HETEROCYCLES

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■ Chiral Benzimidazolium N-Heterocyclic Carbene Ligands with Hydroxyamide- or Hydroxyalkyl-Functionalized Wingtip for Cu-Catalyzed Asymmetric Allylic Alkylation

Satoshi Sakaguchi* and Shota Takayanagi

*Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan

Abstract

An asymmetric allylic alkylation (AAA) reaction of allyl phosphates with dialkylzinc was studied using a combination of a Cu salt and a bis(hydroxyamide)-functionalized benzimidazolium salt ((R,R; S,S)-L1) derived from (1R, 2R)-(−)-1,2-cyclohexanediamine and (S)-leucinol as a catalyst. Treatment of (E)-cinnamyl diethyl phosphate (1) with Et2Zn under the influence of the Cu(acac)2/(R,R; S,S)-L1 catalytic system under ambient conditions without temperature control afforded (R)-3-phenyl-1-pentene ((R)-2) in 91% yield with 83% ee. The enantioselectvity of the reaction was investigated by evaluating the relationship between the catalyst ee and the product ee in the AAA reaction of 1 with Et2Zn, revealing a clear linear relationship. Investigation of the effect of the leaving group in the allylic substrate on the AAA reaction revealed that using a phosphate leaving group was essential to achieve good catalytic performance. 1,2-Diaminocyclohexane-based bis(N-heterocyclic carbene) azolium ligand L1 exhibited higher reactivity than other chiral azolium ligands having a hydroxyamide functional group. Futhermore, it was found that using the Cu(acac)2/hydroxyalkyl-functionalized benzimidazolium salt L7 system to catalyze the AAA reaction of 1 with Et2Zn led to stereoselectivity reversal to afford (S)-2 in 87% yield with 75% ee. A plausible reaction pathway is proposed.