Ruthenium-Lewis acid catalyzed asymmetric Diels-Alder reactions between dienes and alpha,beta-unsaturated ketones

The complex Ru(Cp)(R,R-BIPHOP-F)(acetone)]SbF(6)], (R,R)-1 a, was used as catalyst for asymmetric Diels-Alder reactions between dienes (cyclopentadiene, methylcyclopentadiene, isoprene, 2,3-dimethylbutadiene) and alpha,beta-unsaturated ketones (methyl vinyl ketone (MVK), ethyl vinyl ketone, divinyl ketone, alpha-bromovinyl methyl ketone and alpha-chlorovinyl methyl ketone). The cycloaddition products were obtained in yields of 50-90 % and with enantioselectivities up to 96 % ee. Ethyl vinyl ketone, divinyl ketone and the halogenated vinyl ketones worked best and their reactions with acyclic dienes consistently provided products with >90 % ee. alpha-Chlorovinyl methyl ketone performed better than alpha-bromovinyl methyl ketone. The reaction also provided a 4.3.1]bicyclic ring system in 95 % ee through an intramolecular cycloaddition reaction. Crystal structure determinations of Ru(Cp)((S,S)-BIPHOP-F)(mvk)]SbF(6)], (S,S)-1 b, and Ru(Cp)((R,R)-Me(4)BIPHOP-F)(acrolein)]SbF(6)], (R,R)-2 b, provided the basis for a rationalization of the asymmetric induction.