Dramatic Ligand Effect in Catalytic Asymmetric Reductive Aldol Reaction of Allenic Esters to Ketones

Abstract

A general catalytic asymmetric reductive aldol reaction of allenic esters to ketones is described. Two distinct constitutional isomers were selectively produced depending on the reaction conditions. A combination of CuOAc/(R)-DTBM-SEGPHOS/PCy3 as the catalyst predominantly produced gamma-cis-products in high yield with excellent enantioselectivity (up to 99% ee). The reaction was applicable to both aromatic and aliphatic ketones, including unsaturated ketones. On the other hand, CuF-Taniaphos complexes produced alpha-aldol products with high diastereo- and enantioselectivity (up to 84% ee). The new Taniaphos derivative L3, containing di(3,5-xylyl)phosphine and morpholine units, produced optimum results in the alpha-selective reaction. The products are versatile chiral building blocks in organic synthesis. Furthermore, the basic reaction pattern (i.e., conjugate addition-aldol reaction) was extended to a catalytic enantioselective alkylative aldol reaction to ketones using dialkylzinc reagents as the initiator.