Bis(cyclopentadienyl)titanium Bis(trifluoromethanesulfonate)1

(Cp2Ti(OSO2CF3)2) (1)

[76262-87-8]  · C12H10F6O6S2Ti  · Bis(cyclopentadienyl)titanium Bis(trifluoromethanesulfonate)  · (MW 476.20) (Cp2Zr(OSO2CF3)2) (2)

[94728-52-6]  · C12H10F6O6S2Zr  · Bis(cyclopentadienyl)zirconium Bis(trifluoromethanesulfonate)  · (MW 519.54) (Cp*2Ti(OH2)22+) (3)

[121266-23-7]  · C20H34O2Ti  · Bis(pentamethylcyclopentadienyl)diaquatitanium  · (MW 354.37)

(catalyst for Diels-Alder reaction, Mukaiyama crossed-aldol reaction, and Sakurai reaction)

Physical Data: (1) red crystals; 1H NMR (CD3NO2) d 7.06 (s). An X-ray crystal structure has been reported.2

Solubility: sol CH2Cl2, MeNO2; insol benzene, hexane.

Analysis of Reagent Purity: NMR, elemental analysis.

Preparative Method: 1,2 Cp2Ti(OSO2CF3)2 is prepared by treatment of a 0.06 M THF solution of Dichlorobis(cyclopentadienyl)titanium with 5.5 equiv of a 0.19 M THF solution of Silver(I) Trifluoromethanesulfonate. After stirring for 15 min the reaction mixture is filtered, the red filtrate is layered with hexane, and the resulting red crystals are collected. Recrystallization from CH2Cl2/pentane or THF/hexane affords Cp2Ti(OSO2CF3)2 (25-30%) as red needles. The precursors (Cp2TiCl2 and AgOSO2CF3) are widely available.

Handling, Storage, and Precautions: Cp2Ti(OSO2CF3)2 and Cp2Zr(OSO2CF3)2 can be handled for short periods of time in air, but are best handled and stored in a glove box. Both can be prepared in situ. The pentamethylcyclopentadienyl analog Cp*2Ti(OH2)22+ is air and water stable. Titanium(IV) is reputed to be of low toxicity.

General.

The triflate analog of titanocene dichloride, Cp2Ti(OSO2CF3)2, was first prepared in 1980.2 The trifluoromethanesulfonate ligands were found to be covalently bound to the titanium center in the solid state2 and in CD2Cl2 solution,1,3 but are easily displaced by various neutral ligands such as water,4 methyl vinyl ketone (MVK), and nitromethane.1,3 Recent interest in Cp2Ti(OSO2CF3)2 (1), its zirconium analog Cp2Zr(OSO2CF3)2 (2), and in the pentamethylcyclopentadienyl diaqua complex Cp*2Ti(OH2)22+ (3),4 has focused on the ability of these species to function as Lewis acids in various organic transformations.1,3,5-7 It has been found that very low catalyst loadings are required (0.01-1 mol %) for many transformations which typically require stoichiometric Lewis acids. Although most of the work reported to date has concerned simple substrates and NMR scale reactions, isolated yields are noted to be in the 65-90% range for many of these transformations. The generality of the ability of (1)-(3) to mediate reactions of more complex molecules remains to be determined.

NMR studies of the interaction of Cp2Ti(OSO2CF3)2 with MVK indicate that the ketone can displace triflate from the titanium center, affording a number of different species including cationic complexes such as Cp2Ti(OSO2CF3)(MVK)+ and Cp2Ti(MVK)22+. These cationic complexes are formed reversibly and rapidly, and are believed to account for the observed catalytic activity.3 In addition, the zirconium complex (2), when prepared in THF, is isolated as a THF solvate (2.THF).

Diels-Alder Reactions.1,3,5

The Diels-Alder cycloaddition between simple 1,3-dienes such as cyclopentadiene and isoprene with a,b-unsaturated carbonyl compounds such as MVK and acrolein is efficiently catalyzed by (1)-(3) (eqs 1-3). As is typical of Lewis acid-catalyzed Diels-Alder reactions, enhanced endo selectivity is observed compared to the thermal reactions. An advantage of these catalysts over other Lewis acids such as Titanium(IV) Chloride or Tin(IV) Chloride is that strictly anhydrous conditions need not be maintained, and catalyst loadings of 1 mol % are sufficient to drive the reaction to completion in a reasonable time period (<24 h) for most substrates.1,3 The addition of up to 10 equiv (vs. catalyst) of water to reactions mediated by the diaqua complex (3) has no observed effect on the rate of reaction, nor do the products inhibit catalytic activity.5

Mukaiyama Crossed-Aldol Reactions.1,6

Catalysts (1) and (2).THF have also been shown to catalyze the reaction between silyl enol ethers and carbonyls (eqs 4 and 5) (i.e. the Mukaiyama crossed-aldol reaction8). Catalyst loadings of 0.5% are sufficient except for highly hindered substrates such as pinacolone, and pivaldehyde reacts only slowly under the typical reaction conditions with a 2,2-dimethylsilyl ketene acetal. The primary advantages of (1) and (2).THF over other typical Lewis acids such as TiCl4 and Boron Trifluoride are decreased susceptibility to inhibition by water, and the smooth reaction of silyl enol ethers at low catalyst loadings.

Sakurai Reactions.1,7

The reaction of allylsilanes with ketones and acetals (the Sakurai reaction9) is catalyzed efficiently by (1) and (2).THF (eqs 6 and 7). As with the Diels-Alder and Mukayama reactions discussed above, very low catalyst loadings are required. In addition to ketones and acetals (eqs 6 and 7), a limited number of orthoesters also react with allylsilanes when catalyzed by (1) (eq 8).


1. Hollis, T. K.; Odenkirk, W.; Robinson, N. P.; Whelan, J.; Bosnich, B. T 1993, 49, 5415.
2. Thewalt, U.; Klein, H. P. Z. Kristallogr. 1980, 153, 307.
3. Hollis, T. K.; Robinson, N. P.; Bosnich, B. OM 1992, 11, 2745.
4. Thewalt, U.; Honold, B. JOM 1988, 348, 291.
5. Hollis, T. K.; Robinson, N. P.; Bosnich, B. JACS 1992, 114, 5464.
6. Hollis, T. K.; Robinson, N. P.; Bosnich, B. TL 1992, 33, 6423.
7. Hollis, T. K.; Robinson, N. P.; Whelan, J.; Bosnich, B. TL 1993, 34, 4309.
8. Reviews: (a) Mukaiyama, T. OR 1982, 28, 203. (b) Mukaiyama, T.; Murakami, M. S 1987, 1043.
9. Reviews: (a) Sakurai, H. PAC 1982, 54, 1. (b) Fleming, I.; Dunogués, J.; Smithers, R. OR 1989, 37, 57.

James P. Edwards

Ligand Pharmaceuticals, San Diego, CA, USA



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