Lithium and magnesium triorganozincates, prepared by combination of ZnC12(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 &addition products 2. The approximate reactivity order obtained from the product distribution using unsymmetrical zincates is tBuCH, 4 tBu. Me < Ph, iBu < Et, nBu, iPr, CH=CH2 a MqPhSi. The latter groups are transferred with good selectivity from mixed reagents derived from Me,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 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, M%PhSiZnR,Li, produce the B-silyl ketones from a variety of unhindered or moderately hindered enones in practically useful yields; one example of an a$-unsaturated ester ( 12) is also included.
1,4-Addition von Triorganozinkaten und Silyldiorganozinkaten an a$-ungesiittigte Ketone Lithium-und Magnesiumtriorganozinkate. die aus ZnCl,(TMEDA) oder Dialkylzink und RLi oder RMgX (3 bzw. 1 Molaquivalent) erhalten werden, reagieren mit 2-Cyclohexen-1-on (1) unter milden Bedingungen in mMigen bis guten Ausbeuten zu den 1,4-Additionsprodukten 2. Aus der Produktverteilung bei Verwendung unsymmetrischer Zinkate ergibt sich die ungef&e Reihenfolge der Reaktivitat: tBuCH, 4 tBu, Me < Ph, iBu < Et, nBu, iPr, CH=CH, + Me,PhSi.
Reagenzien auf der Basis von Me2Zn Bbertragen die letzteren Gruppen mit guter Selektivitiit. Die Methylierung mittels Me3ZnLi wird von Cobaltkomplexen katalysiert. Andere Enone (7 -13) geben durchweg schlechte Ausbeuten, und die cobaltkatalysierte Methylierung von Isophoron (3) wird von einer Dekonjugierung im Sinne einer Kharasch-Reaktion begleitet. Gemischte Silyldialkylzinkate, M%PhSiZnR,Li, ergeben ausgehend von ungehinderten oder mB13ig gehinderten Enonen P-Silylketone in brauchbaren Ausbeuten; an einem Beispiel wird auch die Addition an einen a$-ungesattigten Ester (12) belegt.
The 1 ,Caddition of hydrocarbon groups to a$-unsaturated carbonyl compounds is conventionally achieved using organocuprate reagents l). Only little attention has been paid to the related reactions of diorganozinc compounds 2, and triorganozincates 3), a n d their scope remains to be established. W e became interested in zincates in connection with our work on the silylmetalation of acetylenes" and decided to reexamine their behaviour toward a$-unsaturated ketones.