Brucine Diol-Catalyzed Enantioselective Morita-Baylis-Hillman Reaction in the Presence of Brucine N-Oxide

Abstract

Brucine diol (BD) catalyzed asymmetric Morita–Baylis–Hillman (MBH) reaction is observed for the first time. Brucine N-oxide (BNO) was found to not have an effective chiral catalyst. Faster reaction rate was obtained using unsaturated ester or aromatic aldehydes in the presence of BNO. 4-Nitrobenzaldehyde and α,β-unsaturated ketone/ester were converted to the MBH adduct in moderate yields (up to 74%) with 70% ee value by this catalytic system. The mechanism of BD catalysis is probably initiated by conjugating the vicinal diol of BD to the carbonyl group of the aromatic aldehyde through hydrogen bonding. The tertiary amine of BD acts as a nucleophile to activate vinyl ketone for coupling with the carbonyl of aldehyde through an intramolecular carbonylated reaction. Finally, the breakdown of the complex caused the formation of the MBH adduct (a benzyl-allyl alcohol). The chirality of the benzyl-allyl alcohol is likely affected by the interaction of the bulky asymmetric plane of BD.