Synthesis and first application of a new family of monophosphine ferrocene ligands (MOPF)

Abstract

A new family of monophosphine ferrocene ligands (MOPF) has been synthesised in two steps from an optically pure ferrocenyl sulfoxide and the first preliminary studies employing these ligands in asymmetric hydrosilylation of styrene are presented.

References

1. For some examples where monophosphine ligands or ligands with a phosphine and a hemilabile coordinating group are superior to the usual bisphosphines, see e.g.: (a) Asymmetric hydrosilylation: Y. Uozumi and T. Hayashi, J. Am. Chem. Soc., 1991, 113, 9887; (b) Asymmetric hydrovinylation: T. V. BajanBabu, N. Nomura, J. Jin, B. Badetich, H. Park and M. Nandi, Chem. Eur. J., 1999, 5, 1963; (c) Asymmetric Grignard cross-coupling: T. Hayashi, K. Hayashizaki, T. Kiyoi and Y. Ito, J. Am. Chem. Soc., 1988, 110, 8153.
2. For the synthesis and application of MOP and other monophosphine ligands see e.g.: (a) Y. Uozumi, A. Tanahashi, S.-Y. Lee and T. Hayashi, J. Org. Chem., 1993, 58, 1945; (b) K. Kitayama, Y. Uozumi and T. Hayashi, J. Chem. Soc., Chem. Commun., 1995, 1533; (c) H. Tye, D. Smyth, C. Eldred and M. Wills, Chem. Commun., 1997, 1053; (d) C. D. Graf, C. Malan, K. Harms and P. Knochel, J. Org. Chem., 1999, 64, 5581; (e) G. Zhu, Z. Chen, Q. Jiang, D. Xiao, P. Cao and X. Zhang, J. Am. Chem. Soc., 1997, 119, 3836; (f) B. L. Feringa, M. Pineschi, L. A. Arnold, R. Imbos and A. H. M. de Vries, Angew. Chem., Int. Ed. Engl., 1997, 36, 2620; (g) B. Bartels and G. Helmchen, Chem. Commun., 1999, 741.
3. The MOPF ligands are structurally different from known ferrocene monophosphines which have a methylene spacer group next to the phosphine. See e.g.T. Hayashi, in Ferrocenes, ed. A. Togni and T. Hayashi, VCH, Weinheim, 1995, p. 118.
4. (a) O. Riant, G. Argourch, D. Guillaneux, O. Samuel and H. B. Kagan, J. Org. Chem., 1998, 63, 3511; (b) D. H. Hua, N. M. Lagneau, Y. Chen, P. M. Robben, G. Clapham and P. D. Robinson, J. Org. Chem., 1996, 61, 4508; (c) M. Tsukazaki, M. Tinkl, A. Roglans, B. J. Chapell, N. J. Taylor and V. Snieckus, J. Am. Chem. Soc., 1996, 118, 685; (d) For references prior to 1996 see: A. Togni, Angew. Chem., 1996, 108, 1581.
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6. F. Rebière, O. Riant, L. Richard and H. B. Kagan, Angew. Chem., 1993, 105, 644.
7. For other asymmetric hydrosilylation reactions of styrene see e.g.: G. Bringmann, A. Wuzik, M. Breuning, P. Henschel, K. Peters and E.-M. Peters, Tetrahedron: Asymmetry, 1999, 10, 3025; G. Pioda and A. Togni, Tetrahedron: Asymmetry, 1999, 9, 3903; T. Okada, T. Morimoto and K. Achiwa, Chem. Lett., 1990, 999.
8. This selectivity has been shown to be substrate dependent. MeO-MOP generally gave better results than H-MOP using non-styrene-type substrates such as e.g. oct-1-ene (up to 96% ee)[ref. 2(b)]. We are currently investigating the MOPF ligands to see whether they display similar behaviour.
9. A representative hydrosilylation–oxidation procedure: To a flame dried Schlenk tube was added [ClPd(C₃H₅)]₂(6.2 mg, 0.017 mmol) and 2b(30 mg, 0.067 mmol) under N₂. The solids were dried on the vacuum line for 1 h before styrene (0.378 ml, 3.3 mmol) and 2 ml of benzene were added via syringe. After another 30 min the silane (0.404 ml, 4 mmol) was added via syringe and the solution was left with stirring for 72 h. An aliquot of the reaction was used for NMR and it showed that the reaction was completed. The crude reaction mixture was slowly added to a suspension of KF (3.9 g, 67 mmol) in 30 ml of MeOH and the resulting solution was stirred for 30 min. The solvent was evaporated in vacuo and the resulting solid was suspended in 40 ml of DMF. Finally 4 ml H₂O₂(35%) was added at room temperature and the reaction was heated to 60 °C for 1 h before quenching with 30 ml saturated Na₂S₂O₃(aq). After aqueous workup and extraction the product was purified by flash column chromatography (15% EtOAc in pentane) giving 6 as a colorless oil in 41% yield (167 mg) and 64% ee detected by HLPC using a Chiralcel OD-H column (10% PrⁱOH–90% hexane, 0.5 ml min^–1). [α]^rt_D–36 (c 1.065, CHCl₃). Spectroscopic data were in accordance with those from a commercial sample.
10. During the course of our investigation we be came aware of a paper from another group synthesising related benzene chromium tricarbonyl complexes. This prompted us to report our findings at this early stage. See: S. C. Nelson and M. A. Hilfiker, Org. Lett., 1999, 1, 1379.