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Enantioselective Cu-catalyzed 1,4-addition of Grignard reagents to cyclohexenone using taddol-derived phosphine-phosphite ligands and 2-methyl-THF as a solvent.

The 1,4-addition (conjugate addition) of C nucleophiles to a,b-unsaturated carbonyl compounds and related activated olefins belongs to the most powerful and reliable tools for C C bond formation. Accordingly, it is frequently exploited in the synthesis of natural products and other complex organic molecules. Three decades of research on asymmetric and, in particular, asymmetric catalytic versions of conjugate addition reactions have resulted in the development of a broad variety of methods. However, the important task of performing the 1,4-addition of simple Grignard reagents, the most common type of organometallic reagents, in an enantioselective fashion still remains a particular challenge. As recently reviewed, several Cu-based catalyst systems have been suggested in the past for the catalytic asymmetric 1,4-addition of Grignard reagents. Nevertheless, high enantioselectivities ( 90% ee) were achieved only in a few special cases, and the reported methods did not find much application owing to limited substrate scope, operational convenience, and accessibility of the chiral ligands required. In 2004, an important advance was made by Feringa and co-workers who, by screening a set of commercially available chiral P,P ligands, identified ferrocene-based diphosphines, in particular Taniaphos (1) and Josiphos (2), as promising ligands for such transformations.