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Open AccessArticle

(±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Ligand for Cu-Catalyzed Asymmetric Conjugate Addition Reaction

Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
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Catalysts 2019, 9(9), 780; https://doi.org/10.3390/catal9090780
Received: 31 August 2019 / Revised: 12 September 2019 / Accepted: 16 September 2019 / Published: 19 September 2019
(This article belongs to the Special Issue Transition Metal Complexes as Catalysts in Organic Chemistry)
Bis(NHC) ligand precursors, L1, based on trans-1,2-diaminocyclohexane were designed and synthesized. To introduce chirality at the hydroxyamide side arm on the NHC of L1, a chiral β-amino alcohol, such as enantiopure leucinol, was used. Cu-catalyzed asymmetric conjugate addition reactions of cyclic and acyclic enones with Et2Zn were selected to evaluate the performance of L1 as a chiral ligand. For the reaction of cyclic enone, a combination of [bis(trimethylsilyl)acetylene]-(hexafluoroacetylacetonato)copper(I) (Cu(hfacac)(btmsa)) with a (±)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (rac; S,S)-L1, which was prepared from (S)-leucinol, was the most effective. Thus, treating 2-cyclohexen-1-one (3) with Et2Zn in the presence of catalytic amounts of Cu(hfacac)(btmsa) and (rac; S,S)-L1 afforded (R)-3-ethylcyclohexanone ((R)-4) with 97% ee. Similarly, use of (rac; R,R)-L1, which was prepared from (R)-leucinol, produced (S)-4 with 97% ee. Conversely, for the asymmetric 1,4-addition reaction of the acyclic enone, optically pure (−)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (R,R; S,S)-L1, worked efficiently. For example, 3-nonen-2-one (5) was reacted with Et2Zn using the CuOAc/(R,R; S,S)-L1 catalytic system to afford (R)-4-ethylnonan-2-one ((R)-6) with 90% ee. Furthermore, initially changing the counterion of the Cu precatalyst between an OAc and a ClO4 ligand on the metal reversed the facial selectivity of the approach of the substrates. Thus, the conjugate addition reaction of 5 with Et2Zn using the Cu(ClO4)2/(R,R; S,S)-L1 catalytic system, afforded (S)-6 with 75% ee. View Full-Text
Keywords: asymmetric catalysis; conjugate addition; reversal of enantioselectivity; N-heterocyclic carbene; ligand design asymmetric catalysis; conjugate addition; reversal of enantioselectivity; N-heterocyclic carbene; ligand design
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MDPI and ACS Style

Ishibashi, A.; Kamihigashi, S.; Iwai, Y.; Sakaguchi, S. (±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Ligand for Cu-Catalyzed Asymmetric Conjugate Addition Reaction. Catalysts 2019, 9, 780.

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