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

• Published in: Journal of the American Chemical Society Vol. 123; no. 11; pp. 2525 - 2529
• Main Authors: Faller, J. W, Grimmond, Brian J, D'Alliessi, Darlene G
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 21.03.2001; Amer Chemical Soc
• Subjects: Catalysis; Chemical Phenomena; Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; Stereoisomerism; MONOXIDE LIGANDS; CHIRAL RUTHENIUM COMPLEXES; DESIGN; EPIMERIZATION; LEWIS-ACID; DIASTEREOSELECTIVITY; HYDROGENATIONS; HYDROCYANATION; ALDEHYDE COMPLEXES; SELECTIVITY
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja003528c

The compounds (R Ru )-[CyRuCl(S)-BINPO]SbF6 and [CyRuCl(S)-TolBINPO]SbF6 (Cy = η6-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(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 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.