Synthesis of Yb Complexes with Amino‐Acid‐Armed Ligands for Direct Asymmetric Tandem Aldol Reduction Reactions

Abstract

AbstractA synthetic route to a series of C2‐symmetric chiral ligands armed with selectively protected amino acids have been developed with the aim to study the potential of the corresponding Yb(III) complexes for enantioselective direct aldol reactions. These ligands, which contain chiral bis(ester) or bis(amide) moieties, were readily prepared in enantiomerically pure form by the reaction of (S,S)‐hydrobenzoin or (S,S)‐diphenylethylenediamine with various chiral amino acids. In this article, the asymmetric aldol‐reduction reaction leading to 1,3‐diols (known as the aldol–Tishchenko reaction) has been performed with an elaborated family of ligands. This unique tandem reaction was catalysed by chiral Yb complexes that promote both the aldol reaction of unactivated carbonyl compounds and the Evans–Tishchenko reduction of the aldol intermediates. 1,3‐anti‐Diols with three stereogenic centers have been isolated as a result of the condensation of aliphatic ketones with aromatic aldehydes with up to 64 % ee. Additional detailed investigations of the nature of the binding of both class of ligands have also been carried out with high‐resolution 1H‐, 13C‐, and 14N‐NMR techniques.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)