Two methods for catalytic generation of reactive enolates promoted by a chiral poly gd complex: application to catalytic enantioselective protonation reactions.J Am Chem Soc. 2009 Mar 25; 131(11):3858-9.JA
A chiral polynuclear Gd complex derived from Gd(O(i)Pr)(3) and FujiCAPO (2 or 3) catalytically generated Gd enolates through two distinct methods; transmetalation from enol silyl ethers and conjugate addition of cyanide to alpha,beta-unsaturated N-acyl pyrroles. These chiral enolates can be enantioselectively protonated by a proton in an asymmetric environment in the polynuclear catalyst. Thus, catalytic enantioselective protonation of enol silyl ethers was promoted by the Gd catalyst (5-10 mol %) in the presence of a stoichiometric amount of 2,6-dimethylphenol. Kinetic studies and dependencies of the enantioselectivity on the silyl group structure and the proton source suggest that the reaction proceeds through a Gd enolate generated through transmetalation. Moreover, the same Gd complex (5-10 mol %) promoted conjugate addition of a cyanide-enantioselective protonation sequential reaction from alpha,beta-unsaturated N-acyl pyrroles. Because Gd isocyanide was determined to be the active nucleophile in the conjugate addition catalyzed by the Gd complex, enantioselective protonation likely proceeded through a Gd enolate in this case as well. The products are versatile dual functional chiral building blocks for organic synthesis.