Zirconocene dichloride
Chemical compound
From Wikipedia, the free encyclopedia
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.
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| ECHA InfoCard | 100.013.697 | ||
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| Properties | |||
| C10H10Cl2Zr | |||
| Molar mass | 292.31 g·mol−1 | ||
| Appearance | white solid | ||
| Soluble (Hydrolysis) | |||
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| Safety data sheet (SDS) | CAMEO Chemicals MSDS | ||
| Related compounds | |||
Related compounds |
Titanocene dichloride Hafnocene dichloride Vanadocene dichloride Niobocene dichloride Tantalocene dichloride Tungstenocene 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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Preparation and structure
Zirconocene dichloride may be prepared from zirconium(IV) chloride-tetrahydrofuran 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 trimethylaluminium 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 trimethylaluminium, followed by palladium-catalyzed coupling of the resultant vinylaluminium reagent with geranyl chloride.[5]
The use of trimethylaluminium 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 alkylaluminium 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.
Zr-walk
Zirconocene dichloride together with a reducing reagent can form the zirconocene hydride catalyst in situ, which allows a positional isomerization (so-called "Zr-walk"[7]), and ends up with a cleavage of allylic bonds. Not only individual steps under stoichiometric conditions were described with Schwartz reagent,[8] and Negishi reagent,[9] but also catalytic applications on alkene hydroaluminations,[10] radical cyclisation,[11] polybutadiene cleavage,[12] and reductive removal of functional groups[13] were reported.
