Organic Syntheses, CV 9, 162
SCHWARTZ'S REAGENT
[Zirconium, chlorobis(η5-2,4-cyclopentadien-1-yl)hydro-]
Submitted by Stephen L. Buchwald
1, Susan J. LaMaire, Ralph B. Nielsen, Brett T. Watson, and Susan M. King.
Checked by Daniel J. Keavy and Robert K. Boeckman, Jr..
1. Procedure
CAUTION! The washing procedure described in (Note
8) should be followed carefully. On one occasion, allowing a contact time of ca. 1 hr while attempting to use a cannula filter to remove the
methylene chloride led to an exothermic decomposition reaction.
To a dry,
1-L Schlenk flask equipped with a
magnetic stirring bar is added under
argon zirconocene dichloride (100 g, 0.342 mol) (Note
1), followed by dry
tetrahydrofuran (650 mL) (Note
2). Dissolution of the solid is accomplished by gentle heating with a heat gun. To the solution at

35°C (Note
3) is added dropwise, over a 45-min period, a filtered solution (Note
4),(Note
5) of
lithium aluminum hydride (3.6 g, 94 mmol) (Note
4) in
ethyl ether (100 mL) (Note
6),(Note
7). The resulting suspension is stirred at room temperature for 90 min. The mixture is then Schlenk-filtered (
f.htmigure 1) under
argon using a "D" frit. The resulting white solid is washed with
tetrahydrofuran (4 × 75 mL),
methylene chloride (2 × 100 mL) (Note
8) with stirring or agitation of the stirbar immersed in the slurry, and then with
ether (4 × 50 mL). The resulting white solid is dried under reduced pressure to give a white powder (Note
9),
66 g,
75% yield (Note
10).
Figure 1
2. Notes
1.
Zirconocene dichloride is purchased from Boulder Scientific Co., and used without any further purification.
3. Crystallization occurs if the temperature is allowed to fall below 35°C. The reaction can be conducted successfully even if a small amount of solid forms.
4. This solution is prepared by adding
lithium aluminum hydride to
ethyl ether, stirring the suspension for 10 min, and allowing the undissolved material to settle to the bottom of the flask. The clear solution is then filtered under
argon using a modified cannula (
f.htmigure 2) fitted with a piece of glass fiber filter (Number 34 Glass, 24 mm, purchased from Schleicher & Schuell); a Schlenk-filtered or commercial clear solution would work as well.
Figure 2
6. A slightly exothermic reaction results that maintains the temperature of the reaction mixture at

35°C during the addition.
7.
Ethyl ether is distilled from
sodium/
benzophenone ketyl under
argon at atmospheric pressure immediately before use.
9.
Schwartz's Reagent is an air, moisture, and moderately light sensitive compound that should be dried in the dark, and protected from moisture and light during storage.
10. A small sample of the hydride is suspended in benzene-d
6 and assayed in a 5-mm NMR tube by treatment with a known amount of excess
acetone (Equation 1). The relative areas of the signal for the mono- and diisopropoxides are determined by
1H NMR (300 MHz, C
6D
6 integrating the methyl doublets): Cp
2Zr(H)Cl: 94–96%, Cp
2ZrH
2: 4–6%.
2
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Schwartz's Reagent3 is available commercially (from the Aldrich Chemical Company, Inc.) although it is quite expensive. Two literature preparations of this important reagent are available. The first utilizes
LiAl(OtBu)3H to reduce
zirconocene dichloride.
4 5 The second method utilizes
sodium bis(2-methoxyethoxy) aluminum hydride (RED-AL) as the reducing agent.
2 The disadvantages of these procedures have been discussed.
3
Wailes, in his original report on the preparation of
Cp2Zr(H)Cl, investigated the
lithium aluminum hydride reduction of
zirconocene dichloride and found that this leads to considerable overreduction to give
Cp2ZrH2.
4,5 Later it was found that treatment of
Cp2ZrH2 with
methylene chloride converts the dihydride into
Schwartz's Reagent.
3
In the procedure described here,
zirconocene dichloride is reduced to a mixture of
Cp2Zr(H)Cl, and
Cp2ZrH2 using a solution of
lithium aluminum hydride. Washing the mixture with
methylene chloride converts the
Cp2ZrH2 into the desired
Cp2Zr(H)Cl. This method circumvents the need for expensive reducing agents and the use of the filtered
lithium aluminum hydride solution substantially simplifies the product isolation. The procedure can be performed in 3–4 hr and does not require the use of a glove box thus making it an experimentally simple, inexpensive preparation for large scale batches of
Schwartz's Reagent.
The utility of
Cp2ZrHCl for hydrozirconation was discovered by Schwartz.
2,6 7 8 9 Many subsequent applications of this useful reagent have been documented.
10 One such application is illustrated in the conjugate addition of a vinylzirconium reagent to form
3-(1-octen-1-yl)cyclopentanone.
11 In some cases
Schwartz's Reagent (or its equivalent) can be prepared and used in situ.
12
This preparation is referenced from:
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