acid and water as a solvent were expected. First, a model reaction of benzaldehyde, o-methoxyaniline, and 1-phenyl-1-trimethylsiloxyethene was performed in the presence of 0.05 eq. of Sc(OTf) 3 in an aqueous solution of SDS (SDS, 0.2 eq., 35 mM). In a typical procedure, the amine and 1 were combined in the presence of Sc(OTf) 3 in 35 mM SDS, and then the aldehyde was added. 8 The reaction proceeded smoothly at room temperature to afford the corresponding b-amino ketone derivative 2 in 87% yield. It is noted that the dehydration (imine formation) and the coupling reaction between two waterunstable substrates, the imine and the silyl enol ether, occurred successfully in water. 9