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]
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.
^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
^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.
^Negishi; Yoshida (1980). "A novel zirconium- catalyzed hydroalumination of olefins". Tetrahedron Lett. 21 (16): 1501–1504. doi:10.1016/S0040-4039(00)92757-6.
^Fujita; Nakamura; Oshima (2001). "Triethylborane-Induced Radical Reaction with Schwartz Reagent". J. Am. Chem. Soc. 123 (13): 3137–3138. doi:10.1021/ja0032428.
