· (h5,h5-1S,2R,4S,5R-1,4-Bis(indenyl)-2,5-diisopropylcyclohexane)titanium Dichloride
· (MW 513.38)
(alkene isomerization; titanium;4 ansa-bis(indenyl) ligand; chiral ligand; C2 symmetric ligand; asymmetric catalysis8-10)
Physical Data: mp 264-265 °C (dec 240 °C); d 1.339 g cm-3. Dark green crystals from hexane-CH2Cl2. [a]23D = +2900° (c 0.0202, CH2Cl2).
Solubility: sol CHCl3 and CH2Cl2; insol hexane.
Preparative Methods: 1 the enantiomerically pure, chiral ligand (2) is prepared in two steps from the known (1S,2R,4S,5R)-2,5-diisopropylcyclohexane-1,4-diol (3) (eq 1).2,3 It is obtained in 60% yield as a mixture of alkene regioisomers. The chiral titanocene reagent (1) is prepared by treatment of (2) with 2 equiv of n-Butyllithium followed by reaction with Titanium(III) Chloride and subsequent oxidation (HCl/air in CHCl3) of the product (eq 2). It is obtained in 80% yield as a single stereoisomer.
Asymmetric Alkene Isomerization.1
The chiral titanocene reagent (1) serves as precatalyst for the isomerization of alkene (4) (eq 3). Active isomerization catalyst is obtained by in situ reduction of (1) with Lithium Aluminum Hydride (164 °C, 30 min). Treatment of the achiral substrate (4) with 2 mol % catalyst produced axially dissymmetric product (S)-(5) in 44-76% ee (100% yield). The reaction is slow at room temperature (120 h required for complete reaction); faster rates are obtained at higher temperatures, but at the expense of lower product enantiomeric purity.
Other Chiral Cyclopentadienylmetal Complexes.
The chemistry of chiral cyclopentadienylmetal complexes is covered in a review by Halterman.4 Brintzinger's 1,2-ethylenebis(1-indenyl) ligand is the one most commonly used in the preparation of chiral early metal metallocene catalysts.5,6 Compared to Brintzinger's ligand, (2) has the advantage of producing chiral metallocene complexes as single stereoisomers. A related chiral bis(1-indenyl) ligand developed by Burk and Halterman, possessing the same advantages as (2), incorporates a binaphthyl unit as the chiral, enantiomerically pure bridging group.7
Other Reactions of Chiral Titanocene Derivatives.
Buchwald has recently reported the catalytic asymmetric hydrogenation of imines8,9 and unfunctionalized alkenes10 using chiral titanocene catalysts.
- 1. Chen, Z. L.; Halterman, R. L. JACS 1992, 114, 2276.
- 2. Chen, Z.; Halterman, R. L. SL 1990, 103.
- 3. Chen, Z.; Halterman, R. L. OM 1991, 10, 3449.
- 4. Halterman, R. L. CRV 1992, 92, 965.
- 5. Wild, F. R. W. P.; Wasiucionek, M.; Brintzinger, H. H. JOM 1982, 288, 63.
- 6. Wild, F. R. W. P.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. JOM 1982, 232, 233.
- 7. Burk, M. J.; Colletti, S. L.; Halterman, R. L. OM 1991, 10, 2998.
- 8. Willoughby, C. A.; Buchwald, S. L. JACS 1992, 114, 7562.
- 9. Willoughby, C. A.; Buchwald, S. L. JOC 1993, 58, 7627.
- 10. Broene, R. D.; Buchwald, S. L. JACS 1993, 115, 12 569.
William E. Crowe
Emory University, Atlanta, GA, USA
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