Vinyl zirconium reagents

A particularly clear example of syn versus anti addition was carried out in stoichiometric fashion using vinyl-zirconium reagents, in direct comparison with the stereochemical outcome of the catalytic ally lie alkylation with soft anions (Scheme 17). The zirconium reagent gives the natural steroid coiffiguration, while the malonate anion gives the opposite... 

Vinyl organometallic reagents can be added to acyl derivatives. Reaction of an alkyne with Cp2ZrEt2 generates the vinyl zirconium reagent, which react with ethyl chloroformate to give an a,p-unsaturated ester. °°°... 

In this method, one alkyne is treated with Schwartz s reagent (see 15-17) to produce a vinylic zirconium intermediate. Addition of MeLi or MeMgBr, followed by the second alkyne, gives another intermediate, which, when treated with aqueous acid, gives the diene in moderate-to-good yields. The stereoisomer shown is the one formed in usually close to 100% purity. If the second intermediate is treated with I2 instead of aqueous acid, the 1,4-diiodo-1,3-diene is obtained instead, in comparable yield and isomeric purity. This reaction can also be done intramolecularly Diynes 56 can be cyclized to ( , E) exocychc dienes 57 by treatment with a zirconium complex. 

Di(cyclopentadienyl)zirconium hydrochloride, Schwartz s reagent , hydrochloride allows hydrozircona-tion. The resulting vinyl zirconium species 2.137 behave similarly to the vinylalanes, and undergo coupling in a similar manner (Scheme 2.47). 

Similarly, copper and zirconium reagents lead to transmetallation reactions with organopalladium compounds. These reagents are particularly useful to introduce a vinyl group into various substrates and has received applications in steroids [30], prostaglandins (Ref. 31) and natural diterpenes [32, 33] synthesis. 

Alkylidene-bridged zirconium complexes 53, long-chain alkylidene analogues of the Tebbe reagent 3, are prepared by the reaction of chlorobis(cydopentadienyl)-(vinyl)zirconium with DIBAL-H, or by addition of the Schwartz reagent, H(Cl)ZrCp2 [116], to a solution of (vinyl)diisobutylaluminum (Scheme 4.49) [117]. Similarly to the Tebbe reagent 3, the complex 53a (R = Bu) itself serves as an... 

The stereochemically controlled addition of organometallic species of copper, tin, silicon, palladium, zirconium, and boron to acetylenes has been investigated as a route to di-, tri-, and tetra-substituted olefins. The carbon-metal bond thus formed is cleaved in a stereoselective manner either directly, or indirectly, via the corresponding vinyl-lithium reagents with a wide variety of electrophiles. In three... 

The addition of the reagent [Cp2Zr(H)Cl] to terminal acetylenes occurs stereospecifically, leading to a vinyl zirconium complex, which in the presence of catalytic amounts of anhydrous [Ni(acac)2] undergoes rapid conjugate addition to a,/8-unsaturated ketones (Scheme 14). ... 

Inteamolecular threearyl iodides and alkenyl-zirconium reagents provided the synthesis of 1,4-dienes [235]. Both vinyl and aryl iodides are active participants for generating highly regio- and stereoselective assembUes. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

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