Vinyl zirconocene

The facile isomerization of alkyl and vinyl zirconocenes complicates the stereocontrol in the transformations of internal acyclic aikenes and allenes, and so further studies are certainly needed to circumvent this problem. 

From the same zirconocene intermediates, Huang and co-workers have prepared vinyl sulfides [24,35] and sulfones [26] through use of the appropriate quenching agents (Scheme 4.6). Treatment of vinyl zirconocenes with an equivalent of a disulfide in THF at 60 °C affords, after work-up and purification, (E)-vinyl sulfides in good isolated yields. Vinyl sulfones, which as a class are generally useful as Michael acceptors and Diels—Alder dienophiles, are obtained in about two hours upon treatment of (fc)-vinyl zirconocenes with various sulfonyl chlorides in THF at 40°C. 

Cross-coupling reactions of vinyl zirconocenes of general structure 41, mainly using group 10 metal catalysts, can be smoothly effected to give a variety of vinyl selenide-con-taining materials, which are amenable to further elaboration through nickel-catalyzed... 

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

Scheme 4.43. Preparation and reactions of a silylated vinyl zirconocene.

Scheme 4.45. Generation of 2,3-dibora-1,3-butadienes by homocoupling of vinyl zirconocenes.

Since the early disclosure by Negishi that zinc halide salts accelerate Pd(0) -catalyzed crosscouplings between vinyl zirconocenes and various halides [78], several methods have been developed that extend the utility of this metathesis process from a zirconium chloride to a zinc chloride (79 Scheme 4.47). Alternatively, routes to more reactive diorganozinc intermediates, e. g., using Me2Zn, convert readily available zinc derivatives to mixed species 80, which selectively couple with various electrophiles [14]. 

Figure 4.10. Non-racemic ligands screened in 1,2-additions of vinyl zirconocenes to aldehydes.

Scheme 4.52. Coupling of an allylic zirconocene with a vinylic zirconocene.

Scheme 4.60. Ni(0)-catalyzed couplings of vinyl zirconocenes with benzylic chlorides.

Ni(0)-catalyzed coupling of a vinyl zirconocene with a chloromethylated heteroaromatic. 

By means of nickel-catalyzed couplings of vinyl zirconocenes, Schwaebe et al. succeeded in obtaining fluorinated materials using a-bromo esters as electrophiles (Scheme 4.62) [113]. The yields achieved, albeit modest (24—65%), were far better using Ni(0) than those obtained in experiments based on several palladium(O) sources (no product observed). Isopropyl esters appear to be crucial, as competing 1,2-addition occurs with both ethyl and n-butyl analogues. Curiously, t-butyl esters were found to completely inhibit both modes of reaction of the zirconocene. 

Hydrozirconation of alkynes with the Schwartz reagent Cp2Zr(H)Cl yields the chlorovinyl zirconocene 17, which is easily converted to the methyl vinyl zirconocene 18 with either methyllithium in THF or methylmagnesium bromide in CH2C12. Compound 18 loses further methane at room temperature to form a zirconacyclopropene intermediate 19, which couples with a second alkyne and forms the metallacyclopentadiene 20 (Scheme 6) [34]. 

When the above-mentioned process is applied to 1-lithio-l-chloroethene 66 [52] and vinyl zirconocene 67, arising from hydrozirconation of terminal alkynes, a stereospecific formation of 2-zirconated dienes 68 is obtained, affording terminal dienes after protonation [53] (Scheme 24). 

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