Ytterbium catalysis, addition reactions

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). 

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. 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

The use of Montmorillonite clay, a very efllcient acidic catalyst, allows a-alkylation of / -substituted indoles ytterbium triflate can also be used to catalyse such alkylations. This efficient catalysis contrasts with the different, but very instructive, reaction pathway followed when mesityl oxide and 1,3-dimethylindole are combined in the presence of sulfuric acid - electrophilic attack at the already substituted /3-position is followed by intramolecular nucleophilic addition of the enol of the side-chain ketone to C-2. ... 

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