Zirconocene dichloride

Zirconocene dichloride
Identifiers
CAS Number	1291-32-3 ;
3D model (JSmol)	Interactive image;
ChemSpider	29081433 ;
ECHA InfoCard	100.013.697
PubChem CID	10891641;
UNII	SQE0U3LG60 ;
CompTox Dashboard (EPA)	DTXSID7026318 ;
	InChI InChI=1S/2C5H5.2ClH.Zr/c21-2-4-5-3-1;;;/h21-5H;21H;/q2-1;;;+4/p-2 Key: QMBQEXOLIRBNPN-UHFFFAOYSA-L ; InChI=1/2C5H5.2ClH.Zr/c21-2-4-5-3-1;;;/h21-5H;21H;/q2-1;;;+4/p-2 Key: QMBQEXOLIRBNPN-NUQVWONBAX;
	SMILES [cH-]1cccc1.[cH-]1cccc1.[Cl-].[Cl-].[Zr+4];
Properties
Chemical formula	C10H10Cl2Zr
Molar mass	292.31 g·mol−1
Appearance	white solid
Solubility in water	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).
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	Infobox references

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.

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

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.
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
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
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.
"Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenylalanes: α-Farnesene". www.orgsyn.org. Retrieved 2019-11-27.
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