Zirconocene dichloride

Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.

Zirconocene dichloride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.697
UNII
Properties
C10H10Cl2Zr
Molar mass 292.31 g·mol−1
Appearance white solid
Soluble (Hydrolysis)
Hazards
Safety data sheet CAMEO Chemicals MSDS
Related compounds
Related compounds
 Titanocene dichloride
Hafnocene dichloride
Vanadocene dichloride
Niobocene dichloride
Tanatalocene dichloride
Tungstenoocene dichloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Preparation and structure

Zirconocene dichloride may be prepared from zirconium(IV) chloride-THF complex and sodium cyclopentadienide:

ZrCl4(THF)2 + 2 NaCp → Cp2ZrCl2 + 2 NaCl + 2 THF

The closely related compound Cp2ZrBr2 was first described by Birmingham and Wilkinson.[1]

The compound is a bent metallocene: the Cp rings are not parallel, the average Cp(centroid)-M-Cp angle being 128°. The Cl-Zr-Cl angle of 97.1° is wider than in niobocene dichloride (85.6°) and molybdocene dichloride (82°). This trend helped to establish the orientation of the HOMO in this class of complex.[2]

Reactions

Schwartz's reagent

Zirconocene dichloride reacts with lithium aluminium hydride to give Cp2ZrHCl Schwartz's reagent:

(C5H5)2ZrCl2 + 1/4 LiAlH4 → (C5H5)2ZrHCl + 1/4 LiAlCl4

Since lithium aluminium hydride is a strong reductant, some over-reduction occurs to give the dihydrido complex, Cp2ZrH2; treatment of the product mixture with methylene chloride converts it to Schwartz's reagent.[3]

Negishi reagent

Zirconocene dichloride can also be used to prepare the Negishi reagent, Cp2Zr(η2-butene), which can be used as a source of Cp2Zr in oxidative cyclisation reactions. The Negishi reagent is prepared by treating zirconocene dichloride with n-BuLi, leading to replacement of the two chloride ligands with butyl groups. The dibutyl compound subsequently undergoes beta-hydride elimination to give one η2-butene ligand, with the other butyl ligand promptly lost as butane via reductive elimination.[4]

Carboalumination

Zirconocene dichloride catalyzes the carboalumination of alkynes by trimethylaluminum to give a (alkenyl)dimethylalane, a versatile intermediate for further cross coupling reactions for the synthesis of stereodefined trisubstituted olefins. For example, α-farnesene can be prepared as a single stereoisomer by carboalumination of 1-buten-3-yne with trimethylaluminum, followed by palladium-catalyzed coupling of the resultant vinylaluminum reagent with geranyl chloride.[5]

The use of trimethylaluminum for this reaction results in exclusive formation of the syn-addition product and, for terminal alkynes, the anti-Markovnikov addition with high selectivity (generally > 10:1). Unfortunately, the use of higher alkylaluminum reagents results in lowered yield, due to the formation of the hydroalumination product (via β-hydrogen elimination of the alkylzirconium intermediate) as side product, and only moderate regioselectivities.[6] Thus, practical applications of the carboalumination reaction are generally confined to the case of methylalumination. Although this is a major limitation, the synthetic utility of this process remains significant, due to the frequent appearance of methyl-substituted alkenes in natural products.


References
  1. G. Wilkinson and J. M. Birmingham (1954). "Bis-cyclopentadienyl Compounds of Ti, Zr, V, Nb and Ta". J. Am. Chem. Soc. 76 (17): 4281–4284. doi:10.1021/ja01646a008.
  2. K. Prout, T. S. Cameron, R. A. Forder, and in parts S. R. Critchley, B. Denton and G. V. Rees "The crystal and molecular structures of bent bis-π-cyclopentadienyl-metal complexes: (a) bis-π-cyclopentadienyldibromorhenium(V) tetrafluoroborate, (b) bis-π-cyclopentadienyldichloromolybdenum(IV), (c) bis-π-cyclopentadienylhydroxomethylaminomolybdenum(IV) hexafluorophosphate, (d) bis-π-cyclopentadienylethylchloromolybdenum(IV), (e) bis-π-cyclopentadienyldichloroniobium(IV), (f) bis-π-cyclopentadienyldichloromolybdenum(V) tetrafluoroborate, (g) μ-oxo-bis[bis-π-cyclopentadienylchloroniobium(IV)] tetrafluoroborate, (h) bis-π-cyclopentadienyldichlorozirconium" Acta Crystallogr. 1974, volume B30, pp. 2290–2304. doi:10.1107/S0567740874007011
  3. S. L. Buchwald; S. J. LaMaire; R. B.; Nielsen; B. T. Watson; S. M. King. "Schwartz's Reagent". Organic Syntheses.; Collective Volume, 9, p. 162
  4. Negishi, E.; Takashi, T. (1994). "Patterns of Stoichiometric and Catalytic Reactions of Organozirconium and Related Complexes of Synthetic Interest". Accounts of Chemical Research. 27 (5): 124–130. doi:10.1021/ar00041a002.
  5. "Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenylalanes: α-Farnesene". www.orgsyn.org. Retrieved 2019-11-27.
  6. Huo, Shouquan (2016-09-19), Rappoport, Zvi (ed.), "Carboalumination Reactions", PATAI'S Chemistry of Functional Groups, Chichester, UK: John Wiley & Sons, Ltd, pp. 1–64, doi:10.1002/9780470682531.pat0834, ISBN 978-0-470-68253-1, retrieved 2021-01-19

Further reading
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