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

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Abstract
Lithium and magnesium triorganozincates, prepared by combination of ZnCl2(TMEDA) with3molar 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 products2. The approximate reactivity order obtained from the product distribution using unsymmetrical zincates istBuCH2Lt;tBu, Me < Ph,iBu < Et,nBu,iPr, CHCH2≪ Me2PhSi. The latter groups are transferred with good selectivity from mixed reagents derived from Me2Zn. This sequence differs strikingly from that exhibited by unsymmetrical cuprates which transfer neopentyl very easily, and alsotert‐butyl more easily than the corresponding zincates. The methylation with Me3ZnLi 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, Me2PhSiZnR2Li, 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.