Asymmetric Baylis-Hillman reactions promoted by chiral imidazolines.

ABSTRACT

The coupling of electrophiles with activated alkenes by using tertiary amines or phosphines is generally known as the Baylis-Hillman reaction. It is a useful and atom-economical carbon-carbon bond-forming reaction that generates multifunctionalized products. This reaction is notoriously slow; yields are often low and substrate-dependent. The asymmetric reaction is still limited especially for unactivated olefins such as acrylates. Imidazolines have been developed as ligands in metal-catalyzed reactions and have also been used as privileged structures in diversity-oriented synthesis. A series of novel chiral imidazolines were prepared and used to develop asymmetric Baylis-Hillman reactions. These imidazolines promote the reactions of various aromatic aldehydes with unactivated acrylates. Enantiomeric excesses of up to 60% and high yields were obtained by using stoichiometric amounts of the promoter. Furthermore, the imidazolines are also suitable promoters for the reactions between aromatic aldehydes and alkyl vinyl ketones. Enantiomeric excesses of up to 78% and high yields were obtained with 50 mol % of an imidazoline with a chiral methylnaphthyl group. These chiral imidazolines are easily prepared from commercially available amino alcohols and can be easily recovered for reuse without loss of product enantioselectivity.