Abstract
The enantioselective Cu-catalyzed 1,6-boration of (E,E)-α,β,γ,δ-unsaturated ketones is described, which gives homoallylic boronates with high enantiomeric purity and unexpectedly high Z-selectivity. By changing the solvent, the outcome can be altered to give E-allylic boronates.
Highlights
Enriched α-stereogenic alkylboron compounds have numerous applications in organic synthesis.[ 1 – 3 ] One important route to these compounds is the catalytic enantioselective boration of electron-deficient alkenes.[ 4 – 7 ] numerous examples of 1,4-boration are known,[4–7] only a few examples of 1,6-boration have been described.[8,9] Aside from promoting 1,6-boration over competing 1,4-boration, the 1,6-boration of electron-deficient conjugated dienes has the potential to give four products which differ in the position and/or the E/Z geometry of the remaining alkene (Scheme 1)
Acyclic α,β,γ,δ-unsaturated esters and ketones that give products of type A [(E)-γ,δ].[8b]. The ability to access products of type B and D would be advantageous to open up additional avenues for post-boration manipulation
During our studies of enantioselective copper-catalyzed 1,6borations that give E-allylic boronates of type A,[8b] we discovered that α,β,γ,δ-unsaturated ketones with a quaternary center adjacent to the carbonyl group unexpectedly gave significant quantities of Z-homoallylic boronates of type D
Summary
Enriched α-stereogenic alkylboron compounds have numerous applications in organic synthesis.[ 1 – 3 ] One important route to these compounds is the catalytic enantioselective boration of electron-deficient alkenes.[ 4 – 7 ] numerous examples of 1,4-boration are known,[4–7] only a few examples of 1,6-boration have been described.[8,9] Aside from promoting 1,6-boration over competing 1,4-boration, the 1,6-boration of electron-deficient conjugated dienes has the potential to give four products which differ in the position and/or the E/Z geometry of the remaining alkene (Scheme 1). We describe enantioselective copper-catalyzed 1,6borations of acyclic α,β,γ,δ-unsaturated ketones that give homoallylic boronates of type D. During our studies of enantioselective copper-catalyzed 1,6borations that give E-allylic boronates of type A,[8b] we discovered that α,β,γ,δ-unsaturated ketones with a quaternary center adjacent to the carbonyl group unexpectedly gave significant quantities of Z-homoallylic boronates of type D.
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