Ketopinic Acid Derived Bis(hydroxy amides) as Cheap, Chiral Ligands for the Enantioselective Ethylation of Aromatic Aldehydes

Readily accessible, C-2- and pseudo-C-2-symmetric bis(hydroxy amides), derived from commercially available (+)-ketopinic acid and protic diamines, are promising, cheap, chiral ligands for the synthetically valuable, enantioselective addition of organozinc reagents to carbon electrophiles. A series of ligands of this type, having key structural differences, has been synthesized and tested in the enantioselective ethylation of benzaldehydes and (E)-cinnamaldehyde, in order to gain information on the origin of ligand efficiency. The results obtained allow for the definition of a privileged structural pattern for the design of improved cheap ligands and support interesting models proposed for both the acting catalytic species and the controlling transition states. The most efficient ligands proved to be less efficient than commercially available (-)-MIB; nevertheless, an impressive efficiency level was obtained, which should sustain interest in this cheap type of ligands.