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Trends in Organic Chemistry   Volumes    Volume 24 
Asymmetric allylic alkylation reaction catalyzed by copper species bearing a chiral bidentate hydroxyalkyl-N-heterocyclic carbene ligand
Satoshi Sakaguchi, Hiromasa Kawashima, Ryo Kamisue, Yoshiki Kashioka, Yuki Nakahigashi, Ryota Usui
Pages: 41 - 54
Number of pages: 14
Trends in Organic Chemistry
Volume 24 

Copyright © 2023 Research Trends. All rights reserved

A highly tunable hydroxyalkyl-functionalized benzimidazolium salt was developed as a chiral N-heterocyclic carbene (NHC) ligand precursor for the Cu-catalyzed asymmetric allylic alkylation (AAA) reaction of allyl phosphates with diethylzinc (Et2Zn). Treatment of (E)-cinnamyl diethylphosphate (1) with Et2Zn in the presence of a catalytic amount of copper(II)acetylacetonate (Cu(acac)2) combined with azolium salt L7 derived from (S)-2-amino-1-propanol (L-alaninol) yielded (S)-3-phenyl-1-pentene ((S)-2) in 84% yield with 92% enantiomeric excess (ee). Subsequently, the combined catalytic system comprising Cu(acac)2 and azolium salt L8 derived from (R)-1-amino-2-propanol induced a stereoselectivity reversal, yielding (R)-2 in 88% yield with 74% ee. A possible cuprate intermediate bearing both carbene and alkoxide was proposed for the Cu/L7- or Cu/L8-catalyzed AAA reactions, respectively. Based on this assumption, a methoxyalkyl-substituted benzimazolium salt L9 was synthesized and evaluated. No reaction was observed in the Cu/L9-catalyzed AAA reaction, underscoring the criticality of the hydroxy group on the chiral ligand precursor L7 in enhancing catalytic efficiency and stereoselectivity. Additionally, the synthesis of NHC-metal complex from the hydroxyalkyl-functionalized benzimidazolium salt was attempted, and a monodentate RuCl2(NHC)(p-cymene) complex was obtained from the reaction between [RuCl2(p-cymene)]2 and azolium salt L10 derived from L-valinol via the Ag2O method.
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