· (MW 379.59)
(starting material to prepare many kinds of hafnocene(II or IV) derivatives;2 in combination with AgI salt, is an effective activator for glycosyl fluoride3)
Physical Data: mp 230-233 °C.
Solubility: sol aromatic solvents, chloroform; slightly sol THF, ether; insol hexanes.
Form Supplied in: colorless crystals; commercially available.
Analysis of Reagent Purity: 1H NMR (THF-d8) d 6.62 (s, C5H5).
Handling, Storage, and Precautions: the dry solid is reasonably stable to air and moisture and can be handled in an ordinary manner. However, it should be stored in a tightly-sealed nitrogen-flushed bottle to preclude moisture.
Transmetalation and Generation of Cp2Hf Equivalent.
Cp2HfCl2 is an important starting material for the preparation of HfIV and HfII derivatives, just as Dichlorobis(cyclopentadienyl)zirconium is used to prepare zirconium compounds. While the chemistry of Cp2ZrCl2 and its derivatives is well-developed, a corresponding surge in the study of hafnium compounds has not ensued. In general, the chemistry of Cp2HfCl2 and its derivatives is similar to that of zirconium. Transmetalation reactions of Cp2HfCl2 with organometallics containing lithium, sodium, potassium, and magnesium are a general method to prepare Cp2Hf(R)Cl and Cp2HfR2 (R = alkyl, alkenyl, aryl, and alkynyl) (eq 1).2,4 Alkylhafnocenes are thermally more stable than the corresponding alkylzirconocenes in the solid state
and in solution. For example, while dibutylzirconocene rapidly eliminates butane to give a zirconocene-butene complex, thermolysis of the hafnocene analog at 80 °C is required for this process.2a Heating n-butyl-t-butylhafnocene at 45 °C in the presence of trimethylphosphine gives a hafnocene-isobutene complex, which reacts with diphenylacetylene to give an alkyne complex. It also promotes C-H bond activation with benzene or alkoxide (eq 2).2a Alkyl-alkene coupling (eq 3)2b and skeletal rearrangement by C-C bond activation5 to form hafnacyclopentanes have been reported. Generation of Cp2Hf equivalent can be achieved by reduction of Cp2HfCl2 with Magnesium-Mercury(II) Chloride6 or 2 equiv n-Butyllithium.2
Cp2Hf reacts with a series of unsaturated compounds (alkyne and alkene) through a ligand exchange process to give a hafnocene complex; this reaction has been applied to bicyclizations of nonconjugated dienes, enynes, and/or diynes (eq 4).6
The combination of metallocene dichloride (Cp2ZrCl2 or Cp2HfCl2) and silver salt (AgX: X = ClO4, OTf, BF4) is a highly effective reagent system for activating glycosyl fluoride to give the O-,7 C-,8 or N-glycoside9 stereoselectively under mild conditions (eq 5). Selection of the center metal (Zr or Hf), the molar ratio of metallocene dichloride and AgI salt, and the solvent used are crucial factors for controlling the reactivity and stereoselectivity in this glycosidation reaction.7a,10 The Cp2HfCl2 based activators are more reactive than the Cp2ZrCl2 ones9 and the former are more effective than the latter in the case of amino sugar
derivatives.11 Regio- and stereocontrolled C-aryl glycosidation of phenols based on the O-C glycoside rearrangement is efficiently carried out by using the Cp2HfCl2-Silver(I) Perchlorate system (eq 6).12 The high b-selectivity achieved without neighboring group assistance in glycosyl fluoride is a key synthetic advantage of these reagent systems.
- 1. (a) Cardin, D. J.; Lappert, M. F.; Raston, C. L.; Riley, P. I. In Comprehensive Organometallic Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 3, pp 549-633. (b) Cardin, D. J.; Lappert, M. F.; Raston, C. L. The Chemistry of Organo-Zirconium and -Hafnium Compounds; Wiley: New York, 1986.
- 2. (a) Buchwald, S. L.; Kreutzer, K. A.; Fisher, R. A. JACS 1990, 112, 4600. (b) Swanson, D. R.; Rousset, C. J.; Negishi, E.; Takahashi, T.; Seki, T.; Saburi, M.; Uchida, Y. JOC 1989, 54, 3521.
- 3. Suzuki, K.; Matsumoto, J. Synth. Org. Chem. Jpn. 1993, 51, 718.
- 4. (a) Negishi, E.; Swanson, D. R.; Rousset, C. J. JOC 1990, 55, 5406. (b) Beshouri, S. M.; Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P. OM 1990, 9, 2375.
- 5. Takahashi, T.; Fujimori, T.; Seki, T.; Saburi, M.; Uchida, Y.; Rousset, C. J.; Negishi, E. CC 1990, 182.
- 6. Yousaf, S. M.; Farona, M. F.; Shively, R. J. Jr.; Youngs, W. J. JOM 1989, 363, 281.
- 7. (a) Matsumoto, T.; Maeta, H.; Suzuki, K.; Tsuchihashi, G. TL 1988, 29, 3567. (b) Matsumoto, T.; Maeta, H.; Suzuki, K.; Tsuchihashi, G. TL 1988, 29, 3575. (c) Suzuki, K.; Maeta, H.; Suzuki, T.; Matsumoto, T. TL 1989, 30, 6879.
- 8. Matsumoto, T.; Katsuki, M.; Suzuki, K. TL 1989, 30, 833.
- 9. Matheu, M. I.; Echarri, R.; Castillón, S. TL 1992, 33, 1093.
- 10. Suzuki, K.; Maeta, H.; Matsumoto, T. TL 1989, 30, 4853.
- 11. Suzuki, K.; Maeta, H.; Matsumoto, T.; Tsuchihashi, G. TL 1988, 29, 3571.
- 12. (a) Matsumoto, T.; Katsuki, M.; Suzuki, K. TL 1988, 29, 6935. (b) Matsumoto, T.; Katsuki, M.; Jona, H.; Suzuki, K. JACS 1991, 113, 6982.
Takeo Taguchi & Yuji Hanzawa
Tokyo College of Pharmacy, Japan
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