· (R)-1,1´-Bi-2,2´-naphthotitanium Diisopropoxide
· (MW 450.37)
(mild Lewis acid catalyst for asymmetric oxidations1,2 and allylations7)
Physical Data: mp 127 °C (dec) (pentane/ether).
Solubility: sol dichloromethane, toluene, and ether.
Handling, Storage, and Precautions: titanium is reputed to be of low toxicity.
(R)-1,1´-Bi-2,2´-naphthotitanium diisopropoxide (BINOL-Ti(O-i-Pr)2) (1), most conveniently prepared by the reaction of Titanium Tetraisopropoxide with (R)-1,1´-Bi-2,2´-naphthol through the azeotropic removal of isopropanol, is oxophilic (eq 1). A 1:1 mixture of Ti(O-i-Pr)4 and (R)-BINOL in dichloromethane provides an orange-yellow solution and on removal of solvent gives a pale yellow solid. The molecularity of the 1:1 titanium-binaphthol species has been determined to be 2.3. However, the X-ray crystal structure is trimeric, containing a C2 axis of symmetry.3 This 1:1 complex provides, however, only low enantiomeric excess (ee) in the asymmetric epoxidation of allylic alcohols (eq 2).1,3 However, the oxidation of sulfides
to sulfoxides by t-Butyl Hydroperoxide proceeds catalytically with (1) to afford higher enantioselectivity than Kagan's catalytic method using diethyl tartrate as a chiral ligand (eq 3).4 As Kagan has already reported, the amount of water added exhibits a significant effect upon ee value. A high ee is obtained when 0.5-3.0 equiv of H2O was added to the sulfide, while a decrease of ee is observed when less than 0.5 equiv or more than 3.0 equiv of H2O is used. In the absence of H2O, only a low ee is obtained. Addition of water to (1) provides binaphthol-titanium oxide (BINOL-Ti=O; 2) (eq 4),5 which has been reported to serve as an asymmetric catalyst for the Mukaiyama aldol reaction (eq 5).6
- 1. Finn, M. G.; Sharpless, K. B. In Asymmetric Synthesis, Morrison, J. D., Ed.; Academic: New York, 1985; Vol. 5, pp 247-308.
- 2. Komatsu, N.; Nishibayashi, Y.; Sugita, T.; Uemura, S. TL 1992, 33, 5391. Komatsu, N.; Hashizume, M.; Sugita, T.; Uemura, S. JOC 1993, 58, 4529.
- 3. Martin, C. A. Ph.D. Thesis, Massachusetts Institute of Technology, 1988.
- 4. Kagan, H. B.; Rebiere, F. SL 1990, 643.
- 5. Bradley, D. C.; Gaze, R.; Wardlaw, W. JCS 1955, 721.
- 6. Mukaiyama, T.; Inubushi, A.; Suda, S.; Hara, R.; Kobayashi, S. CL 1990, 1015.
- 7. Keck, G. E., Tarbet, K. H.; Geraci, L. S. JACS 1993, 115, 8467. Keck, G. E.; Krishnamurthy, O.; Grier, M. C. JOC 1993, 58, 6543.
Tokyo Institute of Technology, Japan
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