Dienes 2-zirconated

The above intramolecular diene cyclizations are likely to proceed through a similar set of reactions as shown in Scheme 6.2 for the intermolecular variants. Thus, as depicted in Scheme 6.6, formation of the zirconacyclopropane at the less hindered terminal alkene (—> ii), generation of the tricyclic intermediate iii, Zr—Mg exchange through the intermediacy of zirconate iy and 3-H abstraction and Mg alkoxide elimination in v may lead to the formation of the observed product. Additional kinetic and mechanistic studies are required before a more detailed hypothesis can be put forward. 

As zirconocene derivatives constitute a powerful tool for various synthetic pathways, the preparation of zirconated dienes as synthetic precursors has triggered the development of a plethora of methodologies for their synthesis. In this chapter, we describe the existing approaches with a special emphasis on the problems of regio- and stereoselectivities as well as on their scope and limitations. 

The protonolysis of the equally substituted zirconacycle 27 with weak acids such as ethanol leads to the monoprotonated dienyl zirconocene compound 28, which is subsequently converted into substituted diene 29 by a palladium crosscoupling reaction with aryl iodide (Scheme 14) [44]. Selective halogenation of such zirconacycle leads, as well, to the formation of zirconated diene 30 and then to the 1,4-dihalogenodiene 31 (Scheme 15) [45]. 

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). 

A different approach for the in situ preparation of 2-zirconated 1,3-dienes 68 was reported by Szymoniak and Bertus et al.,by treatment of 2-silyloxy-l,3-di-enes 81 with dialkylzirconocene 21 (Scheme 28) [61]. The zirconocene induced the C-0 bond cleavage of 81 and led to the dienyl zirconium compound 68,... 

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. 

The X-ray crystal structure of the tris( n4-naphthalene)zirconate(2-) anion as the bis[K(2,2,2-crypt)] salt has been determined, revealing a trigonal prismatic coordination geometry of local Csv symmetry about the zirconium atom, as defined by the mid-points of the C-C vectors of the coordinated diene units, similar to that eviously observed for the isoelectronic tris(ii i-butadiene)molybdenum complex. 

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