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.
Highlights
Chiral amines are continuing to play a pivotal role as a catalyst in asymmetric synthesis [1]
As the catalytic load was increased to mol% in in nonp reaction to study the catalytic efficiency of Brucine diol (BD)
The reaction time solvent with 5 mol% of BD led to low yields and poor ee values
Summary
Chiral amines are continuing to play a pivotal role as a catalyst in asymmetric synthesis [1]. In the case of chiral tertiary amines, the asymmetric induction by these catalysts is primarily derived from the nitrogen chiral center and a secondary anchoring group is often necessary. These chiral tertiary amines are usually classified as nucleophilic catalysts [6,7,8,9]. A number of tertiary amine N-oxides [10] have been developed and utilized for various asymmetric reactions They are mainly cyclic aliphatic N-oxides [11] and pyridine N-oxides [12,13,14,15], acting as a ligand to coordinate with low valent transition metals [16] for metal-catalyzed reactions. Chiral natural products with bulky shapes and inflexibility [7,17] are used, such as brucine derivatives (1)
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