An enantioselective synthesis of optically pure azaferrocenyl anions - First general and practical approach to chiral azaferrocenes

Organic Letters

Volumn 3, Issue 4, 2001, Pages 499-501

  Hansen, Jimmi Gerner ^a   SØtofte, Inger ^a   Johannsen, Mogens ^a  

^a TECHNICAL UNIVERSITY OF DENMARK   (Denmark)

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ARTICLE;

EID: 0000800202     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol015502b     Document Type: Article

References (23)

1. (a) Togni, A.; Hayashi, T. Ferrocenes; VCH: Weinheim, 1995.
2. (b) Richards, C. J.; Locke, A. J. Tetrahedron: Asymmetry 1998, 9, 2377.
3. (c) Togni, A.; Halterman, R. L. Metallocenes; Wiley-VCH: Weinheim, 1998; Vol. 2, pp 685-721.
4. Marquarding, D.; Klusacek, H.; Gokel, G.; Hoffmann, P.; Ugi, I. J. Am. Chem. Soc. 1970, 92, 5389; see also ref la-c.
5. (a) Fu, G. C. Acc. Chem. Res. 2000, 33, 412.
6. (b) Dosa, P. I.; Ruble, J. C.; Fu, G. C. J. Org. Chem. 1997, 62, 444.
7. (c) Ruble, J. C.; Fu, G. C. J. Org. Chem. 1996, 61, 7230.
8. (d) Hodous, B. L.; Ruble, J. C.; Fu, G. C. J. Am. Chem. Soc. 1999, 121, 2637.
9. (e) Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 1998, 120, 10270.
10. For the first resolution of a chiral azaferrocene (2-methylazaferrocene) see; Bauer, K.; Falk, H.; Schlögl, K. Angew. Chem., Int. Ed. Engl. 1969, 8, 135.
11. During the final stage of this work Fu et al. reported on the successful flash column separation of two diasteromeric azaferrocenyl complexes, which could be used as precursors for his 2-hydroxymethyl azaferrocene catalysts. No experimental details were included, however. See ref 3a.
12. We have observed that only a few of our azaferrocenes could be resolved on some of the most common analytical HPLC colums (vide infra).
13. Earlier reports on the selective ortho lithiation of azaferrocenes describe problems with concomitant 1′,2-dilithiations and monolithiation of the Cp ring. By introducing a Cp* ring in the azaferrocene only monolithiation of the complex should be possible. For the ealier reports on ortho lithiation of azaferrocenes and related complexes see, e.g., (a) Zakrzewski, J. Heterocycles 1990, 31, 383.
14. (b) Pyshnograeva, N. I.; Setkina, V. N.; Kursanov, D. N. J. Organomet. Chem. 1983, 251, C41.
15. (c) Zakrzewski, J. J. Organomet. Chem. 1989, 362, C31.
16. Previous attempts to add directly lithiated metallocenes and related benzene chromium tricarbonyl complexes to menthyl p-tolyl sulfinate gave partly racemized products with ee's in the range of 6-89%. The best procedure involved the cannulation of the metalated ferrocene to a 2-fold excess of the sulfinate, giving the product in 83% ee: (a) Riant, O.; Argouarch, G.; Guillaneux, D.; Samuel, O.; Kagan, H. B. J. Org. Chem. 1998, 63, 3511
17. (b) Davies, S. G.; Loveridge, T.; Fatima, M.; Teixeira, C. C.; Clough, J. M. J. Chem. Soc., Perkin Trans. 1 1999, 3405.
18. (c) See also: Hua, D. H.; Lagneau, N. M.; Chen, Y.; Robben, P. M.; Clapham, G.; Robinson, P. D. J. Org. Chem. 1996, 61, 4508.
19. The compounds are designated according to a slightly modified procedure of Schlögl. See ref 1a, pp 173-4. The azaferrocenes are viewed from the top, i.e., the ring to be assigned, and the nitrogen atom is given first priority followed by the ortho substituent. The subscript p is used for planar chirality and s for sulfur chirality.
20. 3, Z = 4, R1 = 0.0241, wR2 = 0.0639, data/parameters = 4535/226, GOF = 1.033, Flack parameter 0.001(11). The crystallographic data will be available at Cambridge Crystallographic Database.
21. Initially we used the conditions of Kagan et al. to accomplish the transformation; however, after some optimizations it was found that 2.5 equiv of t-BuLi gave the most gratifying results with respect to ee and yield: 1.1 equiv t-BuLi (15 min), 20% yield, >99.5% ee; 2.5 equiv t-BuLi (5 min), 50% yield, >99% ee; 4 equiv t-BuLi (15 min), 36% yield, 97% ee; 4 equiv n-BuLi (60 min), 22% yield, 40% ee.
22. 2PCl, or TMSCl on our columns (Daicel OD-H, AD, and OJ). The results for these reactions will be reported on a later occasion.
23. p)-4e: Ruble, J. C. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, June, 1999.