1,4‐Addition of triorganozincates and silyldiorganozincates to α,β‐unsaturated ketones

Lithium and magnesium triorganozincates, prepared by combination of ZnCl 2 (TMEDA) with 3 molar equivalents of RLi or RMgX, or from dialkylzinc and 1 molar equivalent of RLi or RMgX, react with 2‐cyclohexen‐1‐one ( 1 ) under mild conditions to produce moderate to good yields of the 1,4‐addition products 2 . The approximate reactivity order obtained from the product distribution using unsymmetrical zincates is t BuCH 2 Lt; t Bu, Me < Ph, i Bu < Et, n Bu, i Pr, CHCH 2 ≪ Me 2 PhSi. The latter groups are transferred with good selectivity from mixed reagents derived from Me 2 Zn. This sequence differs strikingly from that exhibited by unsymmetrical cuprates which transfer neopentyl very easily, and also tert ‐butyl more easily than the corresponding zincates. The methylation with Me 3 ZnLi is catalyzed by cobalt complexes. Other enones ( 7 – 13 ) generally give poor yields, and the cobalt‐catalyzed methylation of isophorone ( 3 ) is complicated by a Kharasch‐type deconjugation. Mixed silyldialkylzincates, Me 2 PhSiZnR 2 Li, produce the β‐silyl ketones from a variety of unhindered or moderately hindered enones in practically useful yields; one example of an α,β‐unsaturated ester ( 12 ) is also included.