[37342-98-6]  · C10H12Zr  · Bis(h5-cyclopentadienyl)dihydridozirconium  · (MW 223.42)

(catalyst for hydrogenation of alkenes and alkynes in combination with hydrogen;2 catalyst for Meerwein-Ponndorf-Verley type reduction and Oppenauer type oxidation;3 catalyst for crossed-aldol condensation;4 catalyst for Tishchenko reaction;5 reducing agent for organohalogen compounds6)

Physical Data: polymeric in solid and dimeric [Cp2ZrH(m-H)]2 in benzene or toluene.7 The polymeric nature of [Cp2ZrH2]n is suggested by its involatility, insolubility, and insusceptibility to reaction in air. The presence of a Zr-H-Zr bridging system is inferred from the intense broad IR absorption band at 1520 cm-1.8

Solubility: insol common solvents.

Form Supplied in: white solid.

Analysis of Reagent Purity: ash (ZrO2), 55.5%; hydrolyzable H2, 1.99 g atm mol-1; 1H NMR (C6D6)7 d 5.75 (C5H5), 3.85 (J = 7.3 Hz), -3.45 (J = 7.3 Hz); IR (KBr disk)8 1520, 1300 cm-1 (Zr-H).

Preparative Methods: the following method is convenient for the preparation of Cp2ZrH2. m-Oxobis[chlorobis(h5-cyclopentadienyl)zirconium] ([Cp2ZrCl]2O) is prepared by the reaction of Dichlorobis(cyclopentadienyl)zirconium (Cp2ZrCl2), available from commercial sources, in dichloromethane and water in aniline.9 To a solution of [Cp2ZrCl]2O in THF is added dropwise Lithium Aluminum Hydride in THF with stirring under an argon atmosphere. The reaction mixture is stirred for ca. 8 h at rt, filtered, and then washed with THF to give the Cp2ZrH2 complex as a white solid (eq 1).9

Handling, Storage, and Precautions: photosensitive and hydrolyzed by water; should be stored in the dark under an argon atmosphere.

Reaction with Alkenes.

Alkyl zirconium derivatives, which can be prepared by the addition of Cp2ZrH2 to alkenes (hydrozirconation), are easily decomposed by hydrogenolysis to give the corresponding alkanes under mild conditions.2,10 As a consequence, Cp2ZrH2 can be used as a catalyst for the hydrogenation of alkenes to alkanes (eq 2).2

Reaction with Alkynes.

Cp2ZrH2-catalyzed hydrogenation of alkynes is also attained (eq 3).2 Cp2ZrH2 reacts with monosubstituted acetylenes to give di-trans-alkenyl zirconium complexes, but with diarylacetylene, tetraarylbutadiene complexes are formed (eqs 4 and 5).11

Meerwein-Ponndorf-Verley Type Reduction.

Cp2ZrH2 catalyzes hydrogen transfer reaction from alcohols to carbonyl compounds.3 The regioselective 1,2-reduction of a,b-unsaturated carbonyl compounds to allylic alcohols is accomplished in the presence of 2-propanol (eq 6).12,13 The reagent mediates the chemoselective reduction of keto aldehydes to hydroxy ketones (eq 7).13 Steroidal diketones, e.g. androst-4-ene-3,17-dione, are reduced to the 17-hydroxy enones with high chemoselectivity (eq 8).13

Oppenauer Type Oxidation.

The above discovery (Meerwein-Ponndorf-Verley type reduction) has been extended to the Oppenauer type oxidation.3 Simple terpenoids, such as geraniol and nerol, are oxidized with retention of stereochemistry of the carbon-carbon double bond to form the corresponding a,b-unsaturated aldehydes (eq 9).14

Crossed Aldol Condensation.

Cp2ZrH2 catalyzes the selective crossed aldol condensation between alicyclic ketones and aliphatic aldehydes to form the corresponding 2-alkylidenecycloalkanones (eq 10).4 The reaction of alicyclic ketones with primary alcohols gives 2-alkyl-2-cycloalken-1-ones via the Oppenauer type oxidation followed by crossed aldol condensation (eq 11).4

Tishchenko Reaction.

Cp2ZrH2 efficiently catalyzes the selective dimerization of aldehydes to esters under mild conditions (eq 12). The crossed dimerization of different kinds of aldehydes is also achieved (eq 13).5

Dehalogenation of Organohalogen Compounds.

Reductive dehalogenation of organohalogen compounds is achieved by the action of Cp2ZrH2.6 Replacement of hydride in Cp2ZrH2 by chloride occurs readily. Thus the reaction of Cp2ZrH2 with dichloromethane gives Cp2ZrCl2 quantitatively (eq 14).15

Reaction with Alcohols.

Cp2ZrH2 readily reacts with alcohols to form the corresponding alkoxy complexes (eq 15).16

Reaction with Carboxylic Acid and Acetone.

The addition of Cp2ZrH2 to excess glacial acetic acid in THF affords Cp2Zr(OAc)2 (eq 16).16 Similarly, the slow addition of acetone to a suspension of Cp2ZrH2 in THF forms Cp2Zr(O-i-Pr)2 (eq 17).17

Reaction with Carbon Monoxide.

The carbonylation of Cp2ZrH2 occurs at room temperature to yield an h-formaldehyde-zirconocene complex in a low yield (eq 18).18 This h-acyl zirconium complex reacts with Cp2ZrH2 to form a dinuclear m-aldehyde-zirconocene hydride complex (eq 19).18

Reaction with Organoborane and Aluminum Complexes.

Cp2ZrH2 with Borane-Tetrahydrofuran or Catecholborane affords quantitatively Cp2Zr(BH4)2 (eqs 20 and 21).16 When Cp2ZrH2 is treated with Triethylaluminum pale blue product, which is not stable in solution, is obtained. The dinuclear structure is proposed on the basis of 1H NMR and IR spectra (eq 22).19

1. (a) Wailes, P. C.; Coutts, R. S. P.; Weigold, H. Organometallic Chemistry of Titanium, Zirconium, and Hafnium; Academic: New York, 1974; p 145. (b) Cardin, D. J.; Lappert, M. F.; Raston, C. L.; Riley, P. I. In Comprehensive Organometallic Chemistry, Wilkinson, G., Ed.; Pergamon: Oxford, 1982; Vol. 3, p 602. (c) Fieser, M.; Smith, J. G. FF 1988, 13, 108.
2. Wailes, P. C.; Weigold, H.; Bell, A. P. JOM 1972, 43, C32
3. Ishii, Y.; Nakano, T.; Inada, A.; Kishigami, Y.; Sakurai, K.; Ogawa, M. JOC 1986, 51, 240.
4. Nakano, T.; Irifune, S.; Umano, S.; Inada, A.; Ishii, Y.; Ogawa, M. JOC 1987, 52, 2239.
5. Morita, K.; Nishiyama, Y.; Ishii, Y. OM 1993, 12, 3748.
6. (a) Strunkina, L. I.; Brainina, E. M.; Minacheva, M. Kh. Metalloorg. Khim. 1989, 2, 677 (CA 1989, 112, 55 102w). (b) Brainina, E. M.; Strunkina, L. I.; Kuz'mina, N. A.; Grechkina, E. M. Izv. Akad. Nauk SSSR, Ser. Khim. 1985, 228 (CA 1985, 102, 166 292w).
7. Bickley, D. G.; Hao, N.; Bougeard, P.; Sayer, B. G.; Burns, R. C.; McGlinchey, M. J. JOM 1983, 246, 257.
8. James, B. D.; Nanda, R. K.; Wallbridge, M. G. H. CC 1966, 849.
9. Wailes, P. C.; Weigold, H. Inorg. Synth. 1990, 28, 257.
10. Gell, K. I.; Posin, B.; Schwartz, J.; Williams, G. M. JACS 1982, 104, 1846.
11. Wailes, P. C.; Weigold, H.; Bell, A. P. JOM 1971, 27, 373
12. Nakano, T.; Kino, Y.; Ishii, Y.; Ogawa, M. Technol. Rep. Kansai Univ. 1987, 29, 69.
13. Nakano, T.; Umano, S.; Kino, Y.; Ishii, Y.; Ogawa, M. JOC 1988, 53, 3752.
14. Nakano, T.; Ishii, Y.; Ogawa, M. JOC 1987, 52, 4855.
15. Wailes, P. C.; Weigold, H. JOM 1970, 24, 405.
16. Männig, D.; Nöth, H. JOM 1984, 275, 169.
17. Wailes, P. C.; Weigold, H. JOM 1970, 24, 413.
18. Erker, G.; Kropp, K.; Krüger, C.; Chiang, A.-P, CB 1982, 115, 2447.
19. Wailes, P. C.; Weigold, H.; Bell, A. P. JOM 1972, 43, C29

Y. Nishiyama & Y. Ishii

Kansai University, Osaka, Japan

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