Titanium(IV) triflates in the catalysis of homoaldol reactions

Organic Letters

Volumn 1, Issue 10, 1999, Pages 1643-1645

  Martins, Evelyn O ^a   Gleason, James L ^a  

^a MCGILL UNIVERSITY   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000225881     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol991017t     Document Type: Article

References (28)

1. (a) Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asymmetric Synthesis; Wiley: New York, 1995.
2. (b) Ojima, I Catalytic Asymmetric Synthesis; VCH: Weinheim, 1993.
3. For reviews on the homoaldol reaction, see: (a) Crimmins, M. T.; Nantermet, P. G. Org. Prep. Proc. 1993, 25, 43-81.
4. (b) Kuwajima, I.; Nakamura, E. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergammon: Oxford, 1991; Vol 2, p 441.
5. For examples of diastereoselective homoaldol reactions, see: (a) Kano, S.; Yokomatsu, T.; Shibuya, S. Tetrahedron Lett. 1991, 32, 233.
6. (b) DeCamp, A.; Kawaguchi, A.; Volante, R.; Shinkai, I. Tetrahedron Lett. 1991, 32, 1867.
7. (c) Armstrong, J.; Hartner, F.; DeCamp, A.; Volante, R.; Shinkai, I. Tetrahedron Lett. 1992, 33, 6599.
8. (d) Houkawa, T.; Ueda, T.; Sakami, S.; Asaoka, M.; Takei, H. Tetrahedron Lett. 1996, 37, 1045. For examples of stereoselective reactions using homoenolate equivalents, see ref 2 and Ahlbrecht, H.; Beyer, U. Synthesis 1999, 365.
9. (d) Houkawa, T.; Ueda, T.; Sakami, S.; Asaoka, M.; Takei, H. Tetrahedron Lett. 1996, 37, 1045. For examples of stereoselective reactions using homoenolate equivalents, see ref 2 and Ahlbrecht, H.; Beyer, U. Synthesis 1999, 365.
10. (a) Nakamura, E.; Oshino, H.; Kuwajima, I. J. Am. Chem. Soc. 1986, 108, 3745-3755.
11. (b) Nakamura, E.; Aoki, S.; Sekiya, K.; Oshino, H.; Kuwajima, I. J. Am. Chem. Soc., 1987, 109, 8056-8066.
12. (a) Noyori, R.; Kitamura, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 49-69.
13. (b) Soai, K.; Niwa, S. Chem. Rev. 1992, 92, 833.
14. Duthaler, R. O.; Hafner, A. Chem. Rev. 1992, 92, 807
15. 4, respectively. See: Reetz, M. T. Organotitanium Reagents in Organic Synthesis, Springer-Verlag: 1986.
16. Bond strengths for Si-X are estimated at 113 and 116 kcal/mol for TMS-Cl and TMS-OEt, respectively. See: Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 371.
17. 4, required stoichiometric amounts of the metal salt.
18. 4. See: Mikami, K.; Terada, M.; Nakai, T. J. Am. Chem. Soc. 1990, 112, 3949.
19. For synthesis of alkoxytitanium(IV) triflates, see: (a) Mikami, K.; Sawa, E.; Terada, M. Tetrahedron: Asymmetry 1991, 2, 1403.
20. (b) Joergensen, K. A.; Gothelf, K. V. J. Chem. Soc., Perkin Trans. 2 1997, 111.
21. iPr)Cl was then added to a suspension of silver triflate in toluene to generate triflate 4b.
22. Prepared by ligand exchange. See: Seebach, D.; Plattner, D. A.; Beck, A. K.; Wang, Y. M.; Hunziker, D.; Petter, W. Helv. Chim. Acta 1992, 75, 2171.
23. Chen, H.; White, P. S.; Gagne; M. R. Organometallics 1998, 17, 5853-5366.
24. 1H NMR spectra of the putative catalysts 4a and 4b indicate the presence of a mixture of interconverting species at room temperature.
25. 2 followed by addition of 1 and benzaldehyde results in complete decomposition of the starting materials.
26. We have found that the catalyst loading can be reduced to 2% or 5% with little change in yield for the reaction with benzaldehyde. However, longer reaction times were required, which was not practical for less reactive substrates.
27. 2, TMSOTf does not promote the homoaldol reaction of 1 with aldehydes. For this reason, and by analogy to ref 4, it is presumed that homoenolate 6 is an intermediate in this process. Thus far, it has not been possible to characterize the homoenolate.
28. Similar activation of aldehydes has been postulated in the zinc-catalyzed process (ref 4).