Dialkyl Sulfide Stabilisation in a Catalytic Dicuprate 1,6-Dienone Addition Process

The catalytic dicuprate addition of 1,9-dibromononane to a sterol 1,6-dienone is strongly enhanced by the presence of dimethylsulfide. Dimethylsulfide appears to have an important stabilising effect on the active organo-dicuprate species formed by reaction of Cu(I)Cl with di(bromomagnesio)nonane. In the absence of dimethlysulfide, 1,2-addition of the Grignard and loss of acetate from the sterol are the dominant processes. The overall reaction yield is also very sensitive to the order of addition of various reagents, with controlled addition of the preformed Grignard reagent to a dienone/Cu(I)Cl/dimethyl sulfide mixture leading to high yields of the desired alkylated bis-enone.