An Application of Electronic Asymmetry to Highly Enantioselective Catalytic Diels−Alder Reactions

Abstract

The compounds (RRu)-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 (Cy = eta6-cymene), were synthesized from (CyRuCl2)(2) and the appropriate non-C2-symmetric bisphosphine monoxide ligands (S)-BINPO and (S)-TolBINPO (BINPO = 2,2'-bis(diphenylphosphino)-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 C2-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 (RRu)-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 can be attributed to the electronic asymmetry at the stereogenic Ru center.