(600i) Asymmetric Hydrosilylation of Aromatic Ketones Catalyzed By an Economical and Effective Copper-Diphosphine Complex

Asymmetric hydrosilylation of prochiral ketones catalyzed by transition metal complexes is an easy way for the access to chiral secondary alcohols, which are very important intermediates in organic synthesis. However, most of the catalytic systems for the hydrosilylation reaction require anhydrous anaerobic atmosphere as well as low temperature, which limit the widespread application. We report here the combination of air and moisture stable Cu(OAc)₂·H₂O and the commercial diphosphine BINAP using the inexpensive polymer siloxane PMHS as reducing agent, which is economical and effective for the asymmetric hydrosilylation of aromatic ketones. The active species of the catalytic system was formed in situ involving Cu(OAc)₂·H₂O, BINAP and PMHS. Basically, 3 mol% Cu²⁺ loading, Cu²⁺∕ligand ratio of 3:1 and 3 equivalents of PMHS were used in these experiments. Although the reaction rate was slower under air atmosphere than nitrogen atmosphere, the enantiometic excesses of (R)-1-arylethanol remained unchanged. The results indicated that the enantiomeric excesses of (R)-1-arylethanol achieved the highest (up to 84% ee) and all of conversion of the aromatic ketones exceeded 98% at 15 ^oC in air atmosphere. Furthermore, the addition of t-BuOK and t-BuOH to the reaction system at room temperature in air atmosphere was also examined. It came out that the reaction fully completed in shorter time and the enantiometic excesses could go up a little bit (up to 89% ee).