Reversal of stereoselectivity in the Cu-catalyzed conjugate addition reaction of dialkylzinc to cyclic enone in the presence of a chiral azolium compound

摘要

Reversal of enantioselectivity in a Cu-catalyzed asymmetric conjugate addition reaction of dialkylzinc to cyclic enone with use of the same chiral ligand was successfully achieved. The reaction of 2-cyclohexen-1-one (30) with Et_2Zn catalyzed by Cu(OTf)_2 in the presence of an azolium salt derived from a chiral β-amino alcohol gave (S)-3-ethylcyclohexanone (31) in good enantioselectivity. Among a series of chiral azolium compounds examined, the benzimidazolium salt (10) having both a tert-butyl group at the stereogenic center and a benzyl substituent at the azolium ring was found to be the best choice of ligand in the Cu(OTf)_2-catalyzed reaction. Good enantioselectivity was observed when the reaction was conducted by employing a benzimidazolium derivative rather than an imidazolium derivative. The influence of the substituent at the azolium ring on the stereoselectivity of the reaction was also examined. In addition, from the results of the reaction catalyzed by Cu(OTf)_2 combined with an azolium compound derived from (S)-leucine methyl ester, it was found that the hydroxy side chain in the chiral ligand is probably crucial for the enantiocontrol of the conjugate addition reaction. On the other hand, it was discovered from a screening test of copper species that the reversal of enantioselectivity was realized by allowing 30 to react with Et_2Zn in the presence of Cu(acac)_2 combined with the same ligand precursor to afford (R)-31 as a major product. The influence of the stereodirecting group at the chiral ligand on the stereoselectivity in the Cu(acac)_2-catalyzed reaction differed completely from that observed in the Cu(OTf)_2-catalyzed reaction. Reaction with a cyclic enone consisting of a seven-membered ring such as 2-cyclohepten-1-one (40) resulted in increasing the enantioselectivity of the reaction. Thus, treatment of 40 with Et_2Zn catalyzed by Cu(OTf)_2 combined with a benzimidazolium salt produced the corresponding (S)-conjugate adduct in a 92:8 enantiomer ratio (er), while the Cu(acac)_2-catalyzed reaction with the same ligand afforded (R)-product in a 9:91 er.