The Quest of Configurationally Stable Chiral Allyl Zinc Compounds

Markus Leibeling, Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa, Israel
Ilan Marek, Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa, Israel

Organometallic species bearing a stereogenic carbon center attached to the metal depict an important class of chiral intermediates. After reaction with different organic electrophiles, a variety of chiral products can be obtained. In this context the preparation of chiral organolithium reagents has been an active research field and led to configurationally stable organometallic compounds, which proved to be of great utility in organic synthesis. The absence of a heteroatom at the a- position led to configurationally unstable organolithium compounds. It is anticipated that organometallic compounds bearing a more covalent carbon-metal bond, such as carbon-zinc bonds, should be configurationally more stable.

Our approach makes use of an unprecedented type of “Allyl Zinc Brook Rearrangement” and subsequent electrophilic trapping of the allyl zinc intermediate. Therefore chiral tertiary allyl alcohols were prepared via an asymmetric copper catalyzed 1,2-addition of Grignard reagents to a variety of acyl silanes. Two major questions arise in the course of this project: 1) Would be the newly formed allyl zinc species configurationally stable? and 2) Would a transfer of chirality occur whilst electrophilic trapping?

This novel methodology would be the first example of a chiral and configurationally stable allyl zinc intermediate and can be considered as enantioselective Umpolung of acyl silanes. The scope and limitations of this process, especially with respect to enantiomeric ratios, will be described in detail in the poster.