Theoretical investigation on mechanism of asymmetric Michael addition of malononitrile to chalcones catalyzed by Cinchona alkaloid aluminium(iii) complex

Abstract

The mechanism of Michael addition of malononitrile to chalcones catalyzed by Cinchona alkaloid aluminium(III) complex has been investigated by DFT and ONIOM methods. Calculations indicate that the reaction proceeds through a dual activation mechanism, in which Al^III acts as a Lewis acid to activate the electrophile α,β-unsaturated carbonyl substrate while the tertiary amine in the Cinchona alkaloid works as a Lewis base to promote the activation of the malononitrile and deprotonation. A stepwise pathway involving C–C bond formation followed by proton transfer from the catalyst to the carbonyl substrate is adopted, and latter step is predicted to be the rate-determining-step in the reaction with an energy barrier of 12.4 kcal mol⁻¹. In the absence of the Al^III-complex, a Cinchona alkaloid activates the carbonyl substrate by a hydrogen bonding of the hydroxyl group, involving a higher energy barrier of 30.4 kcal mol⁻¹. The steric repulsion between the phenyl group attached to the carbonyl group in the chalcone and isopropoxyl groups of the Al^III-complex may play an important role in the control of stereoselectivity. The π–π stacking effect between the quinuclidine ring of the quinine and the phenyl group of the chalcones may also help the stabilization of the preferred molecular complex. These results are in agreement with experimental observations.