A mechanistic study of the Lewis acid–Brønsted base–Brønsted acid catalysed asymmetric Michael addition of diethyl malonate to cyclohexenone

Abstract

The Michael addition of diethyl malonate (Michael Donor, MD) to cyclohexenone (Michael Acceptor, MA) catalysed by the mono-lithium salt of (S)- or (R)-BIMBOL in dichloromethane is shown to exhibit biomimetic behavior. A combination of kinetics, spectroscopic studies, synthesis of catalyst analogues, inhibition studies and DFT calculations are used to show that the catalyst activates both components of the reaction and uses a chain of proton transfers to facilitate the deprotonation of diethyl malonate. The initial reaction rate was first order relative to both MA and MD and 0.5 order relative to the catalyst, indicating that an equilibrium exists between monomeric and dimeric forms of the catalyst, with the dimer predominating, but only the monomeric form being catalytically active. This was supported by DOSY ¹H NMR experiments. The importance of the Lewis acidic lithium cation in the catalytic step was established by complete inhibition of the reaction by lithium complexing agents. The importance of the number of OH-groups and their relative intramolecular orientation and acidities in the polyol catalyst was shown by studying the relative catalytic activities of catalyst analogues. DFT calculations allowed the relative energies and structures of the likely intermediates on the reaction coordinate to be calculated and indicated that the ionisation of MD was facilitated due to the Lewis acidity of the lithium cation and hydrogen bond formation between deprotonated MD (MD⁻¹) and the OH groups of the BIMBOL moiety.