An application of electronic asymmetry to highly enantioselective catalytic Diels-Alder reactions

The compounds (R-Ru)-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 (Cy = eta (6)-cymene), were synthesized from (CyRuCl2)(2) and the appropriate non-C-2-symmetric bisphosphine monoxide ligands (S)-BINPO and (S)-TolBINPO (BINPO = 2,2 ' -bis(dipbenylphosphino)-1,1 ' -binaphthyl) in the presence of NaSbF6. When these complexes were mixed with AgSbF6 the resulting Lewis acids catalyzed the Diels-Alder cycloaddition of cyclopentadiene and methacrolein. The product, (2S)-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde was obtained with excellent diastereoselectivity (up to 99%) and enantioselectivity (up to 99%) in several cases. When the complexes containing the analogous C-2-symmetric bisphosphine;ligands (S)-BINAP and (S)-TolBINAP were employed as catalysts, the Diels-Alder cycloadducts were obtained with much lower enantioselectivity (19 to 50%) for the opposite antipode. Although some of the effect may arise from chelate ring size change, much of the enhanced stereoselectivity of (R-Ru)-[CyRuCl(S)-BINPO] SbF6 and [CyRuCl(S)-TolBINPO]SbF6 can be attributed to the electronic asymmetry at the stereogenic Ru center.