Enantioselective reduction of ketones with triethoxysilane catalyzed by chiral bis-oxazoline titanium complexes

Abstract

Chiral bis-oxazoline titanium complexes [Ti(BOX)₂X₂] prepared from C₂ chiral bis-oxazolines, BuLi and titanium salts, catalyze the enantiosective reduction of ketones in the presence of triethoxysilane.

References

1. R. Hett, C. H. Senanayake and S. Wald, Tetrahedron Lett., 1998, 39, 1705 and references cited therein.
2. Catalytic Asymmetric Synthesis, Ed. I. Ojima, VCH, New York, 1993; R. Noyori, Asymmetric Catalysis In Organic Synthesis, Wiley, New York, 1994; R. A. Sheldon, Chirotechnology, Marcel Dekker, New York, 1993.
3. (a) M. B. Carter, B. Schiøtt, A. Gutiérrez and S. L. Buchwald, J. Am. Chem. Soc., 1994, 116, 11667; (b) X. Verdaguer, U. E. W. Lange, M. T. Reding and S. L. Buchwald, J. Am. Chem. Soc., 1996, 118, 6784; X. Verdaguer, U. E. W. Lange and S. L. Buchwald, Angew. Chem., 1998, 110, 1174; Angew. Chem., Int. Ed., 1998, 37, 1103For the use of other chiral ansa-titanocenes see also: S. Xin and J. F. Harrod, Can. J. Chem., 1995, 73, 999; R. L. Halterman, T. M. Ramsey and Z. Chen, J. Org. Chem., 1994, 59, 2642.
4. G. M. Diamond, R. F. Jordan and J. L. Petersen, J. Am. Chem. Soc., 1996, 118, 8024.
5. B. Chin and S. L. Buchwald, J. Org. Chem., 1996, 61, 5650.
6. For a recent and comprehensive review see: A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron: Asymmetry, 1998, 9, 1.
7. Singh has reported the preparation of various titanium(IV)–bis(oxazoline) complexes, but not their use. The structures of the BOX–Ti complexes, prepared in toluene with a 1:1 ligand to titanium reagent ratio, have been proposed to adopt a trigonal bipyramidal geometry in which two nitrogen atoms of the BOX ligand occupy equatorial sites. The author described such complexes as not quite stable: R. P. Singh, Synth. React. Inorg. Met.-Org. Chem., 1997, 27, 155.
8. M. Nakamura, M. Arai and E. Nakamura, J. Am. Chem. Soc., 1995, 117, 1179; M. Nakamura, A. Hirai and E. Nakamura, J. Am. Chem. Soc., 1996, 118, 8489; S. Hanessian and R.-Y. Yang, Tetrahedron Lett., 1996, 37, 8997.
9. The crystal structures of octahedral titanium(III) and titanium(IV) hydroxyphenyloxazoline complexes have been reported: P. G. Cozzi, C. Floriani, A. Chiesi-Villa and C. Rizzoli, Inorg. Chem., 1995, 34, 2921.
10. In these cases, only racemic alcohol was obtained. Reduction of titanium depending on the ligand is well known problem in metallocene chemistry.
11. Aliphatic, non-branched aromatic and cyclic ketones were reduced with our titanium catalyst in lower ees. For example, the reduction of octan-2-one and indan-2-one afforded the corresponding alcohols in 65% yield, 20% ee and 70% yield, 29% ee, respectively.
12. The absolute configurations of 10a[ref. 3(a)] and 10c(ref. 13) were determined by comparison with the [α]_D values reported in the literature. The absolute configurations of 10b and the other halo ketones were assigned by analogy.
13. S. Itsuno, M. Nakano, K. Miyazaki, H. Masuda, K. Ito, A. Irao and S. Nakahama, J. Chem. Soc., Perkin Trans. 1, 1985, 2039.
14. T. Nakai, M. Mori and H. Imma, Synlett., 1996, 1229.
15. D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118, 9440.