Catalysis Today, Vol.355, 635-644, 2020
Mechanism study on asymmetric Michael addition reaction between alkynone and alpha-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 a-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 alpha-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 C-2-C-gamma 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.