Copper-Free and Copper-Promoted Conjugate Addition Reactions of Bis(triorganosilyl) Zincs and Tris(triorganosilyl) Zincates

Abstract

In the course of our investigations directed towards an asymmetric copper-catalyzed silyl transfer from bis(triorganosilyl) zincs onto α,β-unsaturated carbonyl compounds, the presence of Lewis acidic lithium cations and the uncatalyzed background reaction were identified as major causes thwarting appreciable enantio­selection. The latter finding underlines once more that copper is often not even required in the conjugate addition of bis(triorgano­silyl) zincs and tris(triorganosilyl) zincates alike.

Key words

catalysis - cuprates - silicon - zinc - copper

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19

Analytical HPLC analysis on a chiral stationary phase using a Daicel Chiralpak AD column (n-heptane-i-PrOH, 99:1 at 25 °C) provided baseline separation of enantiomers of 2: 9.69 min (minor enantiomer) and 10.8 min (major enantiomer).

20

The low yield of 20% is in part due to the small scale, on which extremely air- and moisture-sensitive KC₈ was handled.