Asymmetric 1,4-Addition of Arylboronic Acids to β,γ-Unsaturated α-Ketoesters using Heterogeneous Chiral Metal Nanoparticle Systems

Asymmetric 1,4-addition reactions with beta,gamma-unsaturated alpha-ketoesters are valuable because the resulting chiral ketoester compounds can be converted into various useful species that are often used as chiral building blocks in drug and natural product synthesis. However, beta,gamma-unsaturated alpha-ketoesters have two reactive points in terms of nucleophilic additions, which will lead to the 1,4-adduct, the 1,2-adduct and to the combined 1,4- and 1,2-adduct. Therefore, controlling this chemoselectivity is an important factor for the development of these transformations. Here, we developed an asymmetric 1,4-addition of aryl boronic acids to beta,gamma-unsaturated alpha-ketoesters by using heterogeneous chiral rhodium nanoparticle systems with a chiral diene ligand bearing a secondary amide moiety. The newly developed polydimethylsilane-immobilized rhodium nanoparticle catalysts showed high activity, high chemoselectivity, and excellent enantioselectivity, and this is the first heterogeneous catalytic system for this asymmetric reaction. Metal nanoparticle catalysts were recovered and reused without loss of activity or leaching of metal.