Alkyne Zirconation

Stable zirconate complexes of type 31 have also been described by the [3 + 2] cycloaddition of a-phosphino zirconaindene with alkyne derivatives, and the method has been extended to the synthesis of zirconium complexes from various heterocumulenes X=C=Y (X,Y=0,S,CyN) (Scheme 15) [69,70]. 

The alkyne hydrozirconation protocol was also applied to acetylenic tellurides furnishing the zirconated vinyl tellnrides in cis fashion and high regioselectivity. Subsequent treatment with tellurenyl halides affords telluro ketene acetals with total retention of configuration. ... 

Among the most prominent examples of this reaction are the carbocupra-tion [5], the zirconium-catalyzed carboalumination [9-12], the nickel-catalyzed carbozincation [13-14], the allylmetalation of metalated alkynes [15], the allyl-zirconation [16], the allylgallation [17], the allylmanganation [18-19], and the alkyllithiation [20] reactions. 

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). 

The modified Negishi protocoi was used in J.S. Panek s total synthesis of (-)-motuporin to couple the left-hand subunit organozinc compound with the right-hand subunit ( )-vinyl iodide. The left-hand subunit was prepared by the Schwartz hydrozirconation of a disubstituted alkyne to give an ( )-trisubstituted zirconate, which was subsequently transmetalated with anhydrous ZnCl2. The resulting vinylzinc species was immediately treated with one equivalent of the ( )-vinyl iodide in the presence of 5 mol% Pd(PPhs)4 to afford the ( , )-diene coupled product with complete stereoselectivity. 

As already mentioned in the introduction, anionic early transition metallocene complexes of the type [Cp2Zr(R1)(R2)(R3)] with R, R2, R3 = alkyl, alkenyl, or alkynyl, are unstable. Most of them exhibit electrostatic anion-cation pairing resulting in dimer, trimer, oligomer, or polymeric structures [21-25]. In marked contrast, stable 18 electron-zirconate complexes were prepared via a formal [3 + 2] cycloaddition reaction between 2-phosphino-zirconaindene 16a and alkyne derivatives (Scheme 5) [26,27]. 

Scheme 5. [3+2] cycloadditions involving a-zirconated phosphines and alkynes... 

Transmetalation of Zirconated Compounds. The reaction of allylzirconation derivatives with allyl chloride can be catalyzed by CuCl to give stereodefined 1,4,7-trienes. Similarly, zirconacyclopentadienes react with 2 equiv of allyl chloride to give 1,4,6,9-decatetraenes in good yields. Reaction of zirconacyclopentadienes with alkynes such as dimethyl acetylenedicar-hoxylate can produce a hexa-substituted benzene ring system in the presence of CuCl/LiCl (eqs 34 and 35). ... 

Takahashi s group has also reported hydrosilation of a number of alkynes with various silanes, using the [Cp2TiCl2/2 n-BuLi] combination, which leads to products with excellent regio- and -selectivity [Eq. (13)]. It is of note that known zircon-ocene and hafnocene species do not catalyze the hydrosilation of alkynes. 

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