Copper-Catalyzed Enantioselective Coupling between Allyl­boronates and Phosphates Using a Phenol–Carbene Chiral Ligand: Asymmetric Synthesis of Chiral Branched 1,5-Dienes

Abstract

Details of the Cu-catalyzed enantioselective allyl–allyl coupling reaction between allylboronates and (Z)-allylic phosphates using a new chiral N-heterocyclic carbene (NHC) ligand containing a phenolic hydroxy group are presented. The copper catalysis delivers enantio­enriched chiral 1,5-dienes with a tertiary stereogenic center. Compatibility with various functional groups and the use of earth-abundant and relatively low-toxicity copper as a metal are attractive features of this protocol. The utility of the chiral phenol–NHC ligand for enantioselective copper catalysis with organoboron compounds is demonstrated and enantiodiscrimination models are discussed.

Key words

asymmetric catalysis - allylic substitution - synthetic methods copper catalysis - organoboron compounds

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• 18 The linear α-substitution product was the (E)-isomer.
• 19 Data for Schemes 4 and 5 are taken from ref. 8.

Nakamura and co-workers conducted DFT calculations on the mechanism of the reaction between [MeCu(CN)Li] and allyl acetate to form a square planar, four-coordinate (γ-σ-enyl)copper(III) species [(π-en-σ-yl)copper(III) complex]. Our mechanistic proposal is in accord with Nakamura’s mechanism, in which the (γ-σ-enyl)copper(III) species is not in equilibrium with the corresponding (α-σ-enyl)copper(III) species; the regioselectivity is determined at the oxidative addition step as a consequence of the asymmetric nature of MeCuCN^–. Our proposed mechanism is in accord with Nakamura’s mechanism in that the reaction proceeds through oxidative addition of a cuprate to form the (γ-σ-enyl)copper(III) species followed by reductive elimination. However, the coordination number of copper in the allylcopper(III) complex is different by virtue of bidentate coordination of the anionic phenol–NHC chiral ligand (L). The strongly electron-donating NHC coordination should render the π-en coordination weaker, making the allylic 1,3-copper migration in the allylcopper(III) complex more feasible. See:
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