Mechanism study on asymmetric Michael addition reaction between alkynone and α-angelica lactone catalyzed by chiral N, N'-dioxide-Sc(III) complex

The reaction mechanism and enantioselectivity of asymmetric Michael addition reaction between alkynone (R1) with α-angelica lactone (R2) catalyzed by chiral N, N'-dioxide-Sc(III) complex were investigated at the M06/6-31G(d,p) (acetonitrile, SMD) level. The α-angelica lactone substrate could isomerize to the active enolized form in the presence of Sc(OTf)3 reagent, assisted by the counter trifluoromethanesulfonate anion OTf-. The alkynone substrate and enolized angelica lactone (or its anion) coordinated to Sc(III) center of N,N'-dioxide-Sc(III) complex catalyst simultaneously, forming a high active hexacoordinate-Sc(III) complex. The catalytic reaction occurred via a two-step mechanism, in which C2 Cγ bond formation was predicted to be the chirality-controlling step as well as the rate-determining step (RDS), affording predominant S-enantiomer. The counterion OTf- facilitated C H construction as a proton-shuttle, producing mainly E-configuration product observed in experiment. The steric repulsion from the ortho-substituent of amide moiety as well as the chiral backbone of N, N'-dioxide-Sc(III) catalyst played the key role for chiral induction in the asymmetric reaction. The less destabilizing Pauli repulsion and more stabilizing attractive interaction, especially the orbital interaction, along the si-face attack pathway enhanced the enantiodifference of the two competing pathways for high enantioselectivity. The energy barriers for E/Z isomerization of S or R-enantiomer assisted by HOTf was as high as 34.6–35.0 kcal mol−1, indicating that the product with Z-conformation was difficult to be obtained. These results were in good agreement with experimental observations.