Enantioselective Protonation of Silyl Enolates Catalyzed by a Binap-AgF Complex

asymmetric catalysis - ketones - protonation - silver - silyl enolates

Significance

<P>A catalytic enantioselective protonation of trimethylsilyl enol ethers using AgF·binap under very mild conditions (CH₂Cl₂-MeOH, -20 °C to 0 °C) is reported. Generally good to excellent yields and ee’s were obtained. This method allows for preparation of enantiomerically enriched α-chiral ketones from racemic ketones by silyl enol ether formation followed by enantioselective protonation.</P>

Comments

<P>Cyclic trimethylsilyl enol ethers gave generally good yields and ee’s using methanol as the proton source.</P><P>In the cases when R³ = aryl, excellent yields and up to 99% ee’s were obtained. Both 6- and 7-membered ring systems work well. However, no acyclic silyl enol ethers were reported in this communication. It was rationalized that Ag(I) acts as a Lewis acid to activate MeOH and F^- interacts with the silyl group in the protonation process. Previously, Yamamoto has reported that a chiral Lewis acid SnCl₄/BINOL-catalyzed enantioselective protonation of silyl enol ethers and ketene disilyl acetals using 2,6-dimethylphenol as the proton source (S. Nakamura, M. Kaneeda, K. Ishihara, H. Yamamoto J. Am. Chem. Soc. 2000, 122, 8120-8130). Nakai has reported cationic Pd(II)/BINAP systems to achieve the same reactions using H₂O as the proton source (M. Sugiura, T. Nakai Angew. Chem. Int. Ed. 1997, 36, 2366-2368).</P>