Acetyl-BINOL as mimic for chiral β-diketonates: a building block for new modular ligands

Abstract

α-Acetyl-( S)-BINOL was prepared by ortho-lithiation and subsequent acetylation of acetal-protected ( S)-BINOL. The β-hydroxyketone moiety of this compound is herein a structural mimic for a β-diketonate and forms six-membered chelates with transition metal ions. The second hydroxy-function was submitted to esterification with several carboxylic acids bearing another donor function, thus, new tridentate chiral ligands were obtained. Out of this library the l-proline-α-acetyl-( S)-BINOL-ester was identified to be most effective for the titanium-mediated addition of Et 2Zn to PhCHO yielding the respective secondary alcohol with up to 93% ee, which is better than with using ( S)-BINOL itself. Besides a solvent dependency (use of MeCN is optimal), the proper choice of the counter-ion is crucial: anion exchange of bromide by trifluoroacetate gave a significant increase of enantioselectivity.