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1,4‐Addition of triorganozincates and silyldiorganozincates to α,β‐unsaturated ketones
Author(s) -
Tückmantel Werner,
Oshima Koichiro,
Nozaki Hitosi
Publication year - 1986
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19861190513
Subject(s) - chemistry , cobalt , reagent , medicinal chemistry , silylation , reactivity (psychology) , lithium (medication) , catalysis , methylation , magnesium , molar ratio , organic chemistry , stereochemistry , medicine , biochemistry , alternative medicine , pathology , gene , endocrinology
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.