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Stereoselective Synthesis of some Chiral a-Ferrocenyl Carbenium Ions

  1. For some early works of enhanced stability of a carbenium center by a ferrocene moiety see references 2-4.
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  9. Reaction of ferrocenyl-stabilised carbocations with water: Substituent, medium, salt, and solvent isotope effects on rates and equilibria. J. Chem. Soc. Perkin Trans. II, 1267-1273, 1979.
  10. To our knowledge, only one example of isolated non racemic ferroce-nyl carbocation has been described, ableit giving some racemization in the solid state.9 Some chiral ferrocenyl carbocations have been in situ synthesized from chiral alcohols and studied by chiroptical methods.10
  11. Allenmark, Chem. Scripta, 7, 97 (1975)
  12. a: Direct observation of an optically active carbonium ion and measurement of the free-energy barrier to racemisation. J. Chem. Soc. Chem. Commun., 182-183, 1973.
  13. Loim, Tetrahedron Lett., 33, 3611 (1992)
  14. a: ?-Trityl perchlorate as an efficient catalyst in the aldol-type reaction. Chem. Lett. 1759-1762, 1984.
  15. An efficient method for the preparation of three cross-aldols from silyl enol ethers and aldehydes using trityl perchlorate as a catalyst. Chem. Lett., 447-450, 1985.
  16. b: Trityl salts as efficient catalysts in the aldol reaction Chem. Lett., 1535-1538, 1985.
  17. A convenient method for stereoselective synthesis of ?-aminoesters. Iron(II) iodide or trityl hexachloroantimonate as an effective catalyst in the reaction of ketene silyl acetals with imines. Chem. Lett., 889-892, 1990.
  18. Taudien, Tetrahedron Lett., 36, 3513 (1995)
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  20. paper in preparation.
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  32. Sammakia Tarek, Latham Hallie A., On the use of ferrocenyl cations as chiral lewis acids: Evidence for protic acid catalysis, 10.1016/0040-4039(95)01440-s
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  35. Similar problems were encountered by Hersch et al. in catalysis by W, Mo and Fe complexes.30
  36. Bonnesen, J. Am. Chem. Soc., 111, 6070 (1989)
  37. A covalent linkage is easy to establish with a nucleophilic function of an achiral substrate. The ferrocenyl fragment will afford a stereocon-trol in diastereoselective reactions and will allow to regenerate the chiral product by nucleophilic substitution. It is known that silyl enol ethers add to ?-ferrocenylcarbenium ions.32. It is interesting to point out that cation 18e reacts diastereoselectively with TMS enol ether (only one diastereomer detected), as in reaction with water which gave only alcohol 19. One may anticipate that the same behavior will occur with other heteroatomic nucleophiles, facilitating the use of ferrocenyl-carbocations as chiral auxiliaries. For example a ferrocenyl cation easily reacts with HPPh2 to generate a ferrocenyl phosphine.33.
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Bibliographic reference Brunner, Andreas ; Taudien, Stephan ; Riant, Olivier ; Kagan, Henri B.. Stereoselective Synthesis of some Chiral a-Ferrocenyl Carbenium Ions. In: Chirality, Vol. 9, no. 5-6, p. 478-486 (1997)
Permanent URL http://hdl.handle.net/2078.1/94845