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

Key words

asymmetric catalysis - ketones - protonation - silver - silyl enolates

Significance

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.

Comments

Cyclic trimethylsilyl enol ethers gave generally good yields and ee’s using methanol as the proton source.

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).