Ytterbium triflate, Michael addition

Ytterbium triflate is an extremely effective catalyst for the Michael addition of a-nitro esters to enones in water (Eq. 4.110).149... 

E. Keller, B. L. Feringa, Ytterbium Triflate Catalyzed Michael Additions of P-Ketoesters in Water Tetrahedron Lett. 1996, 37,1879-1882. 

Significant rate acceleration of metal-catalysed Michael addition reactions of fi-dicarbonyls to a,/S-enones in water was observed upon addition of dibasic ligands. Ytterbium triflate and TMEDA was the most effective combination leading to a nearly 20-fold faster reaction than in the absence of ligand.148... 

Ding, R., K. Katebzadeh, L. Roman, K.-E. Bergquist and U.M. Lindstrom, Expanding the Scope of Lewis Add Catalysis in Water Remarkable Ligand Acceleration of Aqueous Ytterbium Triflate Catalyzed Michael Addition Reactions, Journal of Organic Chemistry, 71, 352-355 (2006). 

Ytterbium triflate-catalyzed Michael addition of iV-methylindole to mesityl oxide 452 was carried out using a modification of the Kerr procedure (4% of Yb(OTf>3, 4equiv of compound 452) <1996SL1047, 1998CJC1256> to give the desired 4-(indol-3-yl)-4-methyl-2-pentanone 453 in 81% yield (Scheme 96) <2001TL6835>. Product 453 was converted into the a-diazo- )-keto ester 454 having an iV-methylindole unit. 

However, during the addition of -keto esters onto enals, ytterbium triflate proved to be an efficient catalyst [171] as in the Michael additions of a-nitro esters [172]. Several water-soluble phosphines gave the corresponding phos-phonium salts in good yields when added to a,j9-unsaturated acids [173] or activated alkynes [174]. With alkynes, vinyl phosphine oxides or alkenes were formed depending on the pH of the aqueous solution. Significantly, the reaction of nitroalkanes with buten-2-one is considerably accelerated when going from... 

The rate of above Michael addition (Scheme 25) was enhanced by the addition of ytterbium triflate [yb(OTf)j]. 

A procedure by using enamino ester and alkynone as the substrates was developed as well [2]. 2,3,4,6-Tetrasubstituted pyridines were prepared in a single step. Various acids, such as acetic acid, Amberlyst 15 ion exchange resin, zinc(II) bromide or ytterbium(III) triflate, can be applied as promoter for the cycUzation step of the Michael addition adduct. 4-(3-Oxoalkyl)isoxazoles found could be applied as starting material for pyridine synthesis as well [3],... 

The first Lewis acid-catalyzed asymmetric Michael addition in water was developed by Kobayashi et al, who reported ee s up to 83%. Very recent developments show great promise for further improvement of Michael addition reactions in water. In an elegant study, Kaneda and coworkers used montmorillonite-enwrapped metal triflates to execute C—C bond forming Michael additions. When scandium triflate was employed, adducts were obtained in quantitative yield within a 0.5-3 h at or slightly above room temperature. The catalysts were reusable with no appreciable loss in activity.In another recent study, Lind-strdm and coworkers observed a remarkable ligand acceleration effect in aqueous ytterbium triflate-catalyzed Michael additions. A number of 1,2-diamines and 1,2-aminoalcohols were shown to have a positive influence on the rate of the reaction, the most efficient being tetramethylethylenediamine, which induced a nearly 20-fold rate acceleration. 

Keller, E. and Eeringa, B.L., Ytterbium triflate catalyzed Michael additions of jB-ketoesters in water. Tetrahedron Lett., 1996, 37,1879-1882. 

Ding, R., Katebzadeh, K., Roman, L., Bergquist, K.-E. and Lindstrom, U.M., Expanding the scope of Lewis add catalysis in water remarkable Ugand acceleration of aqueous ytterbium triflate catalyzed Michael addition reactions, /. Org. Chem., 2006,71,352-355. 

Catalytic amounts (5 mol%) of trityl chloride and tin(II) chloride, or antimony(V) chloride and tin(II) triflate, have also been used to promote the Michael additions, with stereoselectivity in favor of the anti isomers and slightly reduced ratios in comparison with the above results. No stereoselectivity was observed with catalytic amounts (10 mol %) of ytterbium(III) trifluoromethanesulfonate. ... 

Ytterbium triflate, as a water-tolerant Lewis acid, catalyzed the Michael additions of various p-ketoesters toward a-enals (Keller and Feringer, 1996). 

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