Rearrangement reactions zirconacycles

Eq. 2.78. Rearrangement reactions of zirconacycles bearing an alkynylsilyl group. 

Eq. 2.79. Rearrangement reaction of zirconacycles containing an alkynyl phosphorus group. 

In contrast to the examples reported above, some other aryl- and vinyl-stabilized lithiooxiranes show a strong electrophilic behavior and undergo rearrangement reactions (Scheme 83, see also Section V.A.2.a for other examples). Lithiated styrene oxide has been engaged in 1,2-metallate rearrangement with zirconacycles . 

The reaction of zirconacyclobutene-silacyclobutene fused compound 46 formed from Si-tethered diyne 45 with Cp2Zr(II) species has been used in the synthesis of fused aromatic heterocycles 47 (Scheme 11.18) [20]. When the zirconacycles 46 reacted with nitriles, the corresponding pyrrolo[3,2-c]pyridines 47 were formed. This indicates the unexpectedly strong effects of the alkynylsilyl groups on this unusual skeletal rearrangement of zirconacycles. 

Zirconacyclopentanes can readily undergo skeletal rearrangements, exemplified by those shown in Scheme 41. Although the process involving Zr reveals only the final zirconacycle, examination by NMR spectroscopy of the corresponding Hf reaction shows the formation and decay of the 2,5- and 2,4-dimethylhafnacyclopentanes. All of the three hafnacycles as well as the zirconacyclic product are >98% dl.229... 

One important aspect of the Cp2Zr-promoted cycliza-tion that is now well-established is the ready reversibility through decarbozirconation, presumably involving an interaction between the empty Zr-orbital and the C/S -Cfi bond of the five-membered zirconacycle and subsequent displacement of ethylene (equation 53). Reaction of 2-azadienes on zirconocene affords via a retro-Brook rearrangement interesting azaoxazirconacycles. ... 

Addition of zircona- and titanacyclopropane metallocenes to conjugated enones was investigated. Analysis of the products shows that the reaction course is strongly controlled by both the metallocyclic and the enone moiety. The zirconacycle affords the rearranged (467). On the other hand, rearranged adducts, 1,3-butadienylphosphonates (468) are formed when titanacycles are used (Scheme 108). ... 

A wide variety of five-membered zirconacycles 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cyclization takes place via the -complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/P-hydrogen elimination) (path b). Two additional routes to zirconocene r 2 -complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

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