C2-symmetric functionalized azolium salt from serine ester for Cu-catalyzed asymmetric conjugate addition reaction

• Functionalized N-heterocyclic carbene ligand precursor have been synthesized.
• The azolium salt derived from serine ester was air-stable and easy to handle.
• A potential application of the chiral ligand was explored in the asymmetric reaction.
• Enantioselective copper-catalyzed conjugate addition reaction was achieved.

C2-symmetric ester-amide functionalized azolium salt was synthesized from readily available α-amino ester such as L-serine methyl ester. The combination of a Cu salt and the chiral azolium salt promoted the asymmetric conjugate addition reaction of enones with dialkylzincs. Thus, treatment of acyclic enone such as chalcone with Et2Zn afforded the corresponding 1,4-adduct with up to 85% ee. An excellent ee value of 93% was obtained when 3-nonen-2-one was reacted with Et2Zn. The present catalytic system was found to be useful for the 1,4-addition reaction of cyclic enone. For example, the reaction of 2-cyclohepten-1-one with Et2Zn produced (R)-3-ethylcycloheptanone with 80% ee.

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